wind webinar series #3: asce 71 0 wind loads for...
TRANSCRIPT
WIND WEBINAR SERIES #3:
ASCE 7�10 Wind Loads for Signs, Other Structures, Roof�Top Structures &
Equipment, and Other Special Conditions
Robert Paullus, P.E., S.E., SECB
Paullus Structural Consultants [email protected]
26 February 2013
Wind Webinar #326 February 2013
Page 1 of 126
Wind Loads for Solid Signs,
Other Structures, Roof-Top
Structures & Equipment, and
Other Special ConditionsBob Paullus, P.E., S.E.
Paullus Structural Consultants
Wind Webinar #326 February 2013
Page 2 of 126
1. 1. Chapter 29– Other Structures (MWFRS
Directional Method)a. Conditions
b. Limitations
c. Solid Freestanding Walls or Signs
d. Solid Attached Signs
e. Design Wind Loads on Other Structures
f. Design Wind Loads on Rooftop Structures
and Equipment on Buildings
g. Parapets
h. Roof Overhangs
i. Minimum Design Wind Loadings
Outline
Wind Webinar #326 February 2013
Page 3 of 126
1. 2. Chapter 30 – Part 6 - Components &
Cladding for Building Appurtenances
and Rooftop Structures and Equipment
(Directional Procedure)a. Parapets
b. Roof Overhangs
c. Rooftop Structures and Equipment for
Buildings with h ≤ 60 ft (18.3 m)
2. 3. Examples
Outline
Wind Webinar #326 February 2013
Page 4 of 126
• 1. The structure is a regular-shaped
structure as defined in Section 26.2.2.
• Section 26.2.2 - BUILDING OR OTHER
STRUCTURE, REGULAR-SHAPED: A
building or other structure having no
unusual geometrical irregularity in spatial
form.
Section 29.1.2 - Conditions
Wind Webinar #326 February 2013
Page 5 of 126
• 2. The structure does not have response
characteristics making it subject to across-
wind loading, vortex shedding, or instability
due to galloping or flutter; or it does not
have a site location for which channeling
effects or buffeting in the wake of upwind
obstructions warrant special consideration.
Section 29.1.2 - Conditions
Wind Webinar #326 February 2013
Page 6 of 126
• 1. This chapter DOES consider: load
magnification effect caused by gusts in
resonance with along-wind vibrations of
flexible structures.
• 2. This chapter DOES NOT consider:
Unusual shapes or configurations that lead
to effects listed in Section 29.1.2 –
Conditions.
Section 29.1.3 – Limitations
Wind Webinar #326 February 2013
Page 7 of 126
• 3. If your structure does not fall within the
listed limitations, it should probably be the
subject of Wind Tunnel Study.
Section 29.1.3 – Limitations
Wind Webinar #326 February 2013
Page 8 of 126
• 1. No reductions allowed for apparent
shielding by buildings, other structures, or
terrain features.a. Individual hills
b. Individual trees or small groves of
trees
c. Individual levees and similar
features.
• 2. Reductions are afforded for Terrain
Features in determining Exposure
Categories in Chapter 26.
Section 29.1.4 – Shielding
Wind Webinar #326 February 2013
Page 9 of 126
Steps
Wind Webinar #326 February 2013
Page 10 of 126
• 1. Steps 1-4 are the same as in Chapters
26-30.a. Chapter 29, like Chapters 26-30, has its
own table, Table 29.3-1, for Kh and Kz
b. Step 5: Eq. 29.3-1
qz = 0.00256KzKztKdV2 (lb/ft2)
• 2. Be careful to use qz or qh, as directed
under each section.
Steps to Determine Wind Loads
Wind Webinar #326 February 2013
Page 11 of 126
• 1. Hollow Signs and Walls are not covered.
a. Signs which have openings that can be
pressurized
1) Boxed signs
2) Signs made from sea containers
3) Signs with large internal areas for lights
with translucent panels
2. Research is being conducted at Texas
Tech University by Douglas Smith, PhD
Section 29.4.1 – Solid Freestanding Walls
and Solid Freestanding Signs
Wind Webinar #326 February 2013
Page 12 of 126
• 3. Solid Signs can have openings up to
30% of the Gross Area.
a. Reduction factor can be applied to solid
signs with openings.
b. Reduction factor (1 - (1 - ε)1.5)
c. ε = ratio of solid area to gross area
4. If the area of openings exceeds 30% of
the gross area, it is an open sign.–Proceed to Section 29.5 - Design Wind
Loads—Other Structures
Section 29.4.1 – Solid Freestanding Walls
and Solid Freestanding Signs
Wind Webinar #326 February 2013
Page 13 of 126
• 5. Basic Equation: (Eq. 29.4-1)
F = qhGCfAs (lb)• a. qh = the velocity pressure evaluated at
height h (defined in Fig. 29.4-1) as
determined in accordance with
Section 29.3.2
» h = top of the wall or sign
» Note: qh is at the top of the sign or wall
and Kd in Eq 29.3-1 is the Kd of Solid
Freestanding Walls and Solid Free-
standing and Attached Signs in Table
26.6-1
Section 29.4.1 – Solid Freestanding Walls
and Solid Freestanding Signs
Wind Webinar #326 February 2013
Page 14 of 126
• b. G = gust-effect factor from Section 26.9
c. Cf = net force coefficient from Fig. 29.4-1
d. As = the gross area of the solid free-
standing wall or freestanding solid sign, in
ft2 (m2)
Section 29.4.1 – Solid Freestanding Walls
and Solid Freestanding Signs
Wind Webinar #326 February 2013
Page 15 of 126
Figure 29.4-1
Wind Webinar #326 February 2013
Page 16 of 126
Figure 29.4-1
Wind Webinar #326 February 2013
Page 17 of 126
Figure 29.4-1
Wind Webinar #326 February 2013
Page 18 of 126
• Load Cases to Consider
• Case A – Load applied to the centroid of
the area
• Case B – Load applied with an
eccentricity of 0.2*B (width of the wall or
sign)
• Case C – Stepped application of reduced
wind pressures as the distance decreases
from the windward edge.
Section 29.4.1 – Solid Freestanding Walls
and Solid Freestanding Signs
Wind Webinar #326 February 2013
Page 19 of 126
• Case C
–Reduction in loads for walls or signs with
returns at the ends
» Up to 40 % reduction
– For Elevated Signs or walls: where s/h > 0.8,
force coefficients shall be multiplied by the
reduction factor (1.8 - s/h)
» Accounts for reduced wind pressures
when free air flow under the wall or sign
is reduced.
