2012 Wood Frame Construction Manual: Wind Speed and Design Pressure Determination According to ASCE 7 10

Transcription

1 2012 Wood Frame Construction Manual: Wind Speed and Design Pressure Determination According to ASCE 7 10 Presented by: William L. Coulbourne, PE Copyright Materials This presentation is protected by US and International Copyright laws. Reproduction, distribution, display and use of the presentation without written permission of the speaker is prohibited. American Wood Council

2 Learning Objectives At the end of this program, participants will: Be able to determine site specific wind speeds using ASCE 7 10 Understand how wind speeds are used for calculating Main Wind Force Resisting System (MWFRS) and Components and Cladding (C&C) loads Understand how to convert from ASCE 7 10 back to ASCE 7 05 wind speeds Understand how to develop loads from wind speeds 3 WFCM Basis for this webinar series is 2012 Wood Frame Construction Manual (WFCM) Basis follows WFCM Prescriptive Provisions (Chapter 3). Prescriptive provisions are provided for: Connections Floor systems Wall systems Roof systems Provisions provide construction details and load tables WFCM also has engineering design in Chapter 2 4 2

3 WFCM and IBC Chapter 16 Wind Loads Section of IBC Indicates wind loads are to be determined in accordance with ASCE 7 Exception is residential structures can be designed using the provisions of the WFCM WFCM can not be used for design of structures located on hills, ridges or escarpments Chapter 23 Wood design Significant coverage of wind design using wood 5 WFCM Prescriptive Parameters Exposure B or C Mean roof height does not exceed 33 ft. 3 stories Length and/or width of building < 80 ft. Joist and rafter span 26 ft. Loadbearing wall height 10 ft. Joist, wall stud, rafter spacing max 24 in. Limitations on shear wall offsets Use of ASD level wind pressures 6 3

4 ASCE 7 10 Wind Speed Maps Speeds are for ultimate event Maps for 3 Risk Categories (I, II, III and IV) Wind Speed metrics are: 3 sec peak gust 33 ft (10 m) above ground Exposure C Importance Factor is now included in the speeds shown on the maps Year RP Winds 8 4

5 Comparison of ASCE 7 10/ 1.6 vs. ASCE Wind Speeds at Selected Locations Location ASCE 7-05 Exposure C V 700 / 1.6 Exposure C Exposure D Bar Harbor, Maine Boston, MA Hyannis, MA New Port, RI Southampton, NY Atlantic City, NJ Wrightsville Beach, NC Folly Beach, SC Miami Beach Clearwater, FL Panama City, FL Biloxi, MS Galveston, TX Port Aransas, TX Hawaii Guam

6 Finding Your Windspeed Users should consult with local building officials to determine if there are community-specific wind speed requirements that govern. 11 Strength Design Load Combinations Wind load factor changed in 2010 Edition: Old: LF = 1.6 New: Load factor from 1.6 to 1.0; load factor is built into the MRI for the maps For ASD design, new load factor is 0.63 (actually it is 0.6), reduced from

7 Converting from old to new (or vice versa) ASCE 7 10 wind speed/ 1.6 = ASCE 7 05 wind speed ASCE 7 10 wind pressures*0.6 = ASD wind pressures Note = an exact equivalent ASD reduction factor = Wind Flow Around Building Pressure at Stagnation Point from Bernoulli s equation, using a standard atmosphere for density = V

8 Flow Separations Greater separation angle = greater void between surface & windstream. Greater void = higher suction (negative pressure). Increasing roof angle decreases void, thus lowering suction. At roof angle = separation angle, pressure becomes positive. 15 Wind Forces 16 8

9 Wind Actions on Buildings Uplift Roof only Entire building Lateral loads (base shear) Connection between building and foundation Racking Pushing building over at the top Overturning Pushing building over when connection to foundation fails 17 Wind Uplift Source: APA 18 9

10 Source: APA 19 Base Shear (Sliding) Source: APA 20 10

11 Source: APA 21 Racking Source: APA 22 11

12 Source: APA 23 Overturning Source: APA 24 12

13 Source: APA 25 Load Path Through Building Wind pressure is collected by walls and roof Pressure is distributed into diaphragms at roof and floor levels Diaphragms take loads into shear walls Shear walls must be stiff enough to not rack and take loads into foundation Shear walls must be tied down to resist overturning 26 13

14 Developing Wind Design Pressures Developing pressures for wind design requires combining: Meteorological aspects of wind Speed Turbulence Interaction of wind with terrain Aerodynamics Interaction of wind with building 27 Basic Wind Equation p = q * G * C p p = Wind Pressure q = Velocity Pressure (Atmospheric Effects). G = Gust Effect Factor (Atmospheric & Aerodynamic Effects). C p = Pressure Coefficient / Shape Factor (Aerodynamic Effects)

