In general, how do engineers calculate how much load wood or metal can take?

Question

When they measure pounds per square inch or whatever the the unit of measurment is, how does it actually get tested?

For example, take a 2 x 4 8ft piece of spruce. If both ends were anchored and there was a machