Concept explainers
E. Practical Applications
Carpentry When selecting the dimensions of floor joists and beams, it is important to know the moment of inertia IL of the load to be supported. We use the following formula to calculate IL in units of in.4:
where w = the weight in pounds per inch that each joist must support
L = the length in inches of the unsupported span
E = the modulus of elasticity in pounds per square inch (psi) of the material
- (a) Find IL to the nearest whole number if w = 12 lb/in., L = 180 in., and E = 1,500,000 psi.
- (b) The moment of inertia IJ of joists with a breadth b and a depth d, both in inches, is given by the formula
The value of IJ must be greater than or equal to the value of IL in order for the joist to provide sufficient support. Will a joist with a breadth of 6.0 in. and a depth of 8.0 in. be sufficient to support the load from part (a)?
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