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TABLE OF CONTENTS

HISTORY OF NGMA DESIGN LOAD STANDARD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I

STANDARD FOR DESIGN LOADS IN GREENHOUSE STRUCTURES . . . . . . . . . . . . . . . . . . . . . . . 1

1. General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2. Combination of Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3. Dead Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

4. Live Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

5. Wind Loads. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

6. Snow Loads. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

COMMENTARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

C1. General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

C2. Combination of Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

C3. Dead Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

C4. Live Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

C5. Wind Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

C6. Snow Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

ACKNOWLEDGEMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Copyright 1985Revised 1994Revised 1996 National Greenhouse Manufacturers Association

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TABLE OF CONTENTS Design Load

HISTORY OF NGMA STANDARD

On November 5, 1968, NGMA adopted its first structural standard. Seven years later on November 12, 1975, a revised version of the first standard was adopted. Both these original standards were brief documents which defined loads to beused in the design of greenhouse structures throughout the country. The load values and requirements of the standardwere based on years of experience in manufacturing and construction of greenhouses, on consideration of characteristicsthat are unique to greenhouse structures and on the history of successful structural performance of thousands of green-houses constructed during the past 50 years.

Following adoption of the revised standard in 1975, NGMA made an effort to have its requirements included in several recognized building codes. However, it was found that specific NGMA requirements adopted by each of these codesoften varied. As a result, NGMA submitted its standard to the American National Standards Institute (ANSI) for incorporationinto ANSI A58.1, “Building Code Requirements for Minimum Design Loads in Buildings and Other Structures”. In adraft of ANSI A58.1 dated March 10, 1982, most of the NGMA requirements were covered either as part of the codeitself or as part of the appendix to the code.

In a continuing effort to improve and further standardize greenhouse design and construction, NGMA has developedthis current expanded structural load standard. The standard is based on the currently proposed ANSI A58.1 and in factfollows the same notation and much of the same wording. However, any ANSI requirements that do not apply specificallytogreenhouse type structures have been deleted. In addition, several sections while keeping in line with the ANSIintent, have been modified and simplified. In 1996, Section 6.0 snow load, was revised to follow the notation andwording of the BOCA National Building Code, 1993.

It is the hope of NGMA that introduction of this standard will provide for uniform and safe greenhouse design and construction.

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Design Load HISTORY

1.0 GENERAL

1.1 SCOPE

This standard provides load requirements for design of greenhouse structures and their components. The loads specifiedherein are to be used in conjunction with the allowable stresses recommended in current design specification for aluminum,steel, wood, glass, concrete or any other conventional structural material used in the construction of greenhouses.

1.1.1 DEFINITIONS

The following definitions are intended to apply only to greenhouse structures and their components.

Free-Standing Greenhouse - an independently erected greenhouse set totally apart from other buildings and structures. Free-standing greenhouses are usually symmetrical about a longitudinal centerline (even-span) witheither a pitched or an arched roof.

Attached Even-Span Greenhouse - a greenhouse structure similar to a free-standing greenhouse except that oneor both gable ends or sides are eliminated and are attached to an adjacent structure.

Lean-to Greenhouse - a greenhouse structure which depends on its attachment to another building for much of itssupport. A lean-to greenhouse appears as a free-standing greenhouse bisected in half along its longitudinal centerline with the missing side provided by the building against which it is supported.

Gutter-Connected Greenhouse - a series of two or more free-standing greenhouses joined together at their eavesline. A gutter is provided at the common eaves of adjacent greenhouses to allow collection and run-off of rain ormelting snow. Usually the common sides of two adjacent gutter-connected greenhouses are omitted to provide greater uninterrupted interior growing space.

Gable Ends - the two exterior walls of a free-standing greenhouse which are oriented perpendicular to the longitudinal axis of the greenhouse.

Sides - the two exterior walls of a free-standing greenhouse which are oriented parallel to the longitudinal axis ofthe greenhouse.

Eaves - the intersection of the roof and the side of a typical greenhouse.

Hobby House - a greenhouse used by an individual or family for growing flowers and plants as a hobby. A hobbyhouse may be free-standing, attached even-span or lean-to.

Production Greenhouse - a greenhouse used for growing large numbers of flowers and plants on a production basisor for research. Generally there is no public access to a production greenhouse. Included in this category are privately owned greenhouses used for research purposes.

Retail Greenhouse - similar to a production greenhouse in that it is used for growing large numbers of flowers andplants. However, in a commercial greenhouse, general public access for the purpose of viewing and purchasing thevarious products is permitted. Included in this category are greenhouses used by colleges or universities for teaching purposes or for research.

Glazing Material - any rigid material such as glass or fiberglass, rigid plastics or any flexible plastic material suchas polyethylene used to enclose a greenhouse while at the same time permitting the entrance of natural light.

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GENERAL Design Load

1.1.2 LIMITATIONS

This standard applies to free-standing, attached even-span and lean-to greenhouses whose individual foundations are atground level. Greenhouses constructed on top of other structures, solar domes, skylights and similar greenhouse-typestructures are not specifically covered.

1.2 BASIC REQUIREMENTS

1. Safety - Greenhouse structures and all parts thereof shall be designed and constructed to safely support all loads, including dead load without exceeding the allowable stresses for the materials from which the greenhouseis constructed.

2. Serviceability - Greenhouse structures and their components shall have adequate stiffness to limit vertical andtransverse deflection, vibrations or any other deformation that may adversely affect their serviceability.

3. Analysis - Load effect on the individual components and connections of greenhouse structures shall be determinedby accepted methods of structural analysis.

1.3 GENERAL STRUCTURAL INTEGRITY

Through accident or misuse, a greenhouse structure capable of safely supporting the required design loads may sufferlocal damage, i.e., the loss of load resistance in an element or small portion of the structure. In recognition of this, thegreenhouse structure shall possess general structure integrity, i.e., the quality of being able to sustain local damage withthe structure as a whole remaining stable and not damaged to an extent disproportionate to the original local damage.

1.4 ADDITIONS TO EXISTING STRUCTURES

When a lean-to or attached even-span greenhouse is added to an existing building, provision shall be made to adequatelystrengthen the existing structure, where necessary, to withstand existing loads as well as any additional loads imposedon it by the greenhouse.

2.0 COMBINATION OF LOADS

2.1 COMBINING LOADS

Except when applicable codes make other provisions, all loads listed herein shall be considered to act in the followingcombinations. The governing case shall be that which produces the most unfavorable effects in the structure, foundationor member under consideration.

Where:1. D2. D + L D = Dead Load3. D + S L = Live Load4. D + W S = Snow Load5. D + L + W W = Wind Load6. D + S + W

2.2 LOAD COMBINATION FACTORS

Allowable stresses may be increased 33% for any of the above combinations that include wind providing the resultingallowable stress does not exceed the yield stress.

2.3 COUNTERACTING LOADS

When the effects of design loads counteract one another in a structural member or joint, care shall be taken to ensureadequate safety for possible stress reversal.

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Design Load STRUCTURAL LOADS

3.0 DEAD LOADS

3.1 DEFINITION

The weight of all permanent construction including but not limited to walls, roofs, glazing materials and fixed serviceequipment.

3.2 WEIGHTS OF BUILDING MATERIALS

In estimating dead loads for the purpose of design, the actual weights of pertinent building materials shall be used. Inthe absence of definite information, values satisfactory to the authority having jurisdiction shall be used.

3.3 WEIGHT OF FIXED SERVICE EQUIPMENT

In estimating dead loads for the purpose of design, the weight of fixed service equipment such as heating, ventilating andcooling systems, electrical and lighting systems and watering and humidification systems shall be included whenever itis supported by structural members.

