ASCE 7-16 SNOW LOADS-2017 Snow Loads guide to the Snow Load Provisions of ASCE 7-16.pdf

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1、Guide to the Snow Load Provisions of ASCE 7-16 Snow Loads Michael ORourke, Ph.D., P.E.Snow LoadsOther Titles of Interest ASCE 7 Hazard Tool Deliversaquick,ef cientwaytolookupkeydesignparametersspeci edbyStandardASCE/SEI7- 10andStandardASCE/SEI7-16throughaWeb-basedapplicationthatretrievesloaddatafore

2、ach ofsevenhazards,visualizesthemonamap,andgeneratesauni edreportofresults.(ASCE2016) (http:/ascetools.online) Minimum Design Loads and Associated Criteria for Buildings and Other Structures, ASCE/SEI 7-16 Providesrequirementsforgeneralstructuraldesignandincludesmeansfordeterminingdead,live, soil, o

3、od, wind, snow, rain, atmospheric ice, and earthquake loads and their combinations that are suitable for inclusion in building codes and other documents. A detailed commentary of explanatoryandsupplementaryinformationisincluded.(ASCEStandard2016)(ISBN978-0-7844- 1424-8) Snow Loads: Guide to the Snow

4、 Load Provisions of ASCE 7-10 BY MICHAEL OROURKE, PH.D., P.E. Sets forth a detailed and authoritative interpretation of the snow load provisions of Minimum Design Loads for Buildings and Other Structures, Standard ASCE/SEI 7-10. (ASCE Press 2010) (ISBN 978-0-7844-1111-7) Snow Loads on Solar-Paneled

5、Roofs MICHAEL OROURKE, PH.D., P.E., AND NICHOLAS ISYUMOV, PH.D., P.E. Offers guidance for structural engineers regarding the snow load conditions that result from the presence of solar panels on a roof, focusing on the structural design of roof beams, roof girders, and columns that support solar pan

6、els. (ASCE 2016) (ISBN 978-0-7844-8024-3) Snow-RelatedRoofCollapseduringtheWinterof20102011:ImplicationsforBuilding Codes MICHAEL OROURKE, PH.D., P.E., AND JENNIFER WIKOFF Summarizesaninvestigationofnearly500damagedstructurestodeterminewhetherthecollapses and poor roof performance were the result of

7、 snow loads exceeding what was prescribed in building codes or the result of structural capacity that was signi cantly less than required by the building codes. (ASCE 2014) (ISBN 978-0-7844-7824-0)Snow Loads Guide to the Snow Load Provisions of ASCE 7-16 Michael ORourke, Ph.D., P.E.Library of Congre

8、ss Cataloging-in-Publication Data Names: ORourke, Michael J., author. | American Society of Civil Engineers. Title:Snowloads:guidetothesnowloadprovisionsofASCE7-16/MichaelORourke,Ph.D.,P.E. Description: Reston, Virginia : American Society of Civil Engineers, 2017 | Includes bibliographical reference

10、 record available at https:/lccn.loc.gov/2017005716 Published by American Society of Civil Engineers 1801 Alexander Bell Drive Reston, Virginia, 20191-4382 www.asce.org/bookstore | ascelibrary.org Any statements expressed in these materials are those of the individual authors and do not necessarily

11、represent the views of ASCE, which takes no responsibility for any statement made herein.Noreferencemadeinthispublicationtoanyspeci cmethod,product,process,orservice constitutes or implies an endorsement, recommendation, or warranty thereof by ASCE. The materials are for general information only and

12、 do not represent a standard of ASCE, nor are they intended as a reference in purchase speci cations, contracts, regulations, statutes, or any other legal document. ASCE makes no representation or warranty of any kind, whether express or implied, concerning the accuracy, completeness, suitability, o

13、r utility of any information, apparatus, product, or process discussed in this publication, and assumes no liability therefor. Theinformationcontainedinthesematerialsshouldnotbeusedwithout rstsecuringcompetent advice with respect to its suitability for any general or speci c application. Anyone util

14、izing such informationassumesallliabilityarisingfromsuchuse,includingbutnotlimitedtoinfringementof any patent or patents. ASCE andAmerican Society ofCivil EngineersRegistered inU.S.Patentand TrademarkOf ce. Photocopies and permissions. Permission to photocopy or reproduce material from ASCE publicat