Section 29.4.1 – Solid Freestanding Walls
and Solid Freestanding Signs
Wind Webinar #326 February 2013
Page 20 of 126
Section 29.4.1 – Solid Freestanding Walls
and Solid Freestanding Signs
Wind Webinar #326 February 2013
Page 21 of 126
a.1. Requirements to use method in Section
29.4.1
a. The plane of the sign is parallel to and in
contact with the supporting wall
b. Edges of the sign do not extend past the
supporting wall
c. Use Component & Cladding Wall pressures
calculated in Chapter 30
d. Set the Internal Pressure Coefficient (GCpi)
equal to 0
Section 29.4.1 2 - Solid Attached Signs
Wind Webinar #326 February 2013
Page 22 of 126
a. 2. Procedure can also be used for
signs attached to but not in contact
with the supporting wall.
a. Sign must be parallel to the supporting wall.
b. Sign must not be more than three (3) feet
from the wall.
c. Edges of the sign must be at least (3) feet in
from the free edges of the supporting wall:
a. Top of the supporting wall.
b. Bottom of the supporting wall
c. Side Edges of the supporting wall
Section 29.4.1 2 - Solid Attached Signs
Wind Webinar #326 February 2013
Page 23 of 126
Section 29.5: Design Wind Loads—Other
Structures
Wind Webinar #326 February 2013
Page 24 of 126
Basic Equation: (Eq. 29.5-1)
F = qzGCfAf (lb) (N)
a. qz = velocity pressure evaluated at height z as
defined in Section 29.3, of the centroid of area Af
» Note qz is at the centroid of the area and Kd in
Eq 29.3-1 is the Kd of the structure type in
Table 26.6-1
b. G = gust-effect factor from Section 26.9 (these
structures may often be flexible)
c. Cf = force coefficients from Figs. 29.5-1
Section 29.5: Design Wind Loads—Other
Structures
Wind Webinar #326 February 2013
Page 25 of 126
d. Af = projected area normal to the wind except
where Cf is specified for the actual surface area,
in ft2 (m2)
Section 29.5: Design Wind Loads—Other
Structures
Wind Webinar #326 February 2013
Page 26 of 126
Figure 29.5-1
Wind Webinar #326 February 2013
Page 27 of 126
Figure 29.5-1
Wind Webinar #326 February 2013
Page 28 of 126
Figure 29.5-2
Wind Webinar #326 February 2013
Page 29 of 126
Figure 29.5-2
Wind Webinar #326 February 2013
Page 30 of 126
Figure 29.5-3
Wind Webinar #326 February 2013
Page 31 of 126
Figure 29.5-3
Wind Webinar #326 February 2013
Page 32 of 126
1. No guidance is given for rooftop
structures on buildings > 60 feet.
2. Research in the ASCE 7 committee
suggests that it is probably acceptable to
use loads from this section for rooftop
structures on buildings > 60 feet, but this
has not been confirmed yet.
3. Equation 29.5-2 gives lateral pressure
on the rooftop structure.
Section 29.5-1 – Rooftop Structures and
Equipment For Buildings with h ≤ 60 feet
Wind Webinar #326 February 2013
Page 33 of 126
• Lateral Wind force on Rooftop Structures
– Basic Equation: (Eq 29.5-2)
Fh = qh(GCr)Af (lb) (N)
» (GCr) = 1.9 for rooftop structures and
equipment with Af less than (0.1Bh). (GCr)
shall be permitted to be reduced linearly
from 1.9 to 1.0 as the value of Af is
increased from (0.1Bh) to (Bh).
» qh = velocity pressure evaluated at mean
roof height of the building
Section 29.5-1 – Rooftop Structures and
Equipment For Buildings with h ≤ 60 feet
Wind Webinar #326 February 2013
Page 34 of 126
• Lateral Wind force on Rooftop Structures
» Note, qh is at the mean roof height and Kd
in Eq 29.3-1 is the Kd of the building, in
Table 26.6-1, on which the rooftop
structure sits.
» Af = vertical projected area of the rooftop
structure or equipment on a plane normal
to the direction of wind, in ft2 (m2)
Section 29.5-1 – Rooftop Structures and
Equipment For Buildings with h ≤ 60 feet
Wind Webinar #326 February 2013
Page 35 of 126
• Vertical Wind force on Rooftop Structures
– Basic Equation: (Eq 29.5-3)
Fv = qh(GCr)Ar (lb) (N)
» (GCr) = 1.5 for rooftop structures and
equipment with Ar less than (0.1BL). (GCr)
shall be permitted to be reduced linearly
from 1.5 to 1.0 as the value of Ar is
increased from (0.1BL) to (BL).
» qh = velocity pressure evaluated at mean
roof height of the building
Section 29.5-1 – Rooftop Structures and
Equipment For Buildings with h ≤ 60 feet
Wind Webinar #326 February 2013
Page 36 of 126
• Vertical Wind force on Rooftop Structures
» Note qh is at the mean roof height and Kd in
Eq 29.3-1 is the Kd of the building, in Table
26.6-1, on which the rooftop structure sits.
» Ar = horizontal projected area of rooftop
structure or equipment, in ft2 (m2)
Section 29.5-1 – Rooftop Structures and
Equipment For Buildings with h ≤ 60 feet
Wind Webinar #326 February 2013
Page 37 of 126
• Lateral C & C pressure (psf) shall be equal
to the lateral force (Lbs) calculated with
equation (29.5-2) DIVIDED BY the
RESPECTIVE WALL Surface area of the
Rooftop Structure considered.
– Forces (psf) shall be considered to act inward
and outward
Section 30.11 – Component & Cladding
Loads for Rooftop Structures and
Equipment for Buildings with h ≤ 60 feet
Wind Webinar #326 February 2013
Page 38 of 126
• Vertical C & C pressure (Lbs) shall be
equal to the vertical force (Lbs) calculated
with equation (29.5-3) DIVIDED BY the
Horizontal projected area of the roof of
the Rooftop Structure considered.
– The pressures are ONLY required to be
considered to act in the UPWARD direction.
Section 30.11 – Component & Cladding
Loads for Rooftop Structures and
Equipment for Buildings with h ≤ 60 feet
Wind Webinar #326 February 2013
Page 39 of 126
• Comment: If the Rooftop Structure is
large (10’x20’ or larger), consider looking
at the downward pressures from the
building C&C loading figures and make
some judgment about downward wind
loading rooftop structures that resemble
small buildings (penthouses for instance).
– Vertical Wind Load would act in addition to
Dead and Roof Live Loads or Snow Loads.
– Use appropriate load combinations.
Section 30.11 – Component & Cladding
Loads for Rooftop Structures and
Equipment for Buildings with h ≤ 60 feet
Wind Webinar #326 February 2013
Page 40 of 126
Other Resources
Wind Webinar #326 February 2013
Page 41 of 126
Other Resources
• Prepared by: Task
Committee on Wind-
Induced Forces of the
Petrochemical Committee
of the Enginery Division of
ASCE
• Several of those on the
Task Committee are on
the ASCE 7 Wind
Subcommittee
• Based on ASCE 7-05
Wind Webinar #326 February 2013
Page 42 of 126
• “Wind loads on parapets are specified in
Section 27.4.5 for buildings of all heights
designed using the Directional Procedure
and in Section 28.4.2 for low-rise
buildings designed using the Envelope
Procedure.”
