Determinate versus indeterminant structures

I know the title sounds scary, but the part you need to know is really simple: almost all single-family houses are determinant structures. They are composed of studs, columns, simply supported joists, common rafters, valley rafters, hip rafters, headers, etc. and they are all determinate structures.

So what is a determinate structure?

A determinate structure can be analyzed just on the basis of simple equilibrium equations. Analyzing an indeterminate structure requires solving equilibrium equations and special methods with strange names you do not really need to know such as moment distribution.

So what does any of this have to do with single-family houses? Just this: if there is foundation distortion, the load-bearing structure will not see an increase or decrease in stress. There may be an increase in stress in the non-load bearing elements such as shown by distress in wall coverings like brick veneer and drywall, but brick veneer and drywall are not load-bearing.

This characteristic of determinate structures is very important when the foundation is expected to settle or distort as will certainly happen if the supporting soil is expensive.

The bottom line:

The bottom line is really simple: Since the house structure is composed of determinate structural elements, foundation movement rarely has a significant adverse effect on the load-bearing structural (load-carrying) elements: rafters, joists, studs, headers, etc.

It should come as no surprise that virtually all foundation distress is cosmetic or minor functional issues such as door issues. 

 

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