15、ions can be requested by sending an e-mail to permissionsasce.org or by locating a title in the ASCE Library (http:/ascelibrary.org) and using the “Permissions” link. Errata: Errata, if any, can be found at https:/doi.org/10.1061/9780784414569. Copyright 2017 by the American Society of Civil Enginee

16、rs. All Rights Reserved. ISBN 978-0-7844-1456-9 (print) ISBN 978-0-7844-8021-2 (PDF) ISBN 978-0-7844-8085-4 (ePUB) Manufactured in the United States of America. 2 42 32 22 12 01 91 81 7 12345Contents Preface ix Unit Conversions xi Chapter 1 Introduction 1 Chapter 2 Ground Snow Loads 5 2.1 The Map: I

17、nuence of Latitude, Elevation, and Coastlines 8 2.2 The Map: Site-Specic Case Studies 9 2.3 State Tables 9 Example 2-1. Ground Snow Loads 11 Chapter 3 Flat Roof Snow Loads 13 3.1 Measured Conversion Factors 13 3.2 Flat Roof Snow Load 17 3.3 Exposure Factor 18 3.4 Thermal Factor 19 3.5 Importance Fac

18、tor 20 3.6 Minimum Snow Loads for Low-Sloped Roofs 21 3.7 Snow Load for Near Ground Surfaces 22 Example 3-1. Roof Exposure 22 Chapter 4 Sloped Roof Snow Loads 25 4.1 Snow Sliding 25 4.2 Air-Supported Structures 31 4.3 Overall Comparison with Observations 32 Example4-1.UniformRoofSnowLoad,MonoslopeRo

19、of(1on12) 33 Example4-2.UniformRoofSnowLoad,MonoslopeRoof(4on12) 34 Example 4-3. Uniform Roof Snow Load, Wide Gable Roof 35 vChapter 5 Partial Loads 37 5.1 Continuous-Beam Systems 37 5.2 Other Structural Systems 40 Example 5-1. Uniform and Partial Snow Loads, Monoslope Roof with Overhang 41 Example

20、5-2. Partial Snow Load, Continuous Purlins in Gable Roof (1 on 12) 44 Example 5-3. Partial Snow Load, Continuous Purlins in Gable Roof (3 on 12) 45 Example5-4.PartialSnowLoad,CantileveredRoofGirderSystem 46 Chapter 6 Unbalanced Loads 49 6.1 Hip and Gable Roofs 49 6.2 Curved Roofs 53 6.3 Sawtooth-Typ

21、e Roofs 54 6.4 Domes 55 Example 6-1. Unbalanced Snow Load, Narrow Gable Roof 55 Example 6-2. Unbalanced Snow Load, Wide Gable Roof 55 Example 6-3. Unbalanced Snow Load, Asymmetric Gable Roof 56 Example6-4.BalancedandUnbalancedSnowLoads,CurvedRoof 58 Example 6-5. Unbalanced Snow Load, Sawtooth Roof 6

22、0 Chapter 7 Drifts on Lower Roofs 63 7.1 Leeward Drift 65 7.2 Windward Drift 70 7.3 Adjacent Roofs 72 7.4 Roof Steps in Series 72 Example 7-1. Roof Step Drift Load 73 Example 7-2. Roof Step Drift, Limited Height 75 Example 7-3. Roof Step Drift, Low Ground Snow Load 76 Example 7-4. Roof Step Drift, A

23、djacent Structure 78 Example 7-5. Roof Steps in Series 79 Chapter 8 Roof Projections 81 Example 8-1. Parapet Wall Drift 84 Example 8-2. Rooftop Unit Drift 87 Example 8-3. Parapet Wall Drift, Low Ground Snow Load 88 Example 8-4. Elimination of RTU Drift 90 Chapter 9 Sliding Snow Loads 91 9.1 Adjacent

24、 Roofs 91 vi Contents9.2 Separated Roofs 93 Example 9-1. Sliding Snow Load, Residential Gable Roof (4 on 12) 94 Example 9-2. Sliding Snow Load, Commercial Gable Roof (1 on 12) 95 Example 9-3. Sliding Load, Separated Roof 97 Chapter 10 Rain-on-Snow Surcharge Loads 99 Example 10-1. Uniform Design Snow