• Method presented is the Directional
Procedure
Section 29.6 – Parapets (MWFRS)
Wind Webinar #326 February 2013
Page 43 of 126
• Chapter 28 – The Envelope Method, is
exactly the same.
– Chapter 28 uses the velocity pressure
determined with the Envelope Method,
rather than the velocity pressure in Chapter
27 using the Directional Method.
Section 29.6 – Parapets (MWFRS)
Wind Webinar #326 February 2013
Page 44 of 126
• MWFRS pressures due to parapets
– Rigid or Flexible Buildings
– Applies to Flat, Gable, or Hip Roofs
Section 27.4.5 - Parapets
Wind Webinar #326 February 2013
Page 45 of 126
• Basic Equation: (Eq 27.4-4)
pp = qp(GCpn) (lb/ft2)
– pp = combined net pressure on the parapet
due to the combination of the net pressures
from the front and back parapet surfaces.
Plus (and minus) signs signify net pressure
acting toward (and away from) the front
(exterior) side of the parapet
Section 27.4.5 – Parapets (MWFRS)
Wind Webinar #326 February 2013
Page 46 of 126
– qp = velocity pressure evaluated at the top of
the parapet
» (GCpn) = combined net pressure
coefficient
– = +1.5 for windward parapet
– = –1.0 for leeward parapet
Section 27.4.5 – Parapets (MWFRS)
Wind Webinar #326 February 2013
Page 47 of 126
Section 27.4.5 – Parapets (MWFRS)
• FIGURE C29.7-1 Design Wind Pressures on Parapets
Wind Webinar #326 February 2013
Page 48 of 126
• Applicable to All Building Types
• Applicable to All Building Heights
– Except Where the Provisions of Part 4
are used (Simplified Method for
Buildings with h ≤ 160 feet)
Section 30.9 – C & C Loading on Parapets
Wind Webinar #326 February 2013
Page 49 of 126
• Basic Equation: (Eq. 30.9-1)
p = qp((GCp) – (GCpi))
– qp = velocity pressure evaluated at the top of
the parapet
– (GCp) = external pressure coefficient given in
» Fig. 30.4-1 for walls with h ≤ 60 ft (48.8 m)
» Figs. 30.4-2A to 30.4-2C for flat roofs,
gable roofs, and hip roofs
» Fig. 30.4-3 for stepped roofs
Section 30.9 – C & C Loading on Parapets
Wind Webinar #326 February 2013
Page 50 of 126
– (GCp) = external pressure coefficient given in
» Fig. 30.4-4 for multispan gable roofs
» Figs. 30.4-5A and 30-5B for monoslope
roofs
» Fig. 30.4-6 for sawtooth roofs
» Fig. 30.4-7 for domed roofs of all heights
» Fig. 30.6-1 for walls and flat roofs with h >
60 ft (18.3 m)
» Fig. 27.4-3 footnote 4 for arched roofs
Section 30.9 – C & C Loading on Parapets
Wind Webinar #326 February 2013
Page 51 of 126
– (GCpi) = internal pressure coefficient from
Table 26.11-1, based on the porosity of the
parapet envelope.
• Consider Two (2) Load Cases when
evaluating C & C pressures on parapets
– Case A – Pressures on the surfaces of the
Windward parapet
– Case B – Pressures on the surfaces of the
Leeward parapet
Section 30.9 – C & C Loading on Parapets
Wind Webinar #326 February 2013
Page 52 of 126
• Specifics of Case A:
– Windward Parapet shall consist of applying the
applicable positive wall pressure from Fig. 30.4-1
(h ≤ 60 ft (18.3 m)) or Fig. 30.6-1 (h > 60 ft (18.3
m)) to the windward surface of the parapet while
applying the applicable negative edge or corner
zone roof pressure from Figs. 30.4-2 (A, B or C),
30.4-3, 30.4-4, 30.4-5 (A or B), 30.4-6, 30.4-7, Fig.
27.4-3 footnote 4, or Fig. 30.6-1 (h > 60 ft (18.3
m)) as applicable to the leeward surface of the
parapet.
Section 30.9 – C & C Loading on Parapets
Wind Webinar #326 February 2013
Page 53 of 126
• Specifics of Case B:
– Leeward Parapet shall consist of applying the
applicable positive wall pressure from Fig. 30.4-1
(h ≤ 60 ft (18.3 m)) or Fig. 30.6-1 (h > 60 ft (18.3
m)) to the windward surface of the parapet, and
applying the applicable negative wall pressure
from Fig. 30.4-1 (h ≤ 60 ft (18.3 m)) or Fig. 30.6-1
(h > 60 ft (18.3 m)) as applicable to the leeward
surface. Edge and corner zones shall be arranged
as shown in the applicable figures. (GCp)
Section 30.9 – C & C Loading on Parapets
Wind Webinar #326 February 2013
Page 54 of 126
Section 30.9 – C & C Loading on Parapets
• FIGURE C29.7-1 Design Wind Pressures on Parapets
• If internal pressure
is present, both
load cases should
be evaluated
under positive and
negative internal
pressure.
Wind Webinar #326 February 2013
Page 55 of 126
• Step 1: Determine risk category of building,
see Table 1.5-1
• Step 2: Determine the basic wind speed, V,
for applicable risk category, see Figure 26.5-
1A, B or C
Table 30.9-1 – Steps to Determine C&C
Wind Loads on Parapets
Wind Webinar #326 February 2013
Page 56 of 126
• Step 3: Determine wind load parameters:
–Wind directionality factor, Kd , see Section 26.6
and Table 26.6-1
» Use Kd for Buildings C&C (0.85)
– Exposure category B, C or D, see Section 26.7
–Topographic factor, Kzt, see Section 26.8 and Fig.
26.8-1
–Enclosure classification, see Section 26.10
– Internal pressure coefficient, (GCpi), see Section
26.11 and Table 26.11-1
» Open, Partially Enclosed, or Enclosed (parapet or bldg)
Table 30.9-1 – Steps to Determine C&C
Wind Loads on Parapets
Wind Webinar #326 February 2013
Page 57 of 126
• Step 4: Determine velocity pressure
exposure coefficient, Kh, at top of the
parapet see Table 30.3-1
• Step 5: Determine velocity pressure, qp, at
the top of the parapet using Eq. 30.3-1
Table 30.9-1 – Steps to Determine C&C
Wind Loads on Parapets
Wind Webinar #326 February 2013
Page 58 of 126
• Step 6: Determine external pressure
coefficient for wall and roof surfaces
adjacent to parapet, (GCp)
–Walls with h ≤ 60 ft., see Fig. 30.4-1
– Flat, gable and hip roofs, see Figs. 30.4-2A to
30.4-2C
– Stepped roofs, see Fig. 30.4-3
–Multispan gable roofs, see Fig. 30.4-4
–Monoslope roofs, see Figs. 30.4-5A and 30.4-5B
– Sawtooth roofs, see Fig. 30.4-6
Table 30.9-1 – Steps to Determine C&C
Wind Loads on Parapets
Wind Webinar #326 February 2013
Page 59 of 126
• Step 6: (Continued)
–Domed roofs of all heights, see Fig. 30.4-7
–Walls and flat roofs with h > 60 ft., see Fig. 30.6-1
–Arched roofs, see footnote 4 of Fig. 27.4-3
• Step 7: Calculate wind pressure, p, using Eq.