3.4 SPECIAL CONSIDERATIONS

Factors that may result in differences between actual and calculated values should be considered when determiningdead loads. In addition, any permanent loads such as hanging baskets, planters, etc., that are to be supported by struc-tural members for an extended time period (Section 4.1) shall be included as part of the dead load.

4.0 LIVE LOADS

4.1 DEFINITIONS

Live loads are temporary loads produced by the use and occupancy of the greenhouse. Live loads do not include wind load,snow load or dead load. Exterior live loads on greenhouse roofs are the temporary loads workmen and temporary equip-ment such as scaffolds. Interior live loads are temporary loads imposed on the structure by hanging objects. Any live loadshall be considered permanent and therefore included as part of the dead load (Section 3.4) if it is imposed on the structurefor a continuous period of 30 days or more.

4.2 MINIMUM ROOF LIVE LOAD

Pitched and arched greenhouse roofs shall be designed to safely support the minimum live load specified in the followingequation or the snow load specified in Section 6, whichever is greater.

L = 20 R1 R2 > 12

where L, the minimum live load, is in pounds per square foot of horizontal projection, and R1 and R2

are reduction factors determined as follows:

R1 = 1.0 for At < 200= 1.2 - 0.001 At for 200 < At < 600= 0.6 for At > 600

in which At is the tributary area in square feet supported by the structural member under consideration:

R2 = 1.0 for F < 4= 1.2 - 0.05 F for 4 < F < 12= 0.6 for F > 12

in which F is equal to the number of inches of rise per foot for a pitched roof and is equal to the rise tospan ratio multiplied by 32 for an arched roof.

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STRUCTURAL LOADS Design Load

4.3 MAXIMUM ROOF LIVE LOAD

The live load determined by the requirements of Section 4.2 shall be limited to a maximum value of 15 PSF.

4.4 CONCENTRATED LOADS

All roof members such as purlins, rafters, truss top members, etc., shall be capable of safely supporting a minimumconcentrated live load of 100 lbs applied downward and normal to the roof surface at their midspan. In addition, bottomchord panel points of roof trusses shall be capable of safely supporting a minimum concentrated live load of 100 lbsapplied at any panel point. See Section C4.4 for further discussion of concentrated loads.

4.5 PARTIAL LOADING

The full intensity of the live load applied only to a portion of a greenhouse structure or to a portion of an individualmember shall be considered if it produces a more unfavorable effect than the full intensity applied over the entire structureor member.

4.6 IMPACT LOADS

The concentrated live load specified in Section 4.4 includes adequate allowance for ordinary impact conditions.

4.7 RESTRICTIONS ON LOADING

It shall be the responsibility of the greenhouse manufacturer to inform the owner of the live loads for which the greenhousewas designed. It shall then be the responsibility of the greenhouse owner to ensure that a live load greater than that forwhich the roof or roof supporting members were designed is not placed upon the roof or supporting members.

5.0 WIND LOADS

5.1 GENERAL

Provisions for the determination of wind loads on greenhouse structures are described in the following subsections.The provisions apply to the calculation of wind loads for both the main wind-force resisting system and the individualcomponents and glazing of the structure.

5.1.1 WIND LOADS DURING ERECTION AND CONSTRUCTION PHASES

Adequate temporary bracing shall be provided to resist wind loading on structural components and structural assemblagesof greenhouses during the construction phase.

5.1.2 OVERTURNING AND SLIDING

The overturning moment due to wind load shall not exceed two-thirds of the dead load stabilizing moment unless thegreenhouse structure is anchored to resist the excess moment. When the total resisting force due to friction is insufficientto prevent sliding, anchorage shall be provided to resist the excess sliding force.

5.1.3 DEFINITIONS

The following definitions apply only to the provisions of Section 5, WIND LOADS.

Main Wind-Force Resisting System - an assemblage of major structural elements assigned and designed to support the design wind force. The system transfers wind load applied to the components and glazing of the greenhouse to its structural foundation. Such systems include combinations of roof trusses and supporting columns, rigid frames,braced frames, etc.

Components and Glazing - local structural elements which are directly loaded by the wind. In greenhouses, examples of such elements are glass, rigid plastics or fiberglass glazing materials and the connection devices usedto attach these materials to the structure. Secondary members that support the glazing materials and transfer thewind loads to main wind-force resisting system (members such as purlins and lintel beams) should be consideredas components.

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Importance Coefficient (I) - a coefficient to account for hazard to human life and damage to property.

Design Pressure (P) - equivalent static pressure to be used in the determination of wind loads on greenhouses. The pressureis assumed to act in a direction normal to the surface under consideration, either as a pressure directed towards the surface(positive value) or as a suction directed away from the surface (negative value). In calculating the design wind loads forcomponents and glazing the pressure difference between opposite faces of the surface shall be taken into consideration.

5.1.4 SYMBOLS AND NOTATIONS

The following symbols and notations apply only to the provisions of Section 5, WIND LOADS.

A: Tributary area for determination of wind loads on components and glazing (sq ft)

a: Width of pressure coefficient zone (ft)

b: Horizontal dimension of greenhouse normal to wind direction (ft)

d: Horizontal dimension of greenhouse parallel to wind direction ridge line (ft)

Cp: External pressure coefficient

Cpi: Internal pressure coefficient

G: Gust response factor

(GCp): Product of external pressure coefficient and gust response factor

(GCpi): Product of internal pressure coefficient and gust response factor

h: Mean roof height of greenhouse (ft). Eaves height may be used for greenhouses having pitched roofs with slopes of less than 10 degrees.

I: Importance coefficient

Kz: Velocity exposure coefficient at height z

P: Design pressure (psf)

Ph: Design pressure at height z = h (psf)

Pz: Design pressure at height z (psf)

q: Velocity pressure (psf)

qh: Velocity pressure at height z = h (psf)

qz: Velocity pressure at height z (psf)

r: Rise to span ratio for arched roofs

V: Basic wind speed (mph)

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z: Height above ground level (ft)

ø: Angle of plane of pitched roof (degrees)

5.2 CALCULATION OF WIND LOADS

5.2.1 GENERAL

The design wind loads for greenhouse structures as a whole or for individual components and glazing shall be determinedby the Analytical Procedure described in Section 5.2.2.

5.2.2 ANALYTICAL PROCEDURE

Design wind pressures for greenhouses shall be determined in accordance with the equations in Table 5.1 using the followingprocedure:

1. A velocity pressure, q, is determined in accordance with Section 5.3.

2. A gust response factor, G, is determined in accordance with the provisions of Section 5.4.

3. Appropriate pressure or force coefficients are selected from Section 5.5.

5.2.2.1 MINIMUM DESIGN WIND LOADING

The wind load to be used in the design of the main wind-force resisting system for greenhouses shall be at least 10 psf.

In the calculation of design wind loads for components and glazing of greenhouses, the pressure difference betweenopposite faces shall be taken into consideration. The combined design pressure shall be at least 10 psf acting eitherinward or outward normal to the surface.

TABLE 5.1

DESIGN WIND PRESSURES (P)

For the main wind-force resisting system:P = qGCp - qh (GCpi)

where: q: qz for windward wall evaluated at height z above groundqh for leeward wall, sidewall and roof evaluated at mean

roof height (h)G: given in Table 5.4Cp: given in Table 5.5 and 5.7

(GCpi): given in Table 5.8

For components and glazing:P = qh (GCp) - qh (GCpi)

where: qh: evaluated using Exposure C for all terrains

(GCp): given in Tables 5.6A, 5.6B and 5.7(GCpi):given in Table 5.8

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5.3 VELOCITY PRESSURE

5.3.1 PROCEDURE FOR CALCULATING VELOCITY PRESSURE

The velocity pressure qz at height z shall be calculated as follows:

qz = 0.00256 Kz (IV)2

where: V: given in Fig. 5.1 in accordance with the provisions of Section 5.3.2

I: given in Table 5.2

Kz: given in Table 5.3 in accordance with the provisions of Section 5.3.3

5.3.2 SELECTION OF BASIC WIND SPEED

The basic wind speeds, V, to be used in determination of design wind loads shall be as given in Fig. 5.1 for the contigu-ous United States and Alaska. The basic wind speed for Hawaii shall be 80 mph. In no case shall the basic wind speedbe less than 70 mph.