25、 Load, Monoslope Roof (1/4 on 12) 103 Example 10-2. Uniform Design Snow Load, Gable Roof (1/4 on 12) 103 Chapter 11 Ponding Instability and Existing Roofs 105 11.1 Ponding Instability 105 11.2 Existing Roofs 107 Example 11-1. Susceptible Bays Purlin-Parallel Geometry 108 Example 11-2. Susceptible Ba

26、ys Purlin-Perpendicular Geometry 108 Chapter 12 Open-Frame Equipment Structures 109 12.1 Floor-Level Snow Loads 109 12.2 Snow Load on Piping and Cable Trays 110 Example 12-1. Isolated Cable Tray 110 Example 12-2. Adjacent Cable Trays 111 Chapter 13 Design Examples 113 Design Example 1 113 Design Exa

27、mple 2 119 Design Example 3 125 Chapter 14 Frequently Asked Questions 131 References 157 Index 159 About the Author 163 Contents viiThis page intentionally left blankPreface Thisguideprovidespracticingstructuralengineerswithadetaileddescriptionof the snow load provisions of Standard ASCE/SEI 7-16, M

28、inimum Design Loads and Associated Criteria for Buildings and Other Structures, published by theAmericanSocietyofCivilEngineers.Theintentofthisguideistopresentthe research and philosophy that underpins the provisions and to illustrate the applicationoftheprovisionsthroughnumerousexamples.Readersandu

29、sersof this guide will know how to use the provisions, as well as the reasoning behind them. In this fashion, users may be able to address nonroutine snow loading issues that are not explicitly covered in ASCE 7-16. This guide introduces provisions that are new to ASCE 7-16, including ground snow lo

30、ad tables for selected western states and New Hampshire as well as snow load requirements for Open Frame Equipment Support structures. Every effort has been made to make the illustrative example problems in this guide correct and accurate. The author welcomes comments regarding inaccu- racies, error

31、s, or different interpretations. The views expressed and the inter- pretationof thesnowloadprovisionsmadein thisguideare thoseof theauthor and not of the ASCE 7 Standards Committee or the ASCE organization. Acknowledgments The author would like to acknowledge the past and present members of the Snow

32、andRainLoadsCommitteeofASCE7.Withouttheircomments,questions, and discussions, the development of Chapter 7 in ASCE/SEI Standard 7-16, and subsequently this guide, would not have been possible. Aswithanydocumentofthistype,manyindividualshavecontributedtheirhard work and effort. The author acknowledge

33、s the work and effort extended by the administrativestaffoftheDepartmentofCivilandEnvironmentalEngineeringat Rensselaer Polytechnic Institute, who assisted in the word processing and preparation of the narrative. The author also would like to acknowledge the sketch work prepared by Christopher Keado

34、, American Institute of Architects, who graciously contributed the hand-drawn illustrations associated with each chapter. ixThis page intentionally left blankUnit Conversions S.I. Unit Abbreviations Customary Unit Abbreviations m = meter (S.I. base unit of length) yd = yard cm = centimeter in. = inc

35、h km = kilometer mi = mile ha = hectare acre L = liter (S.I. base unit of volume) gal = gallon mL = milliliters qt = quart kg = kilogram (S.I. base unit of mass) lb = pound g = gram oz = ounce N = newton (mkgs 2 ) lbf = pound-force (lb/ft) Pa = pascals (N/m 2 )p s i = pounds per square inch kPa = ki

36、lopascals atm = atmosphere J = joule ftlbf = feet per pound-force W = watt Btu = British thermal unit kW = kilowatt hp = horsepower s = second (S.I. base unit of time) s = second min = minute min = minute h = hour h = hour day day C = degrees Celsius F = degrees Fahrenheit ppm = parts per million pp

37、m = parts per million xiMeasurements Length 1 m = 3.2808 ft = 1.0936 yd 1 ft =3yd= 0.3048 m 1cm= 0.3937 in. 1 in. = 2.54 cm 1km= 0.6214 mile 1 mile = 0.869 nautical mile = 1.6093 km Area 1 m 2 = 10.7643 ft 2 1ft 2 = 0.0929 m 2 1km 2 = 0.3861 mi 2 1mi 2 = 2.59 km 2 1ha= 2.4710 acre 1 acre = 43,560 ft