15 Velocity Pressure q = ( V 2 ) K z K zt K d ASCE 7 adds two more factors: Topographic Factor K zt Hills and Escarpments Directionality Factor K d 0.85 for all building structures 29 ASCE 7 Basic Wind Equation For buildings with External and Internal Pressure: p = qgc p q i (GC pi ) Eq q i = Velocity pressure calculated for internal pressure, usually at mean roof height h GC pi = Internal Pressure Coefficient (+/ 0.18 for enclosed conditions) ASCE 7 calls this Directional Procedure (All Heights) 30 15

16 MWFRS Procedure used in WFCM p = q h [(GC pf ) (GC pi )] Eq where: q h = velocity pressure at mean roof height h GC pf = external pressure coefficient GC pi = internal pressure coefficient 31 MWFRS Load Case A ASCE 7 Used with MWFRS procedure in ASCE 7 and for WFCM 32 16

17 Process for Applying Loads Site determine wind speed and exposure Design based on most extreme exposure expected Find q (velocity pressure) for variety of windward heights and for h Determine p (wind pressure) for all surfaces for both + and internal pressure Wind pressures act normal to surfaces Design with the most restrictive pressures 33 Mean Roof Height 34 17

18 Exposure Categories B Suburban, use as DEFAULT unless others apply >60% to 80% of all buildings are in this category C Open country, 1500 ft creates this category D Water, including on hurricane coast! Change in ASCE 7-10 It s about Flow Characteristics vs. Surface Roughness 35 Exposure B Suburban 36 18

19 Exposure C 37 ACSE 7-10 Figure External Pressure Coefficients 38 19

20 Wind Effects on Buildings IBHS wind tunnel tests center/ 39 IBHS Wind Tunnel Test Results 40 20

21 Example For h = 33 ft tall building, 40 ft (windward face) x 20 ft in plan, find: Roof to wall connection load Load taken into shear walls on ends of house Wind speed = 140 mph Exposure B condition 5:12 roof slope (20 0 is taken as worst case) GC pi = +/ 0.18 (enclosed condition) 41 Calculated Roof Pressures h Kz V q ASDq GCpwind +Gcpi Gcpi GCp lee +GCpi GCpi

22 Converting Pressure to Loads Wind pressures determined for roofs and walls must be converted to loads Pressure x tributary area = loads Loads may be reduced at points in the structure because weight is providing resistance Correct distribution of the loads is key to accurate design 43 Sum Moments to Determine Uplift Load 33 ft 20 ft Tension Tension (connector load) = 122 lbs 44 22

23 WFCM Roof to Wall Connection 45 Roof Wall Connector Load Using roof pressures from calculation procedure (see Slide 41) For 20 ft. roof span, connector load is determined by summing moments about one wall/roof joint. Result = 214 lb Reduce for dead load of roof system: WFCM uses 9 psf as reduction for dead load (90 lb at each wall) WFCM result = 165 x 0.75 reduction = 124 lb (reduction allowed when 8 ft away from roof edge) 46 23

24 MKB10 General Lateral Load Path 47 Calculated Lateral Pressures Horizontal roof load distributed to shear walls Wall pressures distributed to shear walls (windward + leeward) Total shear wall load distributed along the wall to foundation connection WFCM result = 218 plf x L/W (40/20) = 436 plf 48 24

25 Slide 47 MKB10 It seems like this slide could be used in conjunction with slide 21. Michelle Kam-Biron, 8/1/2013

26 WFCM Sill Plate to Foundation Connection 49 C&C Pressure Equations Low rise buildings with h 60 ft. based on Envelope Procedure p = q h [(GC p ) (GC pi )] Eq Buildings with h 60 ft. based on Directional Procedure p = q(gc p ) q i (GC pi ) Eq

27 Components & Cladding Walls Roofs 51 Questions?

28 THANK YOU! Follow up with: SurveyMonkey, presentation links and info. on Certificates Instructor: William L. Coulbourne, PE Sept. 4 th 2012 WFCM: Wind Speed and Design Pressure Determination According to ASCE 7 10 Sept. 11 th 2012 WFCM: Wind Load Distribution on Buildings Load Paths Sept. 18 th 2012 WFCM: Connections Sept. 25 th 2012 WFCM: Foundation Design to Resist Flood Loads and WFCM Calculated Wind Loads NEW! Nov. 21 st Prescriptive Residential Wood Deck Construction Guide (DCA 6) NEW! Jan. 16 th AWC s Code Conforming Wood Design