Engineering – For beginners, part 1

[RAB]

Engineering – For beginners, part 1
Key elements for residential engineering

Beams have 3 main qualities you must design for.
1. Deflection
2. Shear
3. Cost

Simple definitions -
Deflection: The distance a beam will bend under load.
Shear: Failure of the fibers that makeup the material the beam is composed of.

    - Design Standards for Deflection-
Deflection should not exceed a ratio of the beam length to the distance deflected under load.
Even if the deflection exceeds that ratio - a failure may not occur.
    
       -Design Standards for Shear Force-
Shear Force should always be less than the strength of the material.
If the Shear Force exceeds the strength of material – rapid failure can occur.

     -Design Standards for Cost-
 Cost and reduction of Deflection can be considered proportional.

Deflection Examples:
Excessive deflection usually happens over longer time periods on longer spans, compared to shear failure. It presents itself in the middle of a beam. Increasing snow load on a roof will eventually bend the rafters enough to crack a plaster ceiling.  

Shear Force Examples:
Excessive Shear Force results most often occur over a short periods of time on a shorter spans. It will most often present itself next to a support. Overloaded deck ledger boards will fail at their connection points.
    
        **General Rules about Deflection and Shear**
     --Deflection most often precedes Shear Force for limits of beam design--
     --Cost and reduction of Deflection can be considered proportional--
     --Due to the speed of failure and lack of visual clues - Shear Force should always be calculated.

Interested readers – this is a multipart series
Engineers please excuse the simplification that this forum dictates.

Part two: http://countryplans.com/cgi-bin/yabb/YaBB.cgi?board=01;action=display;num=1105070370

Ryan B