38、 2 = 0.4047 ha Volume 1 L = 0.2642 gal 1 gal =4qt= 3.7854 L 1mL=1cm 3 1ft 3 = 7.481 gal = 28.32 L Mass 1 g = 0.0353 oz 1 oz = 28.3495 g 1kg= 2.2046 lb 1 lb = 0.4536 kg Force 1 N = 0.2248 lb/ft 1 lbf = 4.4482 N Density 1 kg/m 2 = 0.2048 lb/ft 2 1 lb/ft 2 = 4.882 kg/m 2 1 kg/m 3 = 6.2427 lb/ft 3 1 lb/

39、ft 3 = 16.018 kg/m 3 Pressure 1 kPa = 0.145 psi 1 psi = 6.8948 kPa 1 atm = 14.7 psi = 101.35 kPa Energy and Power 1 J = 1.00 Ws = 0.7376 ftlbf 1 ftlbf = 1.3558 J 1kJ= 0.2778 Wh = 0.948 Btu 1 Btu = 1.0551 kJ 1W= 0.7376 ftlbf/s = 3.4122 Btu/h 1 ftlbf/s = 1.3558 W 1kW= 1,3410 hp 1 hp = 550 ftlb/s = 0.7

40、457 kW Flow 1 L/s = 15.85 gal/min = 2.119 ft 3 /min 1 gal/min = 0.1337 ft 3 /min = 0.0631 L/s Concentration mg/L = ppm (in dilute solutions) Temperature C = (F 32)5/9 F = (C9/5)+32 Fundamental Constants and Relationships Acceleration of gravity 32.2 ft/s 2 = 9.81 m/s 2 Density of water (at 4C) 1,000

41、 kg/m 3 = 1 g/cm 3 Specic weight of water (15C) 62.4 lb/ft 3 = 9,810 N/m 3 Weight of water 1 gal = 8.345 lb = 3.7854 kg xii Unit Conversions1 Introduction The basic objective of structural engineering is to design facilities such that the structural capacity exceeds the expected loads by a desirable

42、 amount. This guideisdirectedatthesecondhalfofthatinequality,speci callyexpectedsnow loads as provided in ASCE/SEI Standard 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures. Roof snow loads are to be considered in the entire United States, with the exception of F

43、lorida. Snow is the controlling roof load, over wind or roof live load, in roughly half the states. Speci cally, when the ground snow load is 20 lb/ft 2 or greater, snow loading typically controls for at least some roof structural components. Snowloadingisafrequentandcostlycauseofstructural-performa

44、nceproblems, including collapse. For example, a ranking from the Insurance Information Institute has the March 1993 East Coast storm as one of the worst natural catastrophes in the United States. In terms of insurance claims paid, the snow eventcostanestimated$1.75billion.This20-statestorm,calledthe

45、 “Blizzardof theCentury,”wasmorecostlythanthe1991OaklandHills,CA, re($1.7billion) and Hurricanes Fran (1996) and Iniki (1992) ($1.6 billion each), as well as the 1989 Loma Prieta earthquake ($960 million). The following winter (19931994) also resulted in substantial losses at a total cost of approxi

46、mately $100 million. More recently, the 20102011 winter in New England resulted in nearly 500 problem roofs, of which 375 were either full or partial collapses (ORourke and Wikoff 2014). Hence, snow loading and snow load provisions are subjects with which structural engineers involved in building de

47、sign need to be familiar. Lightweight roof framing systems are particularly sensitive to snow overload. The ASCE Standard 7-16 Commentary notes the increased importance of snow overloadasthelive-to-deadloadratioincreases.Considerthecaseofa25 lb/ft 2 design snow load and a 15 lb/ft 2 snow overload. I

48、f the dead load is 50 lb/ft 2 1(live-to-deadloadratioof25=50= 0.5),the15 lb/ft 2 snowoverloadcorresponds toa20%overloadintermsofthetotalload(90=75= 1.20).If,ontheotherhand, the dead load is 5 lb/ft 2 (live-to-dead load ratio of 25=5= 5.0), the 15 lb/ft 2 snow overload now corresponds to a 50% overlo

49、ad in terms of total load (45=30= 1.50). Such differences become apparent when loss information is reviewed. For example, a series of mixed precipitation events (snow, ice, and rain) resulted in structural damage tomore than 1,600 facilities in the Paci cNorthwest during the 19961997 holiday season. Detailed information for a subset of 88 structures is availableinareportbytheStructuralEngineersAssociationofWashington(SEAW 1998). For that subset, the roof systems most frequently damaged were at, wood panelized roofs (41%); wood

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