30.9-1 on windward and leeward face of
parapet, considering two load cases (Case A
and Case B) as shown in Fig. 30.9-1.
Table 30.9-1 – Steps to Determine C&C
Wind Loads on Parapets
Wind Webinar #326 February 2013
Page 60 of 126
Section 30.9 – C & C Loading on Parapets
Wind Webinar #326 February 2013
Page 61 of 126
Figure 30.6-1
• Note 7 defines parapets > 3 feet as tall
parapets• Reduced corner pressures on parapet
• Similar note on other figures
Wind Webinar #326 February 2013
Page 62 of 126
• “Wind loads on roof overhangs are
specified in Section 27.4.4 for buildings of
all heights designed using the Directional
Procedure and in Section 28.4.3 for low-
rise buildings designed using the Envelope
Procedure.”
• Present Direction Method in Section 27.4.1
–Envelope Method in Section 28.3.1 is similar
–Uses different factor for Cp
Section 29.7 – Roof Overhangs (MWFRS)
Wind Webinar #326 February 2013
Page 63 of 126
• The positive external pressure on the
bottom surface of windward roof
overhangs shall be determined using Cp =
0.8 and combined with the top surface
pressures determined using Fig. 27.4-1.
Section 27.4 – Roof Overhangs (MWFRS)
Wind Webinar #326 February 2013
Page 64 of 126
Section 27.4 – Roof Overhangs (MWFRS)
Wind Webinar #326 February 2013
Page 65 of 126
Section 27.4 – Roof Overhangs (MWFRS)
• Must consider cases with positive internal
pressure and negative internal pressure
• From Figure 27.6-3
Wind Webinar #326 February 2013
Page 66 of 126
Section 30.10 – C & C Loading on Roof
Overhangs
• Applicable to All Building Types
• Applicable to All Building Heights
– Except Where the Provisions of Part 4
are used (Simplified Method for
Buildings with h ≤ 160 feet)
Wind Webinar #326 February 2013
Page 67 of 126
Section 30.10 – C & C Loading on Roof
Overhangs
• Basic Equation: (Eq. 30.10-1)
p = qh((GCp) – (GCpi))
– qh = velocity pressure from Section 30.3.2
evaluated at mean roof height h using
exposure defined in Section 26.7.3
– (GCp) = external pressure coefficients for
overhangs given in Figs. 30.4-2A to 30.4-2C
(flat roofs, gable roofs, and hip roofs),
including contributions from top and bottom
surfaces of overhang.
Wind Webinar #326 February 2013
Page 68 of 126
Section 30.10 – C & C Loading on Roof
Overhangs» The external pressure coefficient for the
covering on the underside of the roof
overhang is the same as the external
pressure coefficient on the adjacent wall
surface, adjusted for effective wind area,
determined from Figure 30.4-1 or Figure
30.6-1 as applicable
– (GCpi) = internal pressure coefficient
given in Table 26.11-1
Wind Webinar #326 February 2013
Page 69 of 126
Figure 30.4-2A
Wind Webinar #326 February 2013
Page 70 of 126
Figure 30.4-1
Wind Webinar #326 February 2013
Page 71 of 126
• Step 1: Determine risk category of building,
see Table 1.5-1
• Step 2: Determine the basic wind speed, V,
for applicable risk category, see Figure 26.5-
1A, B or C
Table 30.10-1 – Steps to Determine C&C
Wind Loads on Roof Overhangs
Wind Webinar #326 February 2013
Page 72 of 126
• Step 3: Determine wind load parameters:
–Wind directionality factor, Kd , see Section 26.6
and Table 26.6-1
» Use Kd for Buildings C&C (0.85)
– Exposure category B, C or D, see Section 26.7
–Topographic factor, Kzt, see Section 26.8 and Fig.
26.8-1
–Enclosure classification, see Section 26.10
– Internal pressure coefficient, (GCpi), see Section
26.11 and Table 26.11-1
» Open, Partially Enclosed, or Enclosed (overhang or bldg)
Table 30.10-1 – Steps to Determine C&C
Wind Loads on Roof Overhangs
Wind Webinar #326 February 2013
Page 73 of 126
• Step 4: Determine velocity pressure
exposure coefficient, Kh, see Table 30.3-1
• Step 5: Determine velocity pressure, qh, at
mean roof height h using Eq. 30.3-1
Table 30.10-1 – Steps to Determine C&C
Wind Loads on Roof Overhangs
Wind Webinar #326 February 2013
Page 74 of 126
• Step 6: Determine external pressure
coefficient, (GCp), using Figs. 30.4-2A
through C for flat, gabled and hip roofs.
• Step 7: Calculate wind pressure, p, using Eq.
30.10-1. Refer to Figure 30.10-1
Table 30.10-1 – Steps to Determine C&C
Wind Loads on Roof Overhangs
Wind Webinar #326 February 2013
Page 75 of 126
Figure 30.10-1
Wind Webinar #326 February 2013
Page 76 of 126
• The design wind force for other structures
shall be not less than 16 lb/ft2 (0.77 kN/m2)
multiplied by the area Af.
–16 psf is 10 psf from ASCE 7-05 times 1.6 to bring
the load to a strength level load.
–Apply to the full projected area in each
orthogonal direction.
Section 29.8 – Minimum Design Wind
Loading (MWFRS)
Wind Webinar #326 February 2013
Page 77 of 126
• Loads on many shapes in industrial plants
–Tanks, Silos, Pipe racks, Partially Clad
Frames, etc.