5.3.2.1 SPECIAL WIND REGIONS (See section C5.3.2.1)

5.3.3 EXPOSURE CATEGORIES

5.3.3.1 GENERAL

An exposure category shall be determined for the general region in which the greenhouse is to be constructed. Exposurecategories are intended to reflect variations in surrounding ground surface roughness arising from both natural topogra-phy and vegetation as well as existing construction. Each greenhouse shall be assessed as being located in one of thefollowing exposure categories:

TABLE 5.2IMPORTANCE COEFFICIENT (I)

Notes: 1. Hurricane-prone oceanlines are the Atlantic and Gulf of Mexico coastal areas.

2. For regions between the hurricane-prone oceanline and 100 miles inland, the importance coefficient, I, shall be determined by linear interpolation.

TYPE OF GREENHOUSE100 Miles or More from

OceanlineHurricane Prone

Oceanline

Retail Greenhouse withgeneral public access

permitted1.00 1.05

All other greenhouses 0.95 1.00

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TABLE 5.3VELOCITY EXPOSURE COEFFICIENT (Kz)

z (ft)

Note: Linear interpolation for intermediate values of z is acceptable.

Exposure A: large city centers with at least 50 percent of the buildings having a height in excess of 70 ft. Use of thisexposure category shall be limited to those areas for which terrain representative of Exposure A prevails in theupwind direction for a distance of at least one-half mile. Possible channeling effects or increased velocity pressuresdue to the greenhouse being located in the wake of adjacent buildings shall be taken into account.

Exposure B: urban and suburban areas, well wooded areas or other terrain with numerous closely spaced obstructions having the size of single family dwellings or larger. Use of this exposure category shall be limitedto those areas for which terrain representative of Exposure B prevails in the upwind direction for a distance of at least 1500 ft.

Exposure C: open terrain with scattered obstructions having heights generally less that 30 ft. This category includes flat, open country and grasslands.

Exposure D: flat unobstructed coastal areas directly exposed to wind blowing over large bodies of water. This exposure shall be used for those areas representative of Exposure D extending inland from the shoreline adistance of 1500 ft.

5.3.3.2 EXPOSURE CATEGORY FOR DESIGN OF MAIN WIND-FORCE RESISTING SYSTEM

Wind loads for the design of the main wind-force resisting system in greenhouses shall be based on the exposure categoriesdefined in Section 5.3.3.1.

5.3.3.3 EXPOSURE CATEGORY FOR DESIGN OF COMPONENTS AND GLAZING

Components and glazing for greenhouses shall be designed on the basis of Exposure C.

TABLE 5.4GUST RESPONSE FACTOR (G)

h (ft)

Note: Linear interpolation for intermediate values of h is acceptable.

Exposure 0-15 ft 20 ft 25 ft

A 2.36 2.20 2.09

B 1.65 1.59 1.54

C 1.32 1.29 1.27

D 1.15 1.14 1.13

Exposure 0-15 ft 20 ft 25 ft

A 0.12 0.15 0.17

B 0.37 0.42 0.46

C 0.80 0.87 0.93

D 1.20 1.27 1.32

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5.3.4 SHIELDING

Reductions in velocity pressures due to apparent direct shielding afforded by buildings, structures and terrain features is notpermitted.

5.4 GUST RESPONSE FACTORS

Gust response factors are employed to account for the fluctuating nature of the wind and its interaction with the structure. In design of the main windforce resisting system for greenhouses, the gust response factor, G, is taken from Table 5.4 evaluated at the structure’s mean roof height, h. In design of the components and glazing for greenhouses, the gustresponse factors are combined with the pressure coefficients to yield values of (GCp) and (GCpi) as given in Tables 5.6through 5.8.

5.5 PRESSURE COEFFICIENTS

Pressure coefficients for greenhouse structures and their components and glazing are given in Tables 5.5 through 5.8. Inthe tables, + and - signs signify pressures acting toward and away from the surfaces, respectively.

TABLE 5.5EXTERNAL PRESSURE COEFFICIENTS

FOR AVERAGE LOADS ON MAINWIND - FORCE RESISTING SYSTEM

WALL PRESSURE COEFFICIENTS CP

SURFACE d/b Cp

WINDWARD WALLS ALL VALUES 0.8

LEEWARD WALLS 0 -12

> 4

-0.5-0.3-0.2

SIDE WALLS ALL VALUES -0.7

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ROOF PRESSURE COEFFICIENTS CP

* Both values of Cp shall be used in assessing load effects.Notes: 1. Refer to Table 5.7 for arched roofs, Table 5.6A and 5.6B for components and glazing and Table 5.8 for

internal pressure.

2. For G, use appropriate value from Table 5.4.

3. Linear interpolation may be used to obtain intermediate values of ø, h/b, h/d, and d/b not shown.

TABLE 5.6AEXTERNAL PRESSURE COEFFICIENTS FOR LOADS

ON COMPONENTS AND GLAZING (WALLS)

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NOTES: 1. Notes apply toboth Tables 5.6Aand 5.6B2. Vertical scaledenotes (GCp) to

be used with qh.3. Horizontalscale denotestributary area,A (ft2)4. a = smaller of10% of minimumwidth and 0.4h,but larger than 4% of minimum widthand 3 ft

A,(ft2)

TABLE 5.6B EXTERNAL PRESSURE COEFFICIENTS FOR LOADS

ON COMPONENTS AND GLAZING (ROOFS)

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GCp

GCp

GCp

GCp

FIGURE 5.1 BASIC WIND SPEED (MPH)

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TABLE 5.7EXTERNAL COEFFICIENTS (Cp) FOR ARCHED ROOFS

* When the rise to span ratio is (0.2<r<0.3), alternate coefficients given by (6r-2.1) shall also be used for the windward quarter.

Notes: 1. Values listed are for determination of average loads on main wind force resisting system.

2. For components and glazing at roof perimeter use external pressure coefficients in Table 5.6B with ø based on spring-line slope and qh based on Exposure C.

3. For components and glazing in roof areas away from the perimeter use the external pressure coefficients of this table multiplied by 1.2 for (GCp) and qh based on Exposure C.

4. Definition of terms as follows:

TABLE 5.8INTERNAL PRESSURE COEFFICIENTS (GCpi)

Conditions (GCpi)

Percentages of openings in one wall exceeds that of allother walls by 10% or more and openings in all other wallsdo not exceed 20% of respective wall area

+0.75and-0.25

All other cases +/- 0.25

Type of RoofRise-to-SpanRatio

WindwardQuarter

Center Half Leeward Quarter

Roof onelevatedstructure

0<r<0.20.2<r<0.3*0.3<r<0.6

-0.9(1.5r-0.3)(2.75r-0.7)

(-0.7 -r)(-0.7 -r)(-0.7 -r)

-0.5-0.5-0.5

Roof springingfrom groundlevel

0<r<0.6 1.4r (-0.7 -r) -0.5

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6.0 SNOW LOADS

Provisions for the determination of snow loads on greenhouse structures are described in the following subsections. Theprovisions apply to the calculation of snow loads for both continuously heated greenhouses and for intermittently heatedor unheated greenhouses.

6.1 DEFINITIONS

The following definitions apply only to the provisions of the SNOW LOAD section.

Thermal Resistance (R) - A factor which measures a material’s resistance to the transmission of heat. The smaller the R value, the greater the amount of heat a material will transmit.

