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 #3 26 February 2013
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Wind Loads for Solid Signs, Other Structures, Roof-Top Structures & Equipment, and Other Special Conditions Bob Paullus, P.E., S.E. Paullus Structural Consultants
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Outline 1. 1. Chapter 29– Other Structures (MWFRS Directional Method) a. b. c. d. e. f. g. h. i.
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Conditions Limitations Solid Freestanding Walls or Signs Solid Attached Signs Design Wind Loads on Other Structures Design Wind Loads on Rooftop Structures and Equipment on Buildings Parapets Roof Overhangs Minimum Design Wind Loadings Page 3 of 126
Outline 1. 2. Chapter 30 – Part 6 - Components & Cladding for Building Appurtenances and Rooftop Structures and Equipment (Directional Procedure) a. b. c.
Parapets Roof Overhangs Rooftop Structures and Equipment for Buildings with h ≤ 60 ft (18.3 m)
2. 3. Examples
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Section 29.1.2 - Conditions • 1. The structure is a regular-shaped structure as defined in Section 26.2.2. •
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Section 26.2.2 - BUILDING OR OTHER STRUCTURE, REGULAR-SHAPED: A building or other structure having no unusual geometrical irregularity in spatial form.
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Section 29.1.2 - Conditions • 2. The structure does not have response characteristics making it subject to acrosswind 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.
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Section 29.1.3 – Limitations • 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.
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Section 29.1.3 – Limitations • 3. If your structure does not fall within the listed limitations, it should probably be the subject of Wind Tunnel Study.
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Section 29.1.4 – Shielding • 1. No reductions allowed for apparent shielding by buildings, other structures, or terrain features. a. b. c.
Individual hills Individual trees or small groves of trees Individual levees and similar features.
• 2. Reductions are afforded for Terrain Features in determining Exposure Categories in Chapter 26. Wind Webinar #3 26 February 2013
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Steps
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Steps to Determine Wind Loads • 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.
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Section 29.4.1 – Solid Freestanding Walls and Solid Freestanding Signs • 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
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Section 29.4.1 – Solid Freestanding Walls and Solid Freestanding Signs • 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
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Section 29.4.1 – Solid Freestanding Walls and Solid Freestanding Signs • 5. Basic Equation: (Eq. 29.4-1) F = qhGCfAs (lb) •
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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 Freestanding and Attached Signs in Table 26.6-1 Page 14 of 126
Section 29.4.1 – Solid Freestanding Walls and Solid Freestanding Signs •
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 freestanding wall or freestanding solid sign, in ft2 (m2)
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Figure 29.4-1
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Figure 29.4-1
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Figure 29.4-1
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Section 29.4.1 – Solid Freestanding Walls and Solid Freestanding Signs • 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.
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Section 29.4.1 – Solid Freestanding Walls and Solid Freestanding Signs •
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.
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Section 29.4.1 – Solid Freestanding Walls and Solid Freestanding Signs
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Section 29.4.1 2 - Solid Attached Signs a.1. Requirements to use method in Section 29.4.1 a.
b.
c.
d.
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The plane of the sign is parallel to and in contact with the supporting wall Edges of the sign do not extend past the supporting wall Use Component & Cladding Wall pressures calculated in Chapter 30 Set the Internal Pressure Coefficient (GCpi) equal to 0
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Section 29.4.1 2 - Solid Attached Signs a. 2.
Procedure can also be used for signs attached to but not in contact with the supporting wall. a. b.
c.
Sign must be parallel to the supporting wall. Sign must not be more than three (3) feet from the wall. Edges of the sign must be at least (3) feet in from the free edges of the supporting wall: b.
Top of the supporting wall. Bottom of the supporting wall
c.
Side Edges of the supporting wall
a.