–See ASCE report: “Wind Loads for
Petrochemical and Other Industrial
Facilities”
• Wind Loads on roof mounted Solar
Photovoltaic Arrays
–See new SEOC Guide
Material in Chapters 29 Not Covered
Wind Webinar #326 February 2013
Page 78 of 126
Other Ressources
Wind Webinar #326 February 2013
Page 79 of 126
• Office Complex
• Location: Wichita, KS
• Freestanding Sign
• 3-Story Office
–5-foot tall parapet
–Roof Top Unit
• Well Pump & Maintenance Building
–Roof Overhang
• Chemical Storage Silo
Example
Wind Webinar #326 February 2013
Page 80 of 126
• Site Parameters Common to All Examples
• Exposure (Terrain Roughness): C– Location: N 37.7500, W 97.1683
– Section 26.7.3 (Open Farmland)
• Risk Category II Structures – Table 1.5-1
• Wind Velocity: 115 mph– Fig. 26.5-1A
– http://www.atcouncil.org/windspeed/index.php
• Topographic Factor, Kzt: 1.00– Section 26.8
• All Loads are Calculated to LRFD Levels
Example
Wind Webinar #326 February 2013
Page 81 of 126
Example
Wind Webinar #326 February 2013
Page 82 of 126
• Solid Billboard Sign at Ground Level• Dimensions: 30’ Wide x 10’ High
• Reference Figure 29.4-1 (MWFRS)
– s=10’
– B=30’
– h=10’
• Kh = 0.85 (Table 29.3-1)
• Kd = 0.85 (Solid Freestanding Walls & Signs)(Table 26.6-1)
• qh = 0.00256 KzKztKdV2 (psf)
• qh = 0.00256(0.85)(1.00)(0.85)(115)2 = 24.46 psf
• B/s = 30’/10’ = 3.0
• s/h = 10’/10’ = 1.0
Example-Freestanding Sign
Wind Webinar #326 February 2013
Page 83 of 126
• Enter Figure 29.4-1 for Cf
–Applies to Cases A & B– For B/s = 2: Cf = 1.40
– For B/s = 4: Cf = 1.35
– Interpolating for B/s 3.0, Cf = 1.375
• G = 0.85 (Section 26.9 – Rigid Structure)
• F = qhGCfAs (Lb) (Eq 29.4-1)
• F = (24.46 psf)(0.85)(1.375)As = 28.59 psf*As
– As = Af = 30’x10’ = 300 ft2
– 28.59 psf*As > 16 psf * Af (Section 29.8 Min. Load)
• F = 28.59 psf(300 ft2) = 8577 Lbs
– For CASE A, Load is applied at the plan C.L. and at
– (s/2)+(0.05h) = 5.5’ above base
Example-Freestanding Sign
Wind Webinar #326 February 2013
Page 84 of 126
» See Cross-Section View, Figure 29.4-1
– For CASE B, Load is applied @ 5.5’ above base and at 0.2B
offset, either side of plan C.L.
» 0.2B = 0.2(30’) = 6.0’ either side of plan C.L.
• Check to see if CASE C must be considered
– Note 3, Figure 29.4-1
– If B/s ≥ 2.0, CASE C must be considered
– B/s = 30’/10’ = 3.0 > 2.0, therefore consider CASE C
• Enter Figure 29.4-1 for Cf, under CASE C
– 0-s (0’-10’): Cf = 2.60
– s-2s (10’-20’): Cf = 1.70
– 2s-3s (10’-30’): Cf = 1.15
Example-Freestanding Sign
Wind Webinar #326 February 2013
Page 85 of 126
– For CASE C, where s/h > 0.8, Cf may be multiplied by the
reduction factor (1.8 – s/h)
– s/h = 1.0 > 0.8
– (1.8 – s/h) = (1.8 – 1.0) = 0.8
– F = qhGCfAs (Lb) (Eq 29.4-1)
– F1 = (24.46psf)(0.85)(2.60)(0.8)(10’x10’) = 4324 Lbs
– F2 = (24.46psf)(0.85)(1.70)(0.8)(10’x10’) = 2828 Lbs
– F3 = (24.46psf)(0.85)(1.15)(0.8)(10’x10’) = 1913 Lbs
» Apply F1, F2, and F3 at plan C.L. of each plan length, s,
from each end of sign. See Figure.
» Apply F1, F2 and F3 at 5.5’ above base of each plan
length, s
Example-Freestanding Sign
Wind Webinar #326 February 2013
Page 86 of 126
Example-Freestanding Sign
Wind Webinar #326 February 2013
Page 87 of 126
• Parapet (MWFRS) (Section 29.6)
• Office Building– L = 200 ft., B = 100 ft.
–Roof Height: h = 40 ft.
–Parapet Height: hp = 45 ft.
–Roof Slope, Flat: 0.25:12
» Ridge parallel to 200’ side
–Exposure Category: C
• Section 29.6 references Section 27.4.5 for
directional procedure for MWFRS Parapet load
determination.
Example-Parapet (MWFRS)
Wind Webinar #326 February 2013
Page 88 of 126
– pp = qp(GCpn) (psf)
– qp = 0.00256KhKztKdV2 (psf) (Eq 27.3-1)
– Kh @ hp = 45’, Kh = 1.065
– Kzt = 1.00 (for complex)
– Kd = 0.85 (Building MWFRS)(Table 26.6-1)
– V = 115 mph (for complex)
– qp = 0.00256(1.065)(1.00)(0.85)(115)2 = 30.65 psf
– GCpn = +1.5 for windward parapet (Section 27.4.5)
– GCpn = -1.0 for leeward parapet (Section 27.4.5)
• Windward Parapet
– pp = (30.65 psf)(1.5) = 45.98 psf acting toward building
• Leeward Parapet
– pp = (30.65 psf)(-1.0) = -30.65 psf acting away from
building
Example-Parapet (MWFRS)
Wind Webinar #326 February 2013
Page 89 of 126
• Parapet (C & C Loads) (Section 30.9)
• Office Building – same as previous
• qp = 0.00256(1.065)(1.00)(0.85)(115)2 = 30.65 psf
– from parapet MWFRS, above
• p = qp((GCp) – (GCpi)) (Eq 30.9-1)
• Parapet can be pressurized along with building
– See Figure
• GCpi = ±±±± 0.18 (Enclosed Building – Table 26.11-1)
Example-Parapet (C & C)
Wind Webinar #326 February 2013
Page 90 of 126
Example-Parapet (C & C)
Wind Webinar #326 February 2013
Page 91 of 126
• Studs @ 16” o.c. (both faces)
• Determine Effective Wind Area of Studs
– Greater of Tributary Area or Effective Width
» Effective Wind Area Definition (Section 26.2)
– Greater of 16”/12” = 1.33’ or
– Length/3 = 5’/3 = 1.67’ (governs)
» Effective Wind Area: l2/3 = 52/3 = 8.33 ft2
» If Effective Wind Area > 700 ft2, use MFWRS loads
• Determine which figure to reference from Table 30.9-1,
Step 6
– Figure 30.4-1 for wall pressures, h ≤ 60 ft.
