A Norseman Structures Inc. White Paper

Fabric Covered Steel BuildingsCritical Design Factors to Consider

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Not all structures are created equal and because of this we are often

asked, “What makes your product different?” and “What should I look for when

comparing products”? We understand building designs are diffi cult to compare

and so is making decisions around which one will provide you with the best long

term value for your purchase, as well as being the safest and most functional.

When comparing products, the following information will enable you to ask

the right questions, ensuring you have purchased a product that meets all your

requirements, and that your product is safe, minimizes your liability, and is insurable.

In order to compare designs and ensure a safe, long lasting product is

purchased, it is increasingly important that customers understand these

basic engineering topics that relate to fabric covered steel buildings:

• Steel strength

• Bracing

• Snow & wind loading

• Snow shedding

• Use & Occupancy

• Fabric as Bracing

• End truss strength

• Exposure factors

• Steel specifi cations

• Complete Structure Engineering

• Enclosure Category

• Thermal Factor

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Fabric Covered Steel Buildings Critical Design Factors to Consider

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Fabric Covered Steel Buildings: Critical Design Factors to Consider www.norsemanstructures.com | 3

Steel StrengthSignifi cant factors infl uencing the safety and

longevity of a building are the quality, thickness

(gauge) and the total amount of steel used in a

structure. One of the results of inadequate steel is

called “web punching”. Web punching occurs when

the truss webs punch through the walls of the

truss chords if the steel in the truss chord is too

thin. Thin steel can lead to truss chord buckling at

the web connection location. This process is called

“chord plastifi cation”.

Many manufacturers use the least steel

possible when designing their buildings and

often use thin 14 gauge steel for their truss

chords. Light gauge steel like this permits

excess fl exing of the truss and buckling at low

web forces.

Norseman Structures recognizes that

undersized steel has the possibility to fail

in this manner. To combat this, our designs

require the use of larger diameter webs as

well as thicker gauge steel in our truss chords,

thus preventing any “punch through”.

An effective way to compare building

structures is by weight of the structural steel

in the building. As it is unlikely a competitive

design could be safely engineered using ten

percent less steel than our buildings, it is

likely the competitive design does not meet

all building code requirements. Norseman

Structures also utilizes heavy gauge steel in

order to meet the needs of our customers.

The picture below is an example of the dangers

of ”web punching”. As you can see, the truss

webbing has ‘punched’ through the wall of the

thin gauge steel likely causing a ripple effect

throughout the rest of the structure. The end

result of this unfortunate occurrence is at best

an unusable building and at worst, a catastrophic

failure causing a collapse.

Web PunchingTruss webbing punching through the thin guage outer chord of the truss

Webbing

Webbing

Outer Chord

Web punching occurs when the

truss webs punch through the walls

of the truss chords if the steel in

the truss chord is too thin.

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BracingBracing is required to give the structure

multidimensional strength against wind and snow

load forces. It must also be strong enough to be

able to counteract the force of the fabric under

tension. Bracing systems include the use of purlins

(square or round tubes fi xed between truss

sections) and cable systems that can crisscross

multiple trusses.

Many manufacturers underestimate the forces

on the building and use too few and too slender

roof purlins and wind bracing.

Norseman Structures use larger diameter roof

purlins and heavier cables to resist the fabric

tension loads, as well as to provide stability

bracing for the roof trusses.

Snow & Wind LoadsBuilding codes require engineers to consider the

impact of wind and snow accumulation on building

designs. However, some manufacturers treat these

as separate factors and ignore the possibility of

the effects of ‘combined’ snow and wind loads

using the theory that wind will blow snow off the

building.

By not considering these factors together, your

building could be under-engineered allowing

for the use of lighter steel and/or wider truss

spacing as well as reducing the strength and

cost of the building. Unfortunately, due to

these factors, the structure will likely be unsafe

in certain conditions exposing users to undue

safety risk.

Norseman Structures understands wind will

not blow snow off buildings under certain

temperature and humidity conditions and

considers all combinations of combined snow

and wind loads required by the Building Codes.

Snow SheddingSome manufacturers make an assumption that a

sloped roof and slick fabric properties will allow

snow to be shed automatically, reducing the effect

of potential snow loads and allowing for a lighter

and less costly structure. This, “snow shed” design

theory does not always hold true.

Snow shed decreases over time. As fabric ages

and becomes rougher, the possibility of snow

accumulation increases, decreasing the safety

factor of the building.

Certain temperature and humidity conditions

also contribute to greater snow and ice

adhesion to fabrics and eliminate the ability of

the fabric to shed snow effi ciently.

Norseman Structures always designs to the

strict snow load requirements identifi ed in

local building codes. Our designs include larger

diameter and thicker steel than competitors, in

order to provide safe, long lasting structures.

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Use and OccupancyMost building codes allow for the strength of

building designs to be adjusted based on the

level of human occupancy – the more human use

within the structure, the higher the engineering

requirements or “hazard” rating. The choice

of hazard ratings used in the design can have a

dramatic impact on the overall strength, and

therefore safety of the structure.