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Section 29.5: Design Wind Loads—Other Structures
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Section 29.5: Design Wind Loads—Other Structures 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
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Section 29.5: Design Wind Loads—Other Structures d. Af = projected area normal to the wind except where Cf is specified for the actual surface area, in ft2 (m2)
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Figure 29.5-1
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Figure 29.5-1
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Figure 29.5-2
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Figure 29.5-2
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Figure 29.5-3
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Figure 29.5-3
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Section 29.5-1 – Rooftop Structures and Equipment For Buildings with h ≤ 60 feet 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. Wind Webinar #3 26 February 2013
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Section 29.5-1 – Rooftop Structures and Equipment For Buildings with h ≤ 60 feet Lateral Wind force on Rooftop Structures
• –
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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
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Section 29.5-1 – Rooftop Structures and Equipment For Buildings with h ≤ 60 feet •
Lateral Wind force on Rooftop Structures »
»
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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)
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Section 29.5-1 – Rooftop Structures and Equipment For Buildings with h ≤ 60 feet Vertical Wind force on Rooftop Structures
• –
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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
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Section 29.5-1 – Rooftop Structures and Equipment For Buildings with h ≤ 60 feet •
Vertical Wind force on Rooftop Structures »
»
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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)
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Section 30.11 – Component & Cladding Loads for Rooftop Structures and Equipment for Buildings with h ≤ 60 feet 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.
•
–
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Forces (psf) shall be considered to act inward and outward
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Section 30.11 – Component & Cladding Loads for Rooftop Structures and Equipment for Buildings with h ≤ 60 feet 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.
•
–
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The pressures are ONLY required to be considered to act in the UPWARD direction.
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Section 30.11 – Component & Cladding Loads for Rooftop Structures and Equipment for Buildings with h ≤ 60 feet 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).
•
– – Wind Webinar #3 26 February 2013
Vertical Wind Load would act in addition to Dead and Roof Live Loads or Snow Loads. Use appropriate load combinations. Page 40 of 126
Other Resources
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Other Resources • Prepared by: Task Committee on WindInduced 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
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Section 29.6 – Parapets (MWFRS) •
•
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“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
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Section 29.6 – Parapets (MWFRS) Chapter 28 – The Envelope Method, is exactly the same.
• –
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Chapter 28 uses the velocity pressure determined with the Envelope Method, rather than the velocity pressure in Chapter 27 using the Directional Method.
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Section 27.4.5 - Parapets MWFRS pressures due to parapets
•
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–
Rigid or Flexible Buildings
–
Applies to Flat, Gable, or Hip Roofs
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Section 27.4.5 – Parapets (MWFRS) Basic Equation: (Eq 27.4-4) pp = qp(GCpn) (lb/ft2)
• –
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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
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Section 27.4.5 – Parapets (MWFRS) –
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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
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Section 27.4.5 – Parapets (MWFRS) • FIGURE C29.7-1 Design Wind Pressures on Parapets
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Section 30.9 – C & C Loading on Parapets • •
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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)
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Section 30.9 – C & C Loading on Parapets Basic Equation: (Eq. 30.9-1) p = qp((GCp) – (GCpi))
• – –
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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
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Section 30.9 – C & C Loading on Parapets –
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(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
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Section 30.9 – C & C Loading on Parapets –
Consider Two (2) Load Cases when evaluating C & C pressures on parapets
• – –
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(GCpi) = internal pressure coefficient from Table 26.11-1, based on the porosity of the parapet envelope.
Case A – Pressures on the surfaces of the Windward parapet Case B – Pressures on the surfaces of the Leeward parapet
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Section 30.9 – C & C Loading on Parapets Specifics of Case A:
• –
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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.
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Section 30.9 – C & C Loading on Parapets Specifics of Case B:
• –
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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)
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Section 30.9 – C & C Loading on Parapets • FIGURE C29.7-1 Design Wind Pressures on Parapets
•
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If internal pressure is present, both load cases should be evaluated under positive and negative internal pressure.