Example-Parapet (C & C)
Wind Webinar #326 February 2013
Page 92 of 126
– Figure 30.4-2A for roof loads, h ≤ 60 ft. and gable roofs θ
≤ 7°°°°
– Determine “a” distance (Figure 30.4-1 and 30.4-2A)
» Lesser of 10% of B = 0.10(100’) = 10’ and 0.4h =
0.4(40’) = 16’
– 10’ governs
» Not less than the greater of 4% of B = 0.04(100’) = 4’
or 3’
» a = 10’
– Entering Figure 30.4-1 for pressure coefficients on
exterior surfaces of the parapets:
» Zone 4 Positive Pressure: GCp = 1.0
» Zone 5 Positive Pressure: GCp = 1.0
Example-Parapet (C & C)
Wind Webinar #326 February 2013
Page 93 of 126
» Zone 4 Negative Pressure: GCp = -1.1
» Zone 5 Negative Pressure: GCp = -1.4
– Note 5 says that values of GCp may be reduced
by 10% when θ ≤ 10°°°°
» Zone 4 Positive Pressure: GCp = (0.9)1.0 = 0.9
» Zone 5 Positive Pressure: GCp = (0.9)1.0 = 0.9 Zone 4
Negative Pressure: GCp = (0.9)-1.1 = -1.0
» Zone 5 Negative Pressure: GCp = (0.9)-1.4 = -1.26
– Entering Figure 30.4-2A for pressure coefficients on
interior (roof side) surfaces of parapet:
» Effective Wind Area: A= 52/3 = 8.33 ft2
» a = 10 ft. as in Figure 30.4-1
» Zone 1, 2, and 3 Positive Pressure: GCp = 0.3
Example-Parapet (C & C)
Wind Webinar #326 February 2013
Page 94 of 126
» Zone 1 Negative Pressure: GCp = -1.0
» Zone 2 Negative Pressure: GCp = -1.8
» Zone 3 Negative Pressure: GCp = -2.8
– Note 5: “If a parapet equal to or higher than 3 ft
(0.9m) is provided around the perimeter of the
roof with θ ≤ 7°°°°, the negative values of GCp in
Zone 3 shall be equal to those for Zone 2 and
positive values of GCp in Zones 2 and 3 shall be
set equal to those for wall Zones 4 and 5
respectively in Figure 30.4-1.”
» Parapet height hp = 5’ > 3’
– Zone 3 Negative GCp is Not Applicable
– Zone 2 and Zone 3 Positive Pressure GCp are
those of Wall Zone 4 and Zone 5, respectively.
Example-Parapet (C & C)
Wind Webinar #326 February 2013
Page 95 of 126
• Step 7: Calculate wind pressure, p using Eq 30.9-1 on
windward and leeward faces of parapet, considering two
load cases (CASE A and CASE B) as shown in Figure 30.9-1
– Note: As wind direction changes, each parapet with shift
from a windward parapet to a leeward parapet.
• CASE A – Windward Parapet
– Exterior Face Wall Studs
» p = qp((GCp) – (GCpi)) (Eq 30.9-1)
» With Positive Internal Pressure
– Zone 4 = Zone 5
– P = (30.65 psf)((0.9)-(0.18)) = 22.06 psf
» With Negative Internal Pressure
– Zone 4 = Zone 5
– p = (30.65 psf)((0.9)-(-0.18)) = 33.10 psf
Example-Parapet (C & C)
Wind Webinar #326 February 2013
Page 96 of 126
» Apply to Tributary Area, not Effective Wind Area
– 33.10 psf(1.33’) = 44.12 plf
– Interior Face (roof side) Parapet Studs
» p = qp((GCp) – (GCpi)) (Eq 30.9-1)
» With Positive Internal Pressure
– Zone 2 (Zone 3 also treated as Zone 2)
– p = (30.65 psf)((-1.8)-(0.18)) = -60.69 psf
» With Negative Internal Pressure
– Zone 2 (Zone 3 also treated as Zone 2)
– p = (30.65 psf)((-1.8)-(-0.18)) = -40.65 psf
» Apply to Tributary Area, not Effective Wind Area
– -60.69 psf(1.33’) = -80.72 plf
Example-Parapet (C & C)
Wind Webinar #326 February 2013
Page 97 of 126
• CASE B – Leeward Parapet
– Interior Face Parapet Studs (load toward parapet)
» p = qp((GCp) – (GCpi)) (Eq 30.9-1)
» With Positive Internal Pressure
– Substitute Zone 4 and Zone 5 pressures for roof
Zone 2 and Zone 3 pressures, respectively. Zone
4 = Zone 5
– P = (30.65 psf)((0.9)-(0.18)) = 22.06 psf
» With Negative Internal Pressure
– Zone 4 = Zone 5
– p = (30.65 psf)((0.9)-(-0.18)) = 33.10 psf
» Apply to Tributary Area, not Effective Wind Area
– 33.10 psf(1.33’) = 44.12 plf
Example-Parapet (C & C)
Wind Webinar #326 February 2013
Page 98 of 126
– Exterior Face Wall Studs (load away from parapet)
» p = qp((GCp) – (GCpi)) (Eq 30.9-1)
» With Positive Internal Pressure
– Zone 4 pressure
– p = (30.65 psf)((-1.0)-(0.18)) = -36.17 psf
– Zone 5 pressure
– p = (30.65 psf)((-1.26)-(0.18)) = -44.14 psf
» With Negative Internal Pressure
– Zone 4 pressure
– p = (30.65 psf)((-1.0)-(-0.18)) = -25.13 psf
– Zone 5 pressure
– p = (30.65 psf)((-1.26)-(-0.18)) = -33.10 psf
Example-Parapet (C & C)
Wind Webinar #326 February 2013
Page 99 of 126
» Apply to Tributary Area, not Effective Wind Area
– Zone 4
– -36.17 psf(1.33’) = -48.11 plf
– Zone 5
– -44.14 psf(1.33’) = -58.71 plf
– Summary
» Exterior Wall Studs extended past roof into parapet:
– Zone 4: 44.12 plf (toward building)
– Zone 4: -48.11 plf (away from building)
– Zone 5 is anything within 10ft of the corner
– Zone 5: 44.12 plf (toward building)
– Zone 5: -58.71 plf (away from building)
Example-Parapet (C & C)
Wind Webinar #326 February 2013
Page 100 of 126
» Interior (roof side) Parapet Studs
– Zone 4: 44.13 plf (toward parapet)
– Zone 4: -80.72 plf (away from parapet)
– Zone 5 is anything within 10ft of the corner
– Zone 5: 44.13 plf (toward parapet)
– Zone 5: -80.92 plf (away from parapet)
Example-Parapet (C & C)
Wind Webinar #326 February 2013
Page 101 of 126
• Rooftop Equipment for Buildings (MWFRS)
(Section 29.5.1)
• Office Building – same as previous
• Plan Dimensions: 10’ wide x 20’ long
• RTU Height: 4’ over 1’ tall curb
• Projected Height: 4’+1’=5’
• Lateral Force: Fh = qh(GCr)Af (Lb) (Eq 29.5-2)
• Vertical Force: Fv = qh(GCr)Ar (Lb) (Eq 29.5-3)• qh calculated at mean roof height of building
• Kh @ h = 40’, Kh = 1.04 (Table 29.3-1)
• Use Kd for building NOT Kd for rectangular Other Structures
Example-Rooftop Equip. (MWFRS)
Wind Webinar #326 February 2013
Page 102 of 126
• Kd = 0.85 (Table 26.6-1)
• Other parameters as previously defined for building
• qh = 0.00256(1.04)(1.00)(0.85)(115)2 = 29.93 psf
• HORIZONTAL WIND FORCE
– Check projected area of side compared with least
projected area of building
» B*h = 100’(40’) = 8000 ft2
» Af (max) = 20’(5’) = 100 ft2
» Af <0.1Bh: 100 ft2 < 800 ft2
– GCr = 1.9
– Fh = (29.93 psf)(1.9)(Af) = 56.87 psf(Af) < 16 psf(Af)
» Minimum Load from Section 29.8
– Fh = (29.93 psf)(1.9)(100 ft2) = 5687Lbs(Eq 29.5-2)
Example-Rooftop Equip. (MWFRS)
Wind Webinar #326 February 2013
Page 103 of 126
» Perpendicular to long side
– Fh = (29.93 psf)(1.9)(50 ft2) = 2843 Lbs (Eq 29.5-2)
» Parallel to long side
– Horizontal wind forces applied to geometric center of
vertical projected plane of unit
• VERTICAL WIND FORCE
– Check projected area of roof compared with that of
building
» B*L = 100’(200’) = 20,000 ft2
» Ar = 20’(10’) = 200 ft2
» Ar <0.1BL: 200 ft2 < 2,000 ft2
– GCr = 1.5
– Fv = (29.93 psf)(1.5)(200 ft2) = 8979Lbs(Eq 29.5-2)
Example-Rooftop Equip. (MWFRS)
Wind Webinar #326 February 2013
Page 104 of 126
» Vertical Up
– Vertical wind forces applied to geometric center of
horizontal projected plane of unit
• Note: The UPLIFT pressure on the top of the rooftop
equipment acts SIMULTANEOUSLY with either the Lateral
pressure parallel to or perpendicular to the long edge of the
rooftop equipment or structure.