Continuously Heated Greenhouses - A production or retail greenhouse with a constantly maintained temperatureof 50 degrees F or more during winter months. Such a greenhouse must also have a maintenance attendant on dutyat all times or an adequate temperature alarm system to provide warning in the event of a heating system failure.In addition, the greenhouse roof material must have a thermal resistance (R) less than 2.0.

Intermittently Heated or Unheated Greenhouse - Any greenhouse that does not meet the requirements of a continuously heated single or double glazed greenhouse.

6.2 GROUND SNOW LOADS

Ground snow loads, (Pg), to be used in the determination of design snow loads for roofs of greenhouses are given in

Figs. 6.1, 6.2 and 6.3 for the contiguous United States. In some areas, the amount of local variation in snow loads is soextreme as to preclude meaningful mapping. In certain other areas, the snow load zones are meaningful, but the mappedvalues should not be used for certain geographic settings, such as high country within these zones. These areas areshown shaded in the figures.

TABLE 6.1

ALASKAN GROUND SNOW LOADS (Pg)

Adak 30 Kotzebue 70Anchorage 75 McGrath 80Angoon 100 Nenan 95Barrow 40 Nome 130Barter Island 80 Palmer 45Bethel 80 Petersburg 180Big Delta 95 St. Paul Island 55Cold Bay 20 Seward 70Cordova 95 Shemya 25Fairbanks 100 Sitka 60Ft. Yukon 95 Talkeetna 230Galena 70 Unalakleet 75Gulkana 75 Valdez 130Homer 60 Whitter 450Juneau 90 Wrangell 90Kenai 75 Yakutat 230Kodiak 40

Table 6.1 gives ground snow load values, Pg, for many locations in Alaska. Extreme local variations prohibit statewidemapping of the ground snow loads in Alaska. Ground snow load should be taken as zero for Hawaii.

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6.3 FLAT ROOF DESIGN SNOW LOADS

The design snow loads, Pf, on an unobstructed flat roof shall be calculated using one of the following formulas:

Pf = Ctg CcIPg

where: Pf = flat roof design snow load (psf)

Ce = exposure factor

Ctg = thermal factor

I = importance factor

Pg = ground snow load (psf)

6.3.1 EXPOSURE FACTOR (Ce)Exposure factors, which take into account the effect of wind on the design snow loading are given in Table 6.2. The sitecondition chosen should be representative of that which is likely to exist throughout the life of the greenhouse.

TABLE 6.2SNOW EXPOSURE FACTOR (Ce)

6.3.2 THERMAL FACTOR (Ctg)Thermal factors for various thermal conditions are given in Table 6.3. The thermal factor is meant to take into accountthe thermal resistance of the greenhouse roof glazing and the temperature conditions within the structure. The thermalcondition chosen should be representative of that which is likely to exist throughout the life of the structure.

TABLE 6.3THERMAL FACTOR (Ctg)

Note: See Section 6.11 for definition of greenhouse thermal conditions.

Thermal condition Ctg

Continuously heated greenhouse 0.83

Unheated or intermittently greenhouse

1.00

Roofs located in generally open terrain extendingone-half mile from the structure

0.6

Structures located in densely forested or sheltered areas

0.9

All other structures 0.7

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6.3.3 IMPORTANCE FACTOR (I)Importance factors, which take into account the consequences of failure of the various types of greenhouses are givenin Table 6.4.

6.4 SLOPED ROOF DESIGN SNOW LOADS

All snow loads acting on a sloping surface shall be considered to act on the horizontal projection of that surface. Thesloped roof design snow load, Ps, shall be obtained by multiplying the flat roof design snow load, Pf, by the roof slopefactor Cs.

TABLE 6.4IMPORTANCE FACTOR (I)

Ps = Cs x Pf

Values of Cs for greenhouse roofs are given in Sections 6.4.1 and 6.4.2, respectively.

6.4.1 HEATED GREENHOUSE ROOF SLOPE FACTOR (CS)For unobstructed heated greenhouse roofs with slippery surface that will allow snow to slide off at the eaves, the roofslope factor shall be determined by using the formula:

Cs = 1 - [(a-15)/55]Note: a is the angle of slope in degrees from the horizontal. Lesser slopes use no slope factor.

6.4.2 UNHEATED GREENHOUSE ROOF SLOPE FACTOR (CS)For unobstructed unheated greenhouse roofs having a slope from the horizontal of 30 degrees or more, the roof slopefactor shall be determined by using formula:

Cs = 1 - [(a-30)/40]Note: a is the angle of slope in degrees from the horizontal. Lesser slopes use no slope factor.

6.4.3 ROOF SLOPE FACTOR FOR ARCHED ROOFS

Portions of arched greenhouse roofs having a slope exceeding 70 degrees shall be considered free of snow load. Thepoint at which the slope exceeds 70 degrees shall be considered the “eaves” for such roofs. For arched roofs the roofslope factor, Cs, shall be determined from the appropriate formula in Sections 6.4.1 & 6.4.2, by basing the angle ofslope on the slope line from the “eaves” to the crown.

6.4.4 ROOF SLOPE FACTOR FOR MULTIPLE ROOFS

For multiple folded plate, sawtooth and barrel vault roofs with parallel ridge lines, the roof slope factor (Cs) shall be considered to be equal to 1.0 regardless of the slope of the roof. (Section 1610.5.2 BOCA National Building Code, 1993)

6.5 UNBALANCED SNOW LOADS

Winds from all direction shall be considered when determining unbalanced snow loads. (Section 1610.6 BOCA National Building Code, 1993)

Type of Greenhouse 1

Retail greenhouses with general public accesspermitted

1.0

All other greenhouses 0.8

16


6.5.1 UNBALANCED SNOW LOAD FOR HIP AND GABLE ROOFS

For hip and gable roofs with a slope less than 2.4 degrees or exceeding 70 degrees, unbalanced snow loads are not requiredto be considered. For slopes of 2.4 degrees and up to 20 degrees, the roof slope shall be designed to sustain a uniformlydistributed load of 0.5 Pf acting on one slope and 1.0 Pf acting on the opposite slope, where Pf is the flat-roof snow load.For slopes between 20 degrees and 70 degrees, the structure shall be designed to sustain an unbalanced uniform snow loadon the lee side equal to 1.25 times the sloped roof snow load. In the unbalanced situation, the windward side shall beconsidered free of snow.(Section 1610.6.1 BOCA National Building Code, 1993)

6.5.2 UNBALANCED SNOW LOAD FOR CURVED ROOFS

Portions of curved roofs having a slope exceeding 70 degrees shall be considered free of snow. The equivalent slope of acurved roof is equal to the slope of a line from the eaves or the point at which the slope exceeds 70 degrees, to the crown.If the equivalent slope is less than 10 degrees or greater than 60 degrees, unbalanced snow loads are not required to be considered. Unbalanced snow loads shall be determined according to the loading diagrams in Fig. 6.6. In all cases, thewindward side shall be considered clear of snow. If the ground or another roof abuts a Case II or Case III (see Figure6.6) arched roof structure at or within 3 ft of its eaves, the snow load shall not be decreased between the 30 degree point andthe eaves but shall remain constant at 2 Ps, as indicated by the dashed line in Fig. 6.6, where Ps is the sloped roof snow load.(Section 1610.6.2 BOCA National Building Code, 1993)

6.5.3 UNBALANCED SNOW LOAD FOR MULTIPLE ROOFS

For multiple folded plate, sawtooth and barrel vault roofs with parallel ridge lines, the snow load shall be increasedfromone-half of the balanced load at the ridge or crown (0.5Pf), to three times the balanced load at the valley (3.0 Pf), wherePf is the flat-roof snow load. Balanced and unbalanced loading diagrams for a sawtooth roof are presented in Figure 6.7.However, the snow surface above the valley shall not be at an elevation higher than that above the ridge and if this conditionlimits the unbalanced load to less than 3.0 Pf, the minimum design unbalanced load shall be the lesser value.(Section 1610.6.3 BOCA National Building Code, 1993)

6.6 DRIFTS ON LOWER ROOFS

Multilevel roofs, lower roofs and decks of adjacent structures and roofs adjacent to projections shall be designed inaccordance with Sections 6.6.1 to 6.6.2.(Section 1610.7 BOCA National Building Code, 1993)

6.6.1 DESIGN LOADS FOR LOWER ROOFS

The drift loads on lower roofs or decks shall be taken as the triangular loading surcharge superimposed on the uniformroof snow loads (Pf). The geometry of the drifting shall be in accordance with Fig. 6.9 through 6.12. The height of thedrift (hd), in feet, shall be determined by the following formula:

hd=0.433√Wb4√Pg+10-1.5

where: Wb = Horizontal dimension in feet of the upper roof normal to the line of change of the roof level, but notless than 25 feet.

