A Beginner's Guide to ASCE 7-05

Chapter 2 - Load Combinations

© 2007, 2009, T. Bartlett Quimby

Section 2.4.3

Example Problem 2.3

Last Revised: 11/04/2014

Given:  The ASCE 7-05 load combination permutations found in Table 2.1.

Wanted:  An electronic spreadsheet that will apply the load combination equations to a variety of load sources.

Solution:  By applying the equations frequently, you will soon learn that there are times when you know that a given load combination will not control.  For example, if your load sources only include D and L, then it is fairly obvious that LRFD LC2 and ASD LC2 are the controlling equations and all others can be neglected.  When more load types get included, the picture gets a little less clear, but there are combinations that can reduce the number of equations considered.

The other option is to create a computation tool to handle it all.  Click here to see a spreadsheet example that does just that. In this solution, we have highlighted and put borders around the load cases that are likely to control.  Note that each row in the table represents a single complete load case.  The load sources are in the columns.  As you can see, including all the permutations takes some effort!  The nice thing about programming the solution is that, once done and proven, the tool will save you lots of time and guess work in the future.

It is possible to make the spreadsheet much more compact by writing spreadsheet equations that include the load factors directly and that make judicial use of "max" or "min" statements.  The example here uses matrix multiplication to multiply the coefficient matrices to the right side of the sheet against the matrix of load source data to get the resulting load combination results.  This makes debugging the sheet easier since you only need to change the coefficient values to change all the equations.

This spreadsheet can handle up to seven load sources though only three are used here.  The included load sources are the ones used in example problems 2.1 and 2.2.

It should also be pointed out that the example given does not completely implement all the footnotes to the ASCE 7-05.  If you are to create such a spreadsheet you will need to look closely at all the ASCE 7-05 criteria.