• The same procedure is used for a roof-mounted penthouse.
Example-Rooftop Equip. (MWFRS)
Wind Webinar #326 February 2013
Page 105 of 126
• Rooftop Equipment for Buildings (C&C)
(Section 30.11)• Loads for Designing the Equipment cabinet enclosure or the
wall components for a penthouse
• Lateral C & C pressures
• Fh = 5687 Lbs (from previous)
• C & C Lateral Loads: Fh/Af = 5687 Lbs/100 ft2 = 56.87 psf
– Load is applied toward or away from unit on all sides
• C & C Vertical Loads: Fv/Ar = 8979 Lbs/200 ft2 = 44.90 psf
– Load is applied only in the Upward direction, away from
the top of the unit
Example-Rooftop Equip. (C & C)
Wind Webinar #326 February 2013
Page 106 of 126
• Personal Recommendation (Not in the Standard)
– If the unit is large (over 200 ft2), consider a minimum
downward wind load.
– C & C Loading from Figure 30.4-2A
» GCp = 0.2 (downward component)
» Fv = qhGCp = (29.93psf)(0.2) = 6 psf
» For higher wind loads and low snow loads,
particularly less than 10 psf, this may produce a
controlling load combination
Example-Rooftop Equip. (C & C)
Wind Webinar #326 February 2013
Page 107 of 126
• Roof Overhang (MWFRS) (Section 29.7)
• Equipment Building– L = 60 ft., B = 30 ft.
–Eave Height: 10 ft.
–Overhang Width: 3 ft.
–Roof Slope: 4.375:12 (θ = 20°°°°)
» Ridge parallel to 60’ side
–Exposure Category: C
–Average Building Height: h = 13.28’ < 15’
• Section 29.7 references Section 27.4.4 for
directional procedure for MWFRS Roof Overhang
load determination.
Example-Roof Overhang (MWFRS)
Wind Webinar #326 February 2013
Page 108 of 126
– Use Cp = 0.8 in Eq 27.4-1 for determining roof and wall
loads over and adjacent to roof overhang
– p = qGCp-qi(GCpi) (psf) (Eq 27.4-1)
– G = 0.85 (rigid structure) (Section 26.9)
– Kzt = 1.00 (for complex)
– Kd = 0.85 (Building MWFRS)(Table 26.6-1)
– V = 115 mph (for complex)
– Kz @ z = 10’ for soffit, Kz = 0.85 (Table 27.3-1)
– qz = 0.00256KzKztKdV2 (psf) (Eq 27.3-1)
– qz = 0.00256(0.85)(1.00)(0.85)(115)2 = 24.46 psf
– GCpi = ±±±± 0.18 (Enclosed building)(Figure 26.11-1)
– Transverse Wind Loading governs, by Inspection
Example-Roof Overhang (MWFRS)
Wind Webinar #326 February 2013
Page 109 of 126
– Pressure on underside of roof overhang
» p = (24.46 psf)(0.85)(0.8) – (24.46 psf)(0.18)
» p = 12.23 psf (positive internal pressure)
» p = (24.46 psf)(0.85)(0.8) – (24.46 psf)(-0.18)
» p = 21.04 psf (negative internal pressure)
– Enter Figure 27.4-1 for pressures on windward roof
» h/L = 13.28’/60’ = 0.22 < 0.25
» Cp = 0.2 (Condition 1)
» Cp = -0.3 (Condition 2)
» Condition 1
– p = (24.46 psf)(0.85)(0.2) – (24.46 psf)(0.18)
– p = 0.24 psf (positive internal pressure)
– p = (24.46 psf)(0.85)(0.2) – (24.46 psf)(-0.18)
– p = 8.56 psf (negative internal pressure)
Example-Roof Overhang (MWFRS)
Wind Webinar #326 February 2013
Page 110 of 126
» Condition 2
– p = (24.46 psf)(0.85)(-0.3) – (24.46 psf)(0.18)
– p = -10.64 psf (positive internal pressure)
– p = (24.46 psf)(0.85)(-0.3) –(24.46 psf)(-0.18)
– p = -1.83 psf (negative internal pressure)
» Combine Top & Bottom Pressures with Same Internal
Pressure Conditions
– Note: signs indicate pressure toward or away
from surface
– Change signs so (+) is up, globally
– Change signs so (-) is down, globally
Example-Roof Overhang (MWFRS)
Wind Webinar #326 February 2013
Page 111 of 126
– Positive Internal Pressure
– povh = 12.23 psf - 0.24 psf = 11.99 psf
– povh = 12.23 psf + 10.64 psf = 22.87 psf
– overall povh is upward
– Negative Internal Pressure
– povh = 21.04 psf – 8.56 psf = 12.48 psf
– povh = 21.04 psf + 1.83 psf = 22.87 psf
– overall povh is upward
– Note: Net effect of internal pressures is zero so
that the total uplift on the overhang is the same.
Example-Roof Overhang (MWFRS)
Wind Webinar #326 February 2013
Page 112 of 126
– Overall pressure on the Leeward Overhang is
calculated the same way, but uses the negative
pressure on the wall immediately adjacent to
the overhang for downward pressures on soffit.
– By inspection, total force on windward
overhang will control.
Example-Roof Overhang (MWFRS)
Wind Webinar #326 February 2013
Page 113 of 126
Example-Roof Overhang
Wind Webinar #326 February 2013
Page 114 of 126
• Roof Overhang (C & C) (Section 30.10)
• Equipment Building
–Unless otherwise listed, parameters are
identical to those for the MWFRS calculations
–Determine C & C loads for overhangs of roof
trusses, spaced at 2’-0” o.c.
