1 Advanced ASCE 7 Advanced ASCE 7 – With Significant Changes to the With Significant Changes to the 2010 Edition 2010 Edition presented by T. Eric Stafford T. Eric Stafford & Associates, LLC 2 2007 Florida Codes 2007 Florida Codes – Base Codes Base Codes 2007 Florida Building Code, Building – Base code: 2006 International Building Code 2007 Florida Building Code, Residential – Base code: 2006 International Residential Code 2007 Florida Building Code, Existing Building – Base code: 2006 International Existing Building Code
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Advanced ASCE 7 – With Significant Changes to the 2010 Edition
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Advanced ASCE 7 Advanced ASCE 7 ––With Significant Changes to the With Significant Changes to the 2010 Edition2010 Edition
presented byT. Eric Stafford
T. Eric Stafford & Associates, LLC
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2007 Florida Codes 2007 Florida Codes –– Base CodesBase Codes
2007 Florida Building Code, Building– Base code: 2006 International Building Code
2007 Florida Building Code, Residential– Base code: 2006 International Residential Code
2007 Florida Building Code, Existing Building– Base code: 2006 International Existing Building
Code
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2007 Florida Codes2007 Florida Codes
Florida-specific amendments from 2001, and 2004 Florida Codes carried forward into the 2007 editionIncludes changes from the 2003 I-Codes to the 2006 I-CodesEffective Date: March 1, 2009
Buildings, Partially Enclosed:If the following two conditions are satisfied:1. Ao > 1.1Aoi 2. Ao > 4 sq. ft or >0.01Ag, whichever is smaller, & Aoi< 0.2Agi
where:Aoi = The sum of the areas of openings in the building envelope (walls & roof) not including Ao, in sq. ft
Agi = The sum of the gross surface areas of the building envelope (walls & roof) not including Ag, in sq. ft
MWFRS Limitations– 1. The building is a simple diaphragm building – 2. Low-rise building– 3. Enclosed building and conforms to wind-borne debris
provisions– 4. The building is regular shaped– 5. The building is not classified as a flexible building– 6. The building does not have response characteristics
making it subject to across wind loading, vortex shedding, instability due to galloping or flutter; and does not have a site location for which channeling effects or buffeting in the wake of upwind obstructions warrant special consideration
– 7. The building has an approximately symmetrical cross section in each direction with roof slopes not exceeding 45 degrees (0.78 rad.).
Components and Cladding Limitations– 1. Mean roof height less than or equal to 60 ft – 2. Enclosed building and conforms to wind-borne debris
provisions– 3. The building is regular shaped– 4. The building does not have response characteristics
making it subject to across wind loading, vortex shedding, instability due to galloping or flutter; and does not have a site location for which channeling effects or buffeting in the wake of upwind obstructions warrant special consideration
– 5. The building has either a flat roof, a gable roof with roof slopes not exceeding 45 degrees, or a hip roof with roof slopes not exceeding 27 degrees.
8. The building is exempted from the torsional load cases as indicated in Note 5 of Figure 6-10, or the torsional load cases defined in Note 5 do not control the design of any of the MWFRS’s of the building.
(Commentary gives guidance on determining if the torsional load cases would control the design – flexible
Design Pressure – MWFRS – Rigid Buildings of All Height
qi = qh for windward walls, side walls, leeward walls, and roofs of enclosed buildings and for negative pressure evaluation in partially enclosed buildings
qi = qz for positive pressure evaluation in partially enclosed buildings at height z from the ground. Can be conservatively taken as qh
ASCE 7ASCE 7--05056.5.8 Gust Effect Factor, G or G6.5.8 Gust Effect Factor, G or Gff
For rigid structures as defined in Section 6.2, G shall be taken as 0.85 or calculated by Eqs. 6-4, 6-5, 6-6 and 6-7, using Table 6-2.For flexible or dynamically sensitive structures as defined in Section 6.2, Gfshall be calculated by Eqs. 6-8, 6-9, 6-10, 6-11, 6-12, 6-13a, 6-13b and 6-14, using Table 6-2.
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ASCE 7ASCE 7--0505Gust Effect Factor GGust Effect Factor G
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ASCE 7ASCE 7--0505Flexible vs. RigidFlexible vs. Rigid
How to determine if flexible or rigid for wind design?– When to investigate natural frequency:
• H > 4 times least horizontal dimension, or• Reason to believe natural frequency is less
ASCE 7ASCE 7--0505Basic Wind Speeds Using Climatic DataBasic Wind Speeds Using Climatic Data
New language clarifying that basic wind speeds obtained from regional climatic data may be less than Figure 6-1
6.5.4.2 Estimation of Basic Wind Speeds from Regional Climatic Data In areas outside hurricane prone regions, regional climatic data shall only be used in lieu of the basic wind speeds given in Fig. 6-1 when: (1) approved extreme-value statistical-analysis procedures have been employed in reducing the data; and (2) the length of record, sampling error, averaging time, anemometer height, data quality, and terrain exposure of the anemometer have been taken into account. Reduction in basic wind speed below that of Figure 6-1 shall be permitted.