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Table 30.9-1 – Steps to Determine C&C Wind Loads on Parapets
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•
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 26.51A, B or C
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Table 30.9-1 – Steps to Determine C&C Wind Loads on Parapets •
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 »
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Open, Partially Enclosed, or Enclosed (parapet or bldg)
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Table 30.9-1 – Steps to Determine C&C Wind Loads on Parapets
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•
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
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Table 30.9-1 – Steps to Determine C&C Wind Loads on Parapets •
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
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Table 30.9-1 – Steps to Determine C&C Wind Loads on Parapets •
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
•
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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.
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Section 30.9 – C & C Loading on Parapets
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Figure 30.6-1
•
Note 7 defines parapets > 3 feet as tall parapets • •
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Reduced corner pressures on parapet Similar note on other figures
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Section 29.7 – Roof Overhangs (MWFRS) “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 lowrise 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
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Section 27.4 – Roof Overhangs (MWFRS) •
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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.
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Section 27.4 – Roof Overhangs (MWFRS)
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Section 27.4 – Roof Overhangs (MWFRS)
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•
From Figure 27.6-3
•
Must consider cases with positive internal pressure and negative internal pressure
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Section 30.10 – C & C Loading on Roof Overhangs • •
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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)
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Section 30.10 – C & C Loading on Roof Overhangs Basic Equation: (Eq. 30.10-1) p = qh((GCp) – (GCpi))
• –
–
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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.
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Section 30.10 – C & C Loading on Roof Overhangs »
–
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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
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Figure 30.4-2A
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Figure 30.4-1
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Table 30.10-1 – Steps to Determine C&C Wind Loads on Roof Overhangs
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•
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 26.51A, B or C
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Table 30.10-1 – Steps to Determine C&C Wind Loads on Roof Overhangs •
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 »
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Open, Partially Enclosed, or Enclosed (overhang or bldg)
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Table 30.10-1 – Steps to Determine C&C Wind Loads on Roof Overhangs
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•
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
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Table 30.10-1 – Steps to Determine C&C Wind Loads on Roof Overhangs
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•
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
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Figure 30.10-1
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Section 29.8 – Minimum Design Wind Loading (MWFRS) •
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.
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Material in Chapters 29 Not Covered 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 •
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Other Ressources
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Example • 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 Wind Webinar #3 26 February 2013
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Example • 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 –
•
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Section 26.8
All Loads are Calculated to LRFD Levels Page 81 of 126
Example
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Example-Freestanding Sign • Solid Billboard Sign at Ground Level • •
• • • • • •
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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
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Example-Freestanding Sign •
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 –
• • •
•
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Example-Freestanding Sign 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 »
•
•
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Example-Freestanding Sign 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 –
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Example-Freestanding Sign
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Example-Parapet (MWFRS) • 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 •
Wind Webinar #3 26 February 2013
Section 29.6 references Section 27.4.5 for directional procedure for MWFRS Parapet load determination.
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Example-Parapet (MWFRS) 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 –
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Example-Parapet (C & C) • Parapet (C & C Loads) (Section 30.9) •
Office Building – same as previous
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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)
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Example-Parapet (C & C)
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Example-Parapet (C & C) • •
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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.
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Example-Parapet (C & C) 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
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Example-Parapet (C & C) 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 »
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Example-Parapet (C & C) » » »
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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.
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Example-Parapet (C & C) •
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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
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Example-Parapet (C & C) 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 »
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Example-Parapet (C & C) •
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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
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Example-Parapet (C & C) –
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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
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Example-Parapet (C & C) 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) »
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Example-Parapet (C & C) »
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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)
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Example-Rooftop Equip. (MWFRS) • Rooftop Equipment for Buildings (MWFRS) (Section 29.5.1) • • • • • • • • •
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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
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Example-Rooftop Equip. (MWFRS) • • • •
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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 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 –
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Example- Chemical SILO (MWFRS) 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. –
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Questions
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