Pg = Ground snow load expressed in pounds per square foot.

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6.6.1 DESIGN LOADS FOR LOWER ROOFS

Alternatively, hd shall be determined from Fig. 6.8. The value of hd shall not exceed (hr - hb), where hr is the difference inheight between the upper and lower roof or deck (expressed in feet) and hb is the height if the balanced snow load on thelower roof or deck expressed in feet (see Fig. 6.9). For the purposes of evaluating the height of the snow drift, the groundsnow and roof snow, the snow density (D) expressed in pounds per cubic foot (pcf) shall be calculated from the formula:

D = 0.13Pg + 14

but not more than 35 pounds per cubic foot. The width of the drift (Wd) in feet shall be taken as the smaller of 4 hd or 4(hr - hb) (see Fig. 6.9). Drift loads are only required to be considered when:

hr - hb Pf

> 0.2 where hb=

hb D

and the flat-roof snow load (Pf) is evaluated on the basis of the lower roof. The maximum intensity of the snow load atthe high point of the drift (Pm) shall be determined by the formula:

Pm = D (hd + hb)

except that the maximum intensity of the snow load at the high point of the drift (Pm) shall not exceed Dhn where D is

the snow density and hr is the difference in height between the upper and lower roof or deck.(Section 1610.7.1 BOCA National Building Code, 1993)

6.6.2 ROOF OF ADJACENT LOWER STRUCTURE

A drift surcharge shall be applied to lower roofs or structures sited within 20 feet of a higher structure as depicted inFig. 6.10. The height of the surcharge on the lower structure shall be taken as hd multiplied by (1 - S/20) to account for thehorizontal separation between structures (S) in feet.(Section 1610.7.2 BOCA National Building Code, 1993)

6.7 SLIDING SNOW

Lower roofs which are located below roofs having a slope greater than 20 degrees shall be designed for an increase indrift height of 0.4 hd, provided that the total drift surcharge (hd + 0.4hd) shall not exceed the height of the roof above the uniform snow depth (hr - hd) (see Fig. 6.13 for depiction of hd and hr). Sliding snow shall not be considered where thelower roof is horizontally separated from the higher roof by a distance (S) greater than the difference in height betweenthe upper and lower roofs (hr) or 20 feet (see Fig. 6.13).(Section 1610.8 BOCA National Building Code, 1993)

6.8 UNLOADED PORTIONS

For all roofs, the effect of removing half the snow load from any portion of the loaded area shall be investigated.

6.9 EXTRA LOADS FROM RAIN-ON-SNOW

All unheated or intermittently heated greenhouse roofs shall be designed to sustain a temporary surcharge load associatedwith an intense rain while sustaining the design snow load.

6.10 DRAINAGE IN GUTTER CONNECTED ROOFS

All gutters shall be provided with adequate slope and drains to allow for run off of rain and snow melt water and to preventponding.

18


19

FIGURE 6.1 GROUND SNOW LOAD Pg, FOR EASTERN UNITED STATES (PSF)


FIGURE 6.2 GROUND SNOW LOAD Pg, FOR CENTRAL UNITED STATES (PSF)

20


FIGURE 6.3 GROUND SNOW LOAD Pg, FOR WESTERN UNITED STATES (PSF)

21


22


23

FIGURE 6.8DETERMINATION OF THE MAXIMUM HEIGHT OF

DRIFT SURCHARGE (hd) IN FEET(Figure 1610.7 BOCA National Building Code, 1993)

hd=0.433√Wb4√Pg + 10 - 1.5

Note a. If the horizontal dimension in feet of the upper roof (Wb)is

less than 25 feet, use Wb of 25 feet. Note b. If the horizontal

dimension in feet of the upper roof (Wb) is more than 600 feet,

use equation above. Note c. Pg=Ground snow load expressed in

pounds per square foot.

FIGURE 6.11SNOW DRIFTING AT ROOF PROJECTIONS

(Figure 1610.7.3 BOCA National Building Code, 1993)

FIGURE 6.12INTERSECTING SNOW DRIFTS


FIGURE 6.13ADDITIONAL SURCHARGE DUE TO SLIDING SNOW(Figure 1610.7.8 BOCA National Building Code, 1993)

FIGURE 6.10DRIFTING SNOW ONTO ADJACENT LOW STRUCTURES


FIGURE 6.9DRIFTING SNOW ON LOW ROOFS AND DECKS



COMMENTARY TO NATIONAL GREENHOUSE MANUFACTURERS ASSOCIATIONSTANDARD FOR DESIGN LOADS IN GREENHOUSE STRUCTURES

C1 GENERAL

C1.1 SCOPE

The NGMA standard provides only load requirements for the design of greenhouse structures and their components.The effects these loads have on the structure in terms of stresses and deflection should be determined by accepted methodsof analysis. Depending on the material used, the calculated values should then be compared to the allowable values asgiven in the current editions of the following design specifications:

1. Specifications for Aluminum Structures by the Aluminum Association.

2. National Design Specification for Wood Construction by the National Forest Products Association.

3. Steel Construction Manual by the American Institute of Steel Construction.

4. Building Code Requirements for Reinforced Concrete by the American Concrete Institute.

5. Specification for the Design of Cold Formed Steel Structural Members by the American Iron and Steel Institute.

6. Glazing Manual by the Flat Glass Marketing Association.

C1.1.2 LIMITATIONS

The NGMA standard is written to apply specifically to free-standing, attached even-span, lean-to and gutter-connectedgreenhouses constructed at ground level. There are a variety of other greenhouse-type structures such as solar domes, skylights, A-frames, observatories, etc. which, because they are constructed on top of other structures or because oftheir shape, are not specifically covered by the standard. However, many of the recommendations of this standard maystill be used as a guide in designing these other types of structures.

C1.4 ADDITIONS TO EXISTING STRUCTURES

The designer should be aware of potential structural problems that may be created by the addition of any attached even-span or a lean-to greenhouse to an existing structure. These include the possibility of weakening the existingstructure if a common wall is removed to allow access to the greenhouse and the potential for excessive snow buildupon the lower roof of the two adjacent structures due to drifting.

C2 COMBINATIONS OF LOADS

C2.1 COMBINING LOADS

The load combinations listed cover cases of practical interest. The loads are intended for use with design specificationsfor conventional structural materials such as aluminum, steel, wood, concrete, glass, etc., that are used in greenhouse construction. Some of these specifications are based on allowable stress design while others employ strength design.Accordingly, no safety or load factors have been applied to the given load combinations since these depend on thedesign philosophy of the particular material specification.

