Engineering - For beginners, Part 5

Started by RAB, January 08, 2005, 10:12:44 PM

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 Engineering - For beginners, Part 5

Comparing Code/Design books to Shear Force rules mentioned in parts 1-3

Code Book vs. engineered examples-
2x4 supporting roof only, 16" OC, with sheeting, default wind load.

Code book says ok to use –
Engineered deflection ratio – 1/480
Shear Force +150%

2x4 9'
Code book says ok
Engineered deflection ratio – 1/300
Shear Force +90%

2x4 10'
Code book says ok
Engineered deflection ratio – 1/230
Shear Force +50%

2x4 11'
Code book says ok
Engineered deflection ratio – 1/175
Shear Force +30%

2x4 12'
Code book says ok but do not exceed this length
Engineered deflection ratio – 1/135
Shear Force +10%

2x4 16'  8"OC
Code Book says ok
Engineered deflection ratio – 1/150
Shear Force +60%

For plaster and drywall joint compound you should keep the deflection above 1/180 or cracks (failure of system to support) will appear.
Notice the code/design book and the deflection ratio agree (if you add a reduction value for repetitive member use) on maximum length.
Visual clues of excessive load become apparent long before the actual stud is going to fail.
As stated in Part 1 *Excessive Deflection most often precedes Shear Force failure* in residential design.

Ryan B