To reduce costs, some manufacturers use a

Low Hazard importance factor for all buildings.

The use of sub-standard importance factors

allows a building to be under-designed in

order to save on steel. The end result is a more

inexpensive product for the customer, but a

building that may only be safe under certain

weather conditions.

Norseman Structures only uses the Low Hazard

rating for low occupancy buildings such as

cold storage facilities and agricultural storage

buildings. A Normal importance factor of 1.0

provides 25 percent more structural capacity

than Low Hazard importance of 0.8. The chart

below outlines the conditions a building needs

to meet in order to be classifi ed as Low Hazard.

It is important to note if you are unsure about

the classifi cation, Low Hazard should not be

the default.

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Importance Categories for BuildingsImportance Category

Use & Occupancy Factor

Low Hazard(Category 1 USA)

Buildings that represent a low hazard to human life in the event of failure,

including:

• Low human-occupancy buildings where it can be shown that collapse is

not likely to cause injury or other serious consequences

• Minor storage buildings

0.8

Normal (Category II USA)

All buildings except those listed in importance categories Low, High, and

Post-disaster

1.0

High Hazard (Category III USA)

Buildings that are likely to be used as post-disaster shelters, including

buildings whose primary use is:

• As an elementary, middle or secondary school

• As a community center

Manufacturing and storage facilities containing toxic, explosive or other

hazardous substances in suffi cient quantities to be dangerous to the

public if released

1.15

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Fabric as BracingSome manufacturers rely on the fabric to give the building trusses lateral strength.

This allows them to reduce the size of the truss members and the amount of bracing

in the structures. This in turn reduces the cost and strength of the building.

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Fabric Stress

Truss Cross

Section

Truss Cross

Section

Fabric

Lower Brace

Lower Brace

FabricFailure

OutwardForce

OutwardForce

Fabric

Cross Bracing

Competitive Design

Some designs rely on the

fabric cover to brace the

outer truss sections

Competitive Design

Ripping of the fabric

cover may cause failure

of the truss system.

Norseman Design

Norseman’s designs use strong

steel bracing systems to secure

the truss – not the fabric cover.

This does not work if the fabric is

damaged or if snow/wind loads are

uneven, which is often the case.

Norseman Structures’ designs do not rely on fabric to brace the truss chords.

Our buildings are designed to meet all wind and snow load requirements based

on adequate internal bracing.

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End Truss StrengthAs truss sections at each end of the building do not

have bracing on both sides, unlike other trusses

internal to the structure, there are different load

demands on them to be considered.

Many manufacturers do not adequately

consider all loads on the end trusses when

checking the effects of fabric tension. This can

lead to failure of the end truss, which will lead

to failure of the entire structure.

Norseman Structures considers both vertical

and horizontal fabric tension in combination

with design snow and wind loads on the end

trusses.

Exposure FactorsShelter from winds by adjacent buildings, trees or

terrain can have a signifi cant impact on the wind

and snow load requirements of buildings. Correctly

choosing the exposure factor is very important!

If the proper exposure factor is not selected,

a building may use substandard bracing or

truss & web steel and may not meet the

requirements for the specifi c location. For

example, a building located in an area defi ned

as “Open Terrain” may require a 90 mph wind

rating, but may only be rated for 80 mph if

the importance factor used did not meet the

design requirements for the site.

If the building is sheltered from wind by

adjacent structures or trees, it should be

designed for an increased snow load, since

less snow will potentially be blown off the roof.

Adjacent structures can also cause drifting

of snow which increases snow loads on the

building.

Norseman Structures only uses wind and

snow load reductions for sheltered or exposed

conditions in strict accordance with building

codes. Generally, Norseman Structures uses

the ‘Open’ classifi cation in order to ensure

your building meets all current, as well as

future needs. The classifi cations, as well as

brief explanations, are as follows:

Rough Terrain(Exposure C – Canada, Exposure B – USA)

Suburban, urban, wooded areas, center of large

town. Although this generally depicts ‘rough’

terrain, there are areas of ‘open’ terrain. For this

reason, ‘open’ terrain should be selected if there is

any question about the type of terrain.

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Open Terrain(Exposure A – Canada, Exposure C – USA)

Few trees, scattered trees, open water. Buildings

classifi ed as ‘open’ require as much as 30% more

capacity and strength than those classifi ed as

‘rough’.

Steel Specifi cationsMany manufacturers use the steel strengths

advertised by their steel supplier, instead of

the steel strengths stated in the building codes

and steel specifi cations. Use of steel strengths

higher than those specifi ed by the ASTM or

CSA specifi cations should not be used, due to

lack of consistency of strength and elongation

requirements. This could result in a 30 percent

capacity reduction in some cases.

Complete StructureEngineeringIn order to have a fully engineered building, every

component of the building must also be engineered

from screws, to cables, to trusses, to doors. In

order to ensure you have a properly engineered

product, it is important to ascertain whether all

building components are engineered. At Norseman

Structures, we use exclusively engineered parts.

It is important to note that any piece retrofi tted

on site or incorrectly engineered will affect the

structural integrity of your engineered product.