It should be noted that earthquake loads have been omitted from the combinations to be considered in greenhousedesign. This is due to the fact that current practice for earthquake design calls for the application of equivalent lateralforces to the structure. The magnitude of these equivalent lateral forces is based on the weight of the structure to whichthey are applied. Since greenhouses are relatively lightweight structures, the magnitude of the equivalent lateral earth-quake design forces will always be less than the design wind forces and thus they need not be considered.

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Design Load COMMENTARY

C2.2 LOAD COMBINATION FACTORS

Most loads other than dead loads vary significantly with time. When one of these variable loads is combined with permanent load (e.g., combinations 2 through 4, in Section 2.1), its maximum probable value should be used. However,when more than one variable load is considered (e.g. combinations 5 through 6 in Section 2.1), it is very unlikely thatthey will each attain their maximum values at the same time. Accordingly, some reduction in the total combined loadeffects is appropriate. This reduction is accomplished through the specified load combination factors. It should be noted,however, that many design specifications allow a 1/3 increase in allowable stresses for load combinations which includewind. This accomplishes the same result as the factor recommended in Section 2.2. Therefore, if a 1/3 increase isallowed, the load combination factors should not be used.

C3 DEAD LOADS

C3.2 WEIGHTS OF BUILDING MATERIALS

Table C3.1 gives the weights of several materials commonly used in greenhouse construction. The weights given are average values suitable for general use. However, when there is reason to suspect considerable deviation from the valuesshown, the actual weight shall be determined.

Table C3.1WEIGHTS AND DENSITIES OF

COMMON GREENHOUSE MATERIALS

C3.3 WEIGHT OF FIXED SERVICE EQUIPMENT

Weights of fixed service equipment such as heaters, air conditioners, etc. can usually be obtained from manufacturers’literature.

C3.4 SPECIAL CONSIDERATIONS

Oftentimes, bottom chord members of greenhouse roof trusses or other interior structural members are used to permanentlysupport suspended plant material such as hanging baskets. The designer shall anticipate these loads where they mayoccur and increase the design dead load accordingly.

Steel 490 PCF

Aluminum 165 PCF

Wood (dependent on type andmoisture content)

35 PCF

Glass (1/8” thick) 26 oz/ft2

Glass (1/4” thick) 52 oz/ft2

Fiberglass (4 oz) 4 oz/ft2



Polyethylene (6 mil thickness) 2/3 oz/ft2

25

COMMENTARY Design Load

C4 LIVE LOADS

C4.1 DEFINITIONS

Live loads, exterior or interior are temporary loads produced by the use and occupancy of the greenhouse. An exampleof an exterior live load is the weight of workmen and materials which may be applied to a greenhouse roof during constructionor repair. An example of an interior live load is the weight of suspended plant material or other hanging objects whichmay be temporarily supported by an interior structural member. It has been found that often times interior greenhouselive loads tend to become permanent, i.e., they are left in place for extended time periods. For this reason, Section 4.1requires that any load that is anticipated to be added to a greenhouse and left in place for a continuous time period of30 days or more be considered a dead load for design purposes.

C4.2 MINIMUM ROOF LIVE LOAD

Section 4.2 provides a method for determination of a uniformly distributed minimum live load, L, to be applied togreenhouse roofs. All greenhouse roofs must be designed for the live load, L, or the snow load given in Section 6, whichever is greater. The method for calculation of the minimum roof live load, L, in Section 4.2 is taken directlyfrom ANSI A58.1. The value is intended to estimate the distributed live load which might be applied to a normal building’sroof by workers and equipment during construction or repair.

C4.3 MAXIMUM ROOF LIVE LOAD

During their lifetime, most normal roofs require reroofing or roof repairs, in which case significant temporary loadsmay be applied to the structure. These loads are due not only to the weight of workers, but also to the weight of conventional roofing materials (tars, felts, shingles, etc.) and tools which may be stored on the roof prior to their placement or use.

Greenhouse roofs, because they are constructed of relatively thin sheets of glazing material such as glass or fiberglassdo not fall into the category of normal roofs, i.e., they are never subjected to the construction or repair loads of men or materials that a conventional roof might experience. This is due to the fact that during the construction and repair of greenhouse roofs, scaffolding or ladders are used to place the roof glazing materials thereby eliminating the bulk of construction live load. In addition, during normal use repairs to the greenhouse roof are usually limited to local areasand resulting live loads are concentrated in nature (Section 4.4).

Consequently, Section 4.3 limits the distributed live load, L, on greenhouse roofs to a maximum value of 15 psf. Thisvalue is based on 50 years of experience in greenhouse design and construction where a roof design load value of 15psf has been used successfully.

C4.4 CONCENTRATED LOADS

The design of roof members for a concentrated load of 100 lbs is based on the fact that an average worker may have toclimb on the roof during construction or repair of local areas. Similarly, the 100 lb concentrated load requirement fortruss bottom chord panel points is meant to provide additional strength in the event that equipment, plants, etc., are everhung temporarily from the panel points in local areas.

The 100 lb concentrated load value stipulated in Section 4.4 has been used successfully for over 50 years in greenhousedesign and construction. It assumes that special scaffolds meant to distribute the weight of workmen are used to accessgreenhouse roofs for local repairs. (Many greenhouse systems are actually supplied with fasteners used to support these special scaffolds). It also assumes that the weight of a typical interior live load will be less than 100 lbs. (As an example,the weight of a single hanging basket is less than 10 lbs).

In cases where special scaffolding is not supplied for roof access or where an interior live load is anticipated to be greaterthan normal, the concentrated load requirement in Section 4.4 shall be increased to 200 lbs. In addition, in the event itbecomes necessary to hang temporary concentrated live loads from bottom chord members at locations other than panelpoints, adequate temporary bracing members shall be provided to support these loads.

26


C4.5 PARTIAL LOADING

It is intended that the full intensity of the greenhouse roof live load, L, be considered over portions of the structure aswell as over the entire structure. This partial loading requirement is necessary only when its consideration will producehigher loads and stresses in certain members than will application of the full load.

Partial length loads on a simple beam or truss will produce higher shear on a portion of the span than a full-length load.Loads on the half span of arches or on the two central quarters can be critical.

C5 WIND LOADS

C5.1 GENERAL

The wind design procedure given in the standard requires calculation of two separate sets of design forces, those appliedto the main wind force resisting system and those applied to individual components and glazing. The main wind force resisting system is that part of the greenhouse, such as rigid or braced frames or walls which provides lateral support tothe structure when subjected to wind pressures. In attached even-span or lean-to greenhouses, the existing structure towhich the greenhouse is connected may supply the main wind force resisting system. Individual components and glazingare those portions of the structure such as glass or fiberglass panels which resist the wind pressures directly and which may be subjected to locally higher pressures than those applied to the structure as a whole.

C5.3.1 PROCEDURE FOR CALCULATING VELOCITY PRESSURE

The design wind speed is converted to a velocity pressure by use of the formula:

qz = 0.00256 Kz(IV)2

In the above formula, the constant 0.00256 reflects air mass density for certain standard conditions. The constant shallbe used except where sufficient weather data are available to justify a different value. The appendix to ANSI A58.1gives a procedure for calculating the air mass density. Kz in the above formula is the exposure coefficient which takes intoaccount the effect terrain roughness has on velocity pressure. Values of Kz for various exposure conditions and elevationsare given in Table 5.3

C5.3.2 SELECTION OF BASIC WIND SPEED

Values of the basic design wind speed, V, given in Fig. 5.1 (reproduced from ANSI A58.1) were prepared from data collected at 129 U.S. weather stations. They are based upon an annual probability of 0.02 that the wind speed is exceeded(50 year mean recurrence interval). The basic design wind speed, V, is converted to a velocity pressure using the equationin Section 5.3.1.

