A Beginner's Guide to ASCE 7-05

Chapter 2 - Load Combinations

2007, 2009, T. Bartlett Quimby

Overview

The Load Combination Equations

Comparing LRFD & ASD Results

Example Problems

Homework Problems

References


Report Errors or Make Suggestions

 

Section 2.1

Overview

Last Revised: 11/04/2014

Many structures will see most, if not all, of the loads listed in ASCE 7-05 2.2 sometime in their life.  The next challenge becomes how to combine the loads reasonably.  A direct combination of all the loads at their maximum is not considered to be probable. For example, it would not be probable to expect a full live load to occur simultaneously with a full snow load during a design level wind storm. 

ASCE 7-05 Chapter 2 provides load combination equations for both service and factored loads.  Your choice will be based on the design philosophy that you are using.

As is the nature of the industry, ASCE 7 has recently been released as ASCE 7-05 and there are some changes to the load combination equations.  The combinations are discussed in the following sections.

Your structure must be able to withstand ALL the load combinations in order to be considered safe.  Depending on the subset of load types an element may see, this list can be pared down as some combinations will not be controlling, by observation. 

You will see how to select applicable equations as you gain experience with the combination equations.  If you review the example problems and do all the homework problems associated with this chapter, you will quickly learn how to choose the load combination equations that are likely to control in a given situation.

Load Combination Considerations

There are a few things that first time users of the load combination equations tend to do that are not quite right.  To avoid these mistakes, remember the following concepts

  • You cannot use both LRFD and ASD in the same project.  You need to decide at the start which you are going to use or you do a complete analysis/design with both and the decide which to use.
  • Only ONE load combination equation is used per load diagram for your structure or its elements.  You cannot mix values from different combination equations in the same analysis.  The same equation is applied to each load source.
  • The final structure must have sufficient strength to support all load conditions in the philosophy (LRFD or ASD) you have chosen for your project.