Enclosure CategoryA building with signifi cant openings in the fabric

such as air vents, fabric doors, or overhead doors

not designed to resist the specifi ed wind loads is

subject to signifi cant wind pressures or suctions.

This means any overhead doors or man doors

must be rated to the design wind speed. Failure

of a door or window could result in wind forces

as much as 50 percent higher than the design

wind forces. Norseman Structures design all door

frames and components to resist the design wind

speeds. In buildings with large openings or fabric

doors, Norseman Structures designs the building

to resist the additional wind pressures caused

by the openings in accordance with the building

codes.

Thermal FactorSome manufacturers use a thermal factor of 1.0

which is only appropriate for heated buildings,

or even 0.9, which is only appropriate for heated

greenhouses, to reduce the design snow load

on the building. Norseman Structures uses the

correct thermal factor of 1.2 for the design of

unheated fabric buildings.

SummaryWe are confi dent that if these suggestions are

taken into consideration, our customers will have

the knowledge that allows them to ask the right

questions when purchasing a fabric covered

steel building. What was once a very complicated

process, is now something we can all understand

and use as an evaluation tool.

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Fabric Covered Steel Buildings: Critical Design Factors to Consider www.norsemanstructures.com | 80296-11 04232012

Norseman Inc. has been manufacturing and selling building enclosure systems

since the 1960s and is a major supplier of fabric building and drilling rig enclosure

systems to the Oil and Gas and commercial markets. Norseman Structures now

operates over 200,000 square feet of quality controlled manufacturing space. This,

combined with an extensive network of corporate offi ces in Alberta, British Columbia,

Ontario, Saskatchewan, the USA and UK, positions Norseman Structures to be a world

leader in the manufacturing and distribution of steel-framed, fabric covered buildings.

Contact Us:

CANADA3815 Wanuskewin Road

Saskatoon, SK S7P 1A4

Toll Free: 1.855.385.2782

USA2851 South Pike Ave

Allentown, PA 18103

Toll Free: 1.855.385.2782

UNITED KINGDOM1-8 Enterprise Glade

Bath Yard, Moira

Swadlincote

Derbyshire DE12 6BA

UK: 0800 3891490

Outside NA & UK: +44 (0)1283 554120

www.norsemanstructures.com

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Fabric Covered Steel Buildings: Critical Design Factors to Consider0296-11 04232012 NORSEMAN & NORSEMAN AND DESIGN ARE TRADEMARKS OF NORSEMAN GROUP LTD. 05/11

Building Model

Building Size (width x length)

Chord Gauge - What gauge of steel is being used in the chords?

Chord Diameter - What is the diameter of the chord steel?

Web Gauge - What is the gauge of steel being used in the web?

Web Dimension - What are the dimensions of the web steel?

Truss Depth - What is the truss depth?

Purlin Diameter - What is the diameter of the roof purlins?

Wind Bracing - How many bays are wind braced?

Cable Size - What size are the cables being used?

Snow & Wind Loads – Does the building design take the effects of

‘combined’ wind and snow load into account? See Page 4YES

Snow Shedding – Does the building design assume that snow will be

shed from the roof of your building? See Page 4NO

Use & Occupancy – Does your building design assume the appropriate

occupancy rating? Remember, if your building is to be occupied, it

needs to have an occupancy factor of at least 1.0. See Page 5

Exposure Factors – Does the building design take the proper exposure

factor into account? Remember, Open terrain should be used when

building exposure is in question. See Page 7

Use of fabric in the design – Does the building design use the fabric to

give the building trusses lateral strength? See Page 6NO

End Truss Strength – Does your building design consider both vertical

and horizontal fabric tension in combination with snow and wind loads

on the end trusses? See Page 7

YES

Steel Strength – Does your building design account for web punching

or chord plastifi cation? You may require larger web material. See Page 3YES

Bracing – Does your building design use adequate bracing? Compare

purling and cable sizes. See Page 4YES

Steel Specifi cations – Is the steel strength used in the design

specifi ed by the manufacturer or a regulatory organization ie. CSA

or ASTM? See Page 3

YES

Enclosure Category – Does your building design include doors and/or

windows? Have these additional wind pressures been accounted for in

accordance with the building code? See Page 8

YES

Thermal Factor – Is your building unheated? If so, is it using a thermal

factor of 1.2? See Page 8

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Fabric Covered Steel Buildings: Critical Design Factors to Consider www.norsemanstructures.com | Appendix 1

Compare Before You BuyAll buildings are engineered, but are they equal? Co

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Contact Us:

CANADA3815 Wanuskewin Road

Saskatoon, SK S7P 1A4

Toll Free: 1.855.385.2782

USA2851 South Pike Ave

Allentown, PA 18103

Toll Free: 1.855.385.2782

UNITED KINGDOM1-8 Enterprise Glade

Bath Yard, Moira

Swadlincote

Derbyshire DE12 6BA

UK: 0800 3891490

Outside NA & UK: +44 (0)1283 554120

www.norsemanstructures.com

Fabric Covered Steel Buildings: Critical Design Factors to Consider0296-11 04232012