The velocity pressure equation in Section 5.3.1 also contains another factor, the importance coefficient, I. Values of I aregiven in Table 5.2. The coefficient is meant to account for the importance of a greenhouse in terms of hazard to humanlife and damage to property. Application of the importance coefficient value of 0.95 in Table 5.2 adjusts the design windspeed, V, to an annual probability of 0.04 of being exceeded (25 year mean recurrence interval).

C5.3.2.1 SPECIAL WIND REGIONS

Special consideration shall be given to those regions where records or experience indicate that the wind speeds are higherthan those reflected in Fig. 5.1 and Section 5.3.2. Some such regions are indicated in Fig. 5.1; however, all mountainousand hilly terrain, gorges and ocean promontories shall be examined for unusual conditions and the authority having jurisdiction shall, if necessary, adjust the values of V given herein to account for higher local winds. The appendix toANSI A58.1 provides recommendations for making such adjustments.

27


C5.5 PRESSURE COEFFICIENTS

Pressure coefficient values given in Tables 5.5 through 5.8 were taken directly from ANSI A58.1. The values were assembled from the latest boundary layer wind tunnel and full-scale tests and from previously available literature. Morecomplete information of the compilation of each table along with selected references is included in the appendix toANSI A58.1.

Pressure coefficient values given in ANSI do not directly address multispan buildings such as gutter-connected greenhouses.Data compiled from wind tunnel studies and actual measurement of pressure on low rise multispan buildings indicate thatthe second and subsequent roof spans actually experience a reduction in the pressure applied to the first span. Therefore,if warranted, the designer of a gutter-connected greenhouse may use the pressure coefficients given in Tables 5.5 and5.7 for design wind loads on the first roof span and he may reduce the design wind pressures on subsequent spans.

C6 SNOW LOADS

C6.1 GENERAL

The procedure established in Section 6 for the determination of design snow load is a follows:

1. Determine the ground snow load for the geographic location (Section 6.2 and C6.2).

2. Generate a flat roof design value from the ground load with consideration given to:

a. Roof exposure (Sections 6.3.1, C6.3, and C6.3.1)

b. Roof thermal condition (Sections 6.3.2, C6.3, and C6.3.2)

c. Occupancy and function of structure (Sections 6.3.3, C6.3, and C6.3.3)

3. Consider roof slope (Sections 6.4 and C6.4)

4. Consider unbalanced loads if applicable (Sections 6.5 and C6.5.3)

5. Consider snow drifts on lower roofs if applicable (Sections 6.6 and C6.6)

6. Consider sliding snow (Sections 6.8 and C6.8)

7. Consider unloaded portions (Sections 6.9 and C6.9)

The approach to snow load design used by ANSI is to establish a load value that reduces the risk of snow-load-inducedfailure to an acceptably low level. As such, snow loads in excess of the design value may occur and therefore it is necessaryto consider the implications of such “excess” loads. This would seem especially important in greenhouses, which arerelatively lightweight structures and as a result, the percentage increase in total roof load due to an “excess” snow loadmight be substantial. However, past experience has shown that in the case of greenhouses, the “excess” loads are apparentlynever realized, since few, if any, snow-load-induced roof failures of normally operating greenhouses have occurred.This is most likely due to the fact that most greenhouses are continuously heated and the heat loss through the roof glazingcauses snow striking the roof to melt almost immediately. Verification of this occurrence is given by the fact that snow-load-induced greenhouse roof failures that have occurred did so at times when the greenhouses were out of service and thereforewere not heated and had no heat loss through the roof. Even then, the reported failures were localized, breakingthrough individual and isolated roof glazing panels rather than failing any of the main structural support members.

28


Taking advantage of this past experience with greenhouse structures subjected to snow loads, a distinction has beenmade in Section 6 between continuously heated single or double glazed greenhouses and intermittently heated or unheatedgreenhouses. A greenhouse which meets the requirements for continuous heating (Section 6.1.1) will have a substantiallyreduced design snow load based on the fact that the heating will prevent an “excess” snow build-up. Greenhouses notmeeting the continuously heated criteria will have a design snow load in accord with standard ANSI requirements. Toqualify as continuously heated, a greenhouse must satisfy three requirements. First, its interior temperature must be maintainedat a minimum of 50 degrees F at normal planting level during winter months. Second, the greenhouse must have maintenancepersonnel on duty at all times or an adequate temperature alarm system to assure that the minimum temperature is maintained.Third, the total thermal resistance of the roof glazing material must be less than 1.0 for single glazed roofs and less than 2.0for double glazed roofs, i.e., low enough to transmit the heat necessary to melt falling snow. The first two of theserequirements are met by most large-scale greenhouses operations where it is necessary to keep a certain minimum temperaturefor the interior plant life. Virtually every greenhouse meets the third requirement since the thermal resistance of allcommonly used double glazing materials is less than 2.0.

It should be noted that air inflated double polyethylene greenhouse roofs may be considered as single glazed. This is dueto the fact that a minimal amount of snow striking this type of roof causes it to deflate and thus act as a single glazed roof.

C6.2 GROUND SNOW LOADS

The methodology used to determine appropriate values and compile the results along with the results themselves for theground snow load maps given in Figs. 6.1, 6.2 and 6.3 are reported in the following documents:

Redfield, R., and Tobiasson, W.Snow Loads for the United States: Part I, GroundLoad StatisticsU.S. Army Cold Regions Research and Engineering Laboratory(CRREL), Hanover, NH, 1980.

Tobiasson, W. and Redfield, R.Snow Loads for the United States: Part II, Ground and Roof LoadsU.S. Army Cold Regions Research and Engineering Laboratory(CRREL), Hanover, NH., 1980.

The values indicated in the figures are based on an annual probability of 0.02 of being exceeded (50 year mean recurrenceinterval).

In the above mentioned report, Snow Loads for the United States: Part II, Ground and Roof Loads, a methodology isdeveloped for establishing a design snow load for a specific site from meteorological information available at surroundinglocations with consideration given to the orientation, elevation and records available at each location. That methodologyshould be used to establish design values for sites in shaded portions of Figs. 6.1, 6.2 and 6.3. It can also be used toimprove upon the values presented in unshaded portions of the figures. Detailed study of a specific site may generate adesign value lower than that indicated by the generalized national map. It is appropriate in such a situation to use thelower value established by the detailed study. Occasionally, a detailed study may indicate that a higher design valuethan the national map indicates should be used. Again, results of the detailed study should be followed.

29


TABLE C6.1SITE SPECIFIC VS. ZONED

GROUND SNOW LOADS

* Based on a detailed study of information in the vicinity of each location according to the methodology developed in Snow Loads for the USA: Part II, Ground and Roof Loads (Tobiasson and Redfield, 1980)

Table C6.1 is included to emphasize the importance of considering local sitting in the shaded areas of Figs. 6.1, 6.2 and6.3. For some locations in shaded areas of the Northeast, ground snow loads exceed 100 psf. Even in the southern portionof the Appalachian Mountains, not far from the sites where a 15 psf ground snow load is appropriate, ground loadsexceeding 50 psf may be required. Lake effect storms create requirements for ground loads in excess of 75 psf alongportions of the Great Lakes. In some areas of the Rocky Mountains, ground snow loads exceed 200 psf. Local recordsand experience should also be considered when establishing design values.

Ground snow load values in Table 6.1 are for specific Alaskan locations only and generally do not represent appropriatedesign values for other nearby locations. They are presented to illustrate the extreme variability of snow loads withinAlaska.

C6.3 FLAT ROOF DESIGN SNOW LOADS

The minimum allowable values of Pf presented in section 6.3 for unheated or intermittently heated greenhouses acknowledgethat, in some areas, a single major storm can generate loads which exceed those developed from an analysis of weatherrecords and snow load case studies. Factors are included which account for the thermal, aerodynamic and geometriccharacteristics of the greenhouse in its particular setting.