• p = qh[(GCp) – (GCpi)] psf (Eq 30.10-1)
–Kd = 0.85 (Building C&C) (Table 26.6-1)
–All other parameters for qh are same as for
MFWRS
Example-Roof Overhang (C & C)
Wind Webinar #326 February 2013
Page 115 of 126
– qh = 0.00256(0.85)(1.00)(0.85)(115)2 = 24.46 psf
• Enter Figure 30.4-2B for Roof Overhang C & C coefficients
• Determine Effective Wind Area of Studs
– Greater of Tributary Area or Effective Width
» Effective Wind Area Definition (Section 26.2)
– Greater of 2.0’ or (governs)
– Length/3 = 3’/3 = 1.00’
» Effective Wind Area: 3’x2’ = 6.00 ft2
» If Effective Wind Area > 700 ft2, use MFWRS loads
– Determine “a” distance (Figure 30.4-1 and 30.4-2A)
» Lesser of 10% of B = 0.10(30’) = 3’ and 0.4h =
0.4(13.28’) = 5.31’
– 3’ governs
Example-Roof Overhang (C & C)
Wind Webinar #326 February 2013
Page 116 of 126
» Not less than the greater of 4% of B = 0.04(30’) = 1.2’
or 3’ (3’ controls)
» a = 3’ (equal to width of overhang; therefore, Zone 1
pressures are not applicable to any part of the
overhang)
– Zone 2: GCp = -2.2
» p = (24.46 psf)[(-2.2) – (0.18)] = -58.21 psf
– with positive internal building pressure
» p = (24.46 psf)[(-2.2) – (-0.18)] = -49.41 psf
– with negative internal building pressure
– Zone 3: GCp = -3.7
» p = (24.46 psf)[(-3.7) – (0.18)] = -94.90 psf
– with positive internal building pressure
Example-Roof Overhang (C & C)
Wind Webinar #326 February 2013
Page 117 of 126
» p = (24.46 psf)[(-3.7) – (-0.18)] = -86.10 psf
– with negative internal building pressure
» For the overhang portion of the truss:
– Tributary Width = 2’
– Upward force on the entire truss end is:
– Zone 2: (-58.21 psf)(2’) = -116.42 plf
(upward)
– Zone 3: (-94.90 psf)(2’) = -189.90 plf
(upward)
– These are NOT the loads on the soffit material .
Example-Roof Overhang (C & C)
Wind Webinar #326 February 2013
Page 118 of 126
• Section 30.10 states that the external coefficient for the
covering on the underside of the roof overhang (soffit) is the
same as the external pressure coefficient on the adjacent
wall surface, determined from Figure 30.4-1 or Figure 30.6-
1, as applicable.
– Use of the GCp with negative internal pressure yields the
greatest upward load on the material on the underside of
the overhang on the windward wall.
– Use of the GCp with positive internal pressure yields the
greatest downward load on the material on the
underside of the overhang on the leeward wall.
– For this building, assuming effective wind area is the
same as for the truss overhang:
Example-Roof Overhang (C & C)
Wind Webinar #326 February 2013
Page 119 of 126
» Windward Wall Soffit Material (Figure 30.4-1)
– Zone 4 and Zone 5: GCp = +1.0
– p = (24.46 psf)[(1.0) –(-0.18)] = 28.86 psf
– acting upward
» Leeward Wall Soffit Material (Figure 30.4-1)
– Zone 4: GCp = -1.1
– p = (24.46 psf)[(-1.1) –(0.18)] = -31.31 psf
(acting downward)
– Zone 5: GCp = -1.4
– p = (24.46 psf)[(-1.4) –(0.18)] = -38.65 psf
(acting downward)
Example-Roof Overhang (C & C)
Wind Webinar #326 February 2013
Page 120 of 126
• Chemical Silo (Other Structure MWFRS)
(Section 29.5)
• Silo Dimensions: h = 20’, D = 5.0’
• Welded Steel Tank: smooth sides, no ladder
• Roof Slope: 1:12 (conical)
–Maximum rise: 2.5 inches (consider
contribution to wind load, negligible)
• Unless otherwise listed, parameters for
calculation of qz are identical to those for the
MWFRS calculations for equipment building.
• F = qzGCfAf Lbs (Eq 29.5-1)
Example- Chemical SILO (MWFRS)
Wind Webinar #326 February 2013
Page 121 of 126
Example- Chemical SILO (MWFRS)
Wind Webinar #326 February 2013
Page 122 of 126
– qz = 0.00256KzKztKdV2 (psf)
– Kz = 0.90 (Building C&C) (Table 29.3-1)
– Kd = 0.95 (Circular Tanks) (Table 26.6-1)
– G = 0.85 (Rigid Structure) (Section 26.9)
– qz = 0.00256(0.90)(1.00)(0.95)(115)2 = 28.95 psf
– Go to Table 29.5-1
» D/√ qz = 5’/Sqrt(28.95psf) = 0.93 < 2.5
» Go to bottom row
» h/D = 20’/5’ = 4.0
» Must interpolate between h/D=1.0 and h/D= 7.0
» Cf = 0.75
– F = (28.95 psf)(0.85)(0.75) Af = (18.45 psf)Af
– F = (18.45 psf)Af < (16psf)Af
Example- Chemical SILO (MWFRS)
Wind Webinar #326 February 2013
Page 123 of 126
– Af = 5’x20’ = 100 ft2
– F = (18.45 psf)(100 ft2) = 1845 Lbs
» This is conservative, OR calculate F for increase in
pressure as height increases
– q15 = 0.00256(0.85)(1.00)(0.95)1152 = 27.34 psf
– q(15-20)= 0.00253(0.90)(1.00)(0.95) (115)2 = 28.95 psf
» For 0-15’: D/√ qz = 5’/Sqrt(27.34psf) = 0.96 < 2.5
» Cf = 0.75
– F0-15 = (27.34 psf)(0.85)(0.75)Af = 17.43 psf Af > 16 psf Af
– F0-15 = (17.43 psf)(5’)(15’) = 1307 Lbs.
– F15-20 = (18.45 psf)(5’)(5’) = 461 Lbs.
– Total F on Silo: 1307 Lbs + 461 Lbs = 1768 Lbs
Example- Chemical SILO (MWFRS)
Wind Webinar #326 February 2013
Page 124 of 126
– Conservative OTM: (1845 Lbs)(10’) = 18,450 ft-lbs
– More Detailed OTM: (1307 Lbs)(15’/2)+(461
Lbs)(15’+5’/2) = 17,870 ft-lbs
» The taller the structure is, the more important it is to
use the stepped wind force approach.
Example- Chemical SILO (MWFRS)
Wind Webinar #326 February 2013
Page 125 of 126
Questions
Wind Webinar #326 February 2013
Page 126 of 126