Chapter 2 - Load Combinations
© 2007, 2009, T. Bartlett Quimby
Last Revised: 08/02/2008
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