C6.3.1 EXPOSURE FACTOR (Ce)Except in areas where loads are increased by snow drifting, far less snow is present on most roofs than on the ground.Loads in unobstructed areas of conventional flat roofs average less than 50 percent of ground loads. The values in the standard are above average values, chosen to reduce the risk of snow-load induced failures to an acceptably low level.Because of the variability of wind action, a rather conservative approach has been taken when considering load reductionsby wind.

C6.3.2 THERMAL FACTOR (Ctg)Case studies verify that more snow will be present on cold roofs than on warm roofs. Glass, fiberglass or plastic roofs ofcontinuously heated greenhouse structures are seldom subjected to much snow load because their high heat losses cause snowmelt and sliding. The value of the thermal factor, Ctg, given in Table 6.3 for continuously heated greenhouses assumes thatthe total thermal resistance value, R, of the greenhouse roofing material is less than 1.0 for single glazed roofs and less

State LocationZoned Value

(PSF)Site-SpecificValue (PSF)*

California Mount Hamilton 5 45

ArizonaChiracahua Nat.

Mon.10 30

Arizona Palisade R.S. 5 200

Tennessee Monteagle 15 20

West Virginia Fairmont 40 55

Maryland Edgemont 50 65

Pennsylvania Blairsville 50 60

Vermont Vernon 60 75

30


than 2.0 for double glazed roofs. The values are taken directly from ANSI A58.1 which requires that the roofing materialhave a total thermal resistance of 10 or less. For greenhouse roofs the values are therefore conservative.

The NGMA has for years used a 15 psf snow load for design of continuously heated greenhouses having roofing materialswith R values less than 1.0. Based on examination of insurance claims for greenhouse damage over the last 20 years, thisvalue has apparently been realistic. In the isolated cases when snow has damaged greenhouses, they were normally outof operation at the time with no heat, and as discussed in Section C6.1, the failures themselves were limited to localportions of the structures.

C6.3.3 IMPORTANCE FACTOR (Ir)Importance factor, Ir, has been included to account for the need to relate design loads to the consequences of failure.Roofs of retail greenhouses where general public access is permitted, are designed using a risk factor equal to 1.0. Thisequates to unmodified use of the ground snow loads given in Figs. 6.1, 6.2, and 6.3 for an annual probability of beingexceeded of 0.02 (50 year mean recurrence interval). All other greenhouses use a risk factor equal to 0.8. This in effectmodifies the ground snow loads to an annual probability of being exceeded of 0.04 (25 year mean recurrence interval).

C6.4 SLOPED ROOF DESIGN SNOW LOADS

Snow loads decrease as the slope of roofs increases. A portion of the decrease is related to the aerodynamics of snowaccumulation but sliding and improved drainage are also important factors. The ability of a sloped roof to shed snow loadby sliding is related to the absence of obstructions not only on the roof, but also below it, the temperature of the roof andthe slipperiness of its surface. Most materials used in greenhouse roof construction can be considered slippery. All ofthe above factors are considered in the slope reduction factors presented in Fig. 6.4.

If the ground or another flatter roof exists near the eaves of a sloped roof, snow may not be able to slide completely offthe sloped roof. This may result in the elimination of snow loads on upper portions only. Lateral loads induced by such conditions should be considered.

C6.4.4 ROOF SLOPE FACTOR FOR GUTTER-CONNECTED GREENHOUSE ROOFS

Gutter connected roofs on unheated or intermittently heated greenhouses are susceptible to collecting extra snow in theirvalleys by snow creep and sliding and by wind drifting. Therefore, no reduction in the design load because of slope shouldbe applied.

C6.5 UNBALANCED ROOF SNOW LOADS

Unbalanced snow loads may develop on sloped roofs because of sunlight and wind. Winds tend to reduce snow loads onwindward portions and increase snow loads on leeward portions. Since it is not possible to define wind direction with assurance, winds from all directions should generally be considered when establishing unbalanced roof loads.

C6.6 DRIFTS ON LOWER ROOFS

The requirements for drift loads need not be considered on continuously heated greenhouses. For unheated or intermittentlyheated greenhouses, however, it is extremely important to consider localized drift loads in designing roofs. Drifts will accumulate on roofs in the wind shadow of higher roofs. The affected roof may be influenced by a higher portion of thesame structure or by another structure nearby if the separation is 20 ft or less. When a new structure is built within 20 ft ofan existing structure, drifting possibilities should also be investigated for the existing structure. The method presented inSection 6.6.2 will establish reasonable drift loads for most situations. However, in windy tree less areas and in windy areasthat experience heavy snowfalls and blizzards, snowdrifts somewhat larger than those generated by Section 6.6.2 have beenmeasured. Local experience may prove valuable in determining the nature and extent of snow drifts on roofs in such areas.

C6.7 SLIDING SNOW

Situations which permit snow to slide onto lower roofs should be avoided. Where this is not possible, the extra load ofthe sliding snow should be considered. The final resting place of any snow which slides off a higher roof onto a lowerroof will depend on the size, position and orientation of each roof. Distribution of sliding loads might vary from a uniform

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load 5 ft wide, if a significant vertical offset exists between the two roofs, to a 20 ft wide uniform load where a low-slopeupper roof slides its load onto a second roof that is only a few feet lower.

In some instances, a portion of the sliding snow may be expected to slide clear of the lower roof. Nevertheless, it is pru-dent to design the lower roof for a substantial portion of the sliding load to account for any dynamic effects that mightbe associated with sliding snow.

C6.8 UNLOADED PORTIONS

For greenhouse structures, the effect of removing half the design snow load from any portion is usually less severe thanthe effect of the entire snow load. Nevertheless, it should be considered.

C6.9 EXTRA LOADS FROM RAIN-ON-SNOW

The ground snow load measurements on which this standard is based contain the load effects of light rain-on-snow.However, since heavy rains percolate down through snowpacks and drain away, they are not included in the measuredvalues. The temporary roof load contributed by a heavy rain may be significant. Its magnitude will depend on theduration and intensity of the design rainstorm, the drainage characteristics of the snow on the roof, the geometry of theroof and the type of drainage provided.

Loads associated with rain-on-snow are discussed by:

Colbeck, S.C.Snow Loads Resulting From Rain-On-SnowCRREL report 77-12, 1977, Hanover, N.H.

Colbeck, S.C.Roof Loads Resulting From Rain On Snow - Results of a Physical ModelCanadian Journal of Civil Engineering, Volume 4, 1977.

The following rain-on-snow surcharge loads are suggested for design purposes:

Roof Slope Rain-on-Snow Surcharge (psf)<1/4 in./ft 8>1/4 in./ft 5

It is recommended that the appropriate surcharge load be applied to all final roof snow loads for unheated or intermittentlyheated greenhouses except where the minimum allowable flat roof design snow load exceeds Pf in Section 6.3. In thatsituation, the rain-on-snow surcharge load above should be reduced by the difference between the minimum allowableflat roof design snow load and Pf. For example, for a flat roof where Pg = 20 psf and Pf = 18 psf, the minimum allowablevalue of Pf (20 psf) is the design snow load. The rain-on-snow surcharge recommended for this situation would be 8 - (20-

18) = 6 psf. The total design load considering snow (20 psf) and rain-on-snow (6 psf) would be 26 psf. If this roof had aslope of 1/4 in./ft or more, the rain-on-snow surcharge load would equal 3 psf.

ACKNOWLEDGEMENT

The National Greenhouse Manufacturers Association wishes to express its gratitude to the members of ANSICommittee A58, authors of Building Code Requirements for Minimum Design Loads in Buildings and Other Structures.The 1980 draft of that document has been used as a basis for this current NGMA Standard.

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Design Load COMMENTARY & ACKNOWLEDGEMENT

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