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ASCE 7ASCE 7--1010
Available May 2010Most comprehensive update to wind load provisions since ASCE 7-98Approved at Code Development Hearings for inclusion in 2012 IBC and 2012 IRC
ASCE 7ASCE 7--1010
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ASCE 7ASCE 7--1010Wind ProvisionsWind Provisions
Topics Discussed– Reorganization– New wind speed maps– Exposure Categories
– Wind-borne Debris– Roof Overhangs– Minimum Design Wind Loads– Simplified methods for MWFRS and C&C
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ASCE 7ASCE 7--1010ReorganizationReorganization
ASCE 7-05: Chapter 6 contains all wind provisionsASCE 7-10:– 6 new Chapters (Chapters 26-31)– Step by step design procedure provided
at the beginning of each method– Intent is to clarify the applicability of the
Chapter 27 – MWFRS Directional Procedure– Buildings of all heights method– New simplified method for buildings
with h ≤ 160 ftChapter 28 – MWFRS Envelope Procedure– Low-rise buildings method– Simplified method for simple diaphragm
buildings83
ASCE 7ASCE 7--1010ReorganizationReorganization
Chapter 29 – MWFRS Other Structures and Appurtenances– Signs– Rooftop structures– Other structures
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ASCE 7ASCE 7--1010ReorganizationReorganization
Chapter 30 – Components and Cladding– Analytical method for h ≤ 60 ft– Simplified method for h ≤ 60 ft– Analytical method for h > 60 ft– Simplified method for h ≤ 160 ft– Analytical method for free roofs– Building appurtenances
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ASCE 7ASCE 7--1010ReorganizationReorganization
Chapter 31 – Wind Tunnel Procedure
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ASCE 7ASCE 7--1010ReorganizationReorganization
Each chapter further subdivided into Parts for exampleChapter 27 Part 1
Chapter 27 Part 2
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ASCE 7ASCE 7--1010ReorganizationReorganization
How to find provisions in ASCE 7-10 using ASCE 7-05 as the basis.Significant Changes to Wind Load Provisions contains a crosswalk table.
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Excerpt
ASCE 7ASCE 7--1010ReorganizationReorganization
Each method/part also is provided with a tabular step-by-step design procedure
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ASCE 7ASCE 7--1010Chapter 26: General RequirementsChapter 26: General Requirements
Basic wind speed, V, see Figure 26.5-1A, B or CWind directionality factor, Kd , see Section 26.6Exposure category, see Section 26.7Topographic factor, Kzt, see Section 26.8Gust Effect Factor, see Section 26.9Enclosure classification, see Section 26.10Internal pressure coefficient, (GCpi), see Section 26-11
Basic wind speeds haven’t changed at all since ASCE 7-983-sec gust wind speeds introduced in ASCE 7-95 to replace fastest-mile wind speedsMinor adjustments between ASCE 7-95 and ASCE 7-98
– 700 yr MRI– Risk category III and IV – 1700 yr MRI– Risk Category I – 300 yr MRI
Strength design-based or “Ultimate”wind speeds
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In most of the non-hurricane US mainland, the mapped values are exactly a 50 year mean recurrence interval
In hurricane regions, the mapped values vary from 50 to 100 years along the hurricane coastline
Wind speeds along the hurricane coastline have been adjusted upward so that when incorporated with the wind LF, produce a wind load having a consistent hazard level with the interior US (700 MRI)
Strength Design Load Factor = 1.0Allowable Stress Design – multiply W x 0.6Use of different maps for different Risk Categories negates the need for Importance Factors– “I” deleted from wind chapters
ASCE 7ASCE 7--1010Surface Roughness C and DSurface Roughness C and D
Older research and modeling suggested roughness increased with increase in wind speedHence ASCE 7-98, -02, and -05 classified water surfaces in hurricane-prone regions as Exposure CNew research suggests otherwiseRoughness of ocean does not continue to increase with increasing wind speed.
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ASCE 7ASCE 7--1010Surface Roughness C and DSurface Roughness C and D
Sea surface drag coefficient reaches a maximum at wind speeds of nearly 70 mphSome evidence that sea surface drag coefficient actually decreases at higher wind speeds reflecting a “frothing” or smoothingUse of Surface Roughness D now required for all water surfaces in Hurricane Prone Regions
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ASCE 7ASCE 7--1010Surface Roughness C and DSurface Roughness C and D
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ASCE 7ASCE 7--1010Surface Roughness C and DSurface Roughness C and D
ASCE 7-05 requires minimum wind load to not be less than 10 psf multiplied by the vertical projection of buildingParticular impact on low-rise buildingsOften over-rides lateral design provisions from the analytical proceduresCan require same lateral resistance in low wind areas as in the highest hurricane-prone regions for many buildings
Roof loads are tabulated for all wind speeds at various mean roof heights for Exposure Category CNote 1 provides conversion factors for Exposure Categories B or D