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ANALYSIS OF THE INTERNATIONAL RESIDENTIAL CODE 2006 LOW RISE WOOD FRAME BUILDING BRACING REQUIREMENTS FOR PENNSYLVANIA
ABSTRACT
Buildings must be designed structurally for a fifty year wind speed according to the ASCE 7 Standard. Residential buildings in Pennsylvania currently must be designed for 90 mph winds as shown on the ASCE-7 Standard wind speed map. For this research wind speed data from anemometer stations all over PA and its surrounding states are compared statistically using extreme value distributions. An overall lower wind speed than the Standard’s 90mph was found to be statistically accurate. Currently the International Residential Code 2006 Bracing Requirements Table R602.10.1 starts at wind speeds of 100mph or less. The new wind speed found for Pennsylvania is used to redevelop the bracing requirements table to incorporate areas with lower wind speeds than 100mph. The new bracing requirements are found to be stricter than the current code. Further analysis was implemented using an analytical model and a Monte Carlo simulation on two separate residential structures to predict the probability of failure in the walls. It was found that the bracing requirements of the International Residential Code 2006 are inadequate for the state of Pennsylvania.
TABLE OF CONTENTS
LIST OF FIGURES………………………………………………………………………………………….. vi
LIST OF TABLES…………………………………………………………………………………………….. x
ACKNOWLEDGEMENTS………………………………………………………………………………. xii
Chapter 1 Introduction………………………………………………………………………………………….. 1
1.1 Problem Statement………………………………………………………………………………… 2
1.2 Objectives…………………………………………………………………………………………….. 2
1.3 Scope…………………………………………………………………………………………………… 3
Chapter 2 Background……………………………………………………………………………………….. 4
2.1 The ASCE 7 wind speed map………………………………………………………………….. 4
2.2 Using extreme value statistics to find the fifty year wind speed…………………… 5
2.3 Wall Bracing and the International Residential Code…………………………………. 9
2.4 The ASCE 7 Tributary Area Method……………………………………………………… 11
2.5 Analytical modeling for light-frame buildings…………………………………………. 13
2.5.1 The Rigid-Beam Model…………………………………………………………………… 14
2.5.2 The Rigid-Plate Model……………………………………………………………………. 14
2.5.3 Modeling Shear Walls…………………………………………………………………….. 14
Chapter 3 Wind Speed Procedure………………………………………………………………………. 17
Chapter 4 Wind Speed Results………………………………………………………………………….. 23
Chapter 5 ASCE 7 Method……………………………………………………………………………….. 27
5.1 The ASCE 7 Analytical Procedure…………………………………………………………. 27
Chapter 6 ASCE 7 Method Results……………………………………………………………………. 31
Chapter 7 Modeling Procedure………………………………………………………………………….. 33
7.1 Rigid Beam Model Description……………………………………………………………… 33
7.2 Rigid Plate Model Description………………………………………………………………. 35
7.3 Wind Pressure……………………………………………………………………………………… 37
7.4 Random Variables……………………………………………………………………………….. 39
Chapter 8 Analytical Model Results…………………………………………………………………… 49
8.1 The “IRC House”………………………………………………………………………………… 49
8.2 A typical L-shaped house……………………………………………………………………… 62
Chapter 9 Conclusions……………………………………………………………………………………… 68
9.1 Recommendations based on ASCE 7 Analytical Method………………………….. 68
9.2 Recommendations based on Monte Carlo simulations……………………………… 71
References………………………………………………………………………………………………………. 75
Appendix A Plotted wind distributions for grouped stations in PA………………………… 77
Appendix B Calculations for shear forces in tested houses using ASCE 7 Analytical Method 83
Appendix C Results for IRC house simulations…………………………………………………… 89
Appendix D Matlab Programs…………………………………………………………………………… 95
Appendix E Matlab Flowchart…………………………………………………………………………. 131
Chapter 1 Introduction
Wood frame construction bracing provisions are provided in the 2006 International Residential Code (IRC) [1] . However, there is little documentation of the derivation of these provisions and the technical research behind them [2] . Wall bracing systems such as wood let-in braces, wood structural panels, Portland cement stucco, and other bracing methods are used to provide lateral resistance to a structure. The minimum bracing requirements are listed in the IRC and the table begins with wind speeds at or below 100 mph and seismic categories defined as A and B [1] .
The required wind speed used for the design of buildings is that which has a two percent chance of being exceeded. This wind speed is assumed to be the fifty year recurrence interval. The current wind speed map was created using superstations throughout the United States and is not accurate since many superstations contain multiple occurrences of the same wind station.
In areas outside hurricane-prone regions, regional climatic data may be used in lieu of the basic wind speeds given in the ASCE 7 when extreme-value statisticalanalysis procedures have been employed in reducing the data and the length of the record, sampling error, averaging time, anemometer height, data quality, and terrain exposure of the anemometer have been taken into account [3] .
The state of Pennsylvania is located within a 90 mph wind zone on the ASCE 7 wind speed map [3] yet still must adhere to the bracing requirements listed in the IRC for 2 100 mph or less. In this research, extreme value statistics and records of wind data are used to predict the fifty year wind speed for the state of Pennsylvania. The ASCE 7 analytical method for wind loads is used to calculate the shear forces felt in braced walls of various house geometries. These results are compared with the bracing requirements of the IRC. Simple analytical models are then used to evaluate the probability of failure of light-frame buildings under random wind loads following an extreme value distribution for the wind in Pennsylvania.
1.1 Problem Statement
This study will determine if a reduced fifty year design wind speed is warranted for the state of Pennsylvania. If the reduced wind speed is feasible through statistical analysis, it will be further determined if the reduced wind speed will comply with the IRC bracing requirements for a typical two story home. Modified bracing requirements will be provided if warranted.
1.2 Objectives
The primary objective of this research is to determine whether a lower wind speed will reduce the bracing requirements provided in the IRC. The secondary objective is to determine whether a lower wind speed is feasible for Pennsylvania. The tertiary objective is to design a new bracing table similar to the IRC bracing table with requirements listed for lower wind speeds.
1.3 Scope
The scope of this research includes using extreme value statistics on recorded wind data for the state of Pennsylvania to develop a fifty year wind speed. Analytical models will then be used to evaluate this wind speed on a one, two and three story home. The probability of failure in each of the walls will be calculated using random multipliers to account for the variability of the structure of the wall, the uncertainty of the stiffness and the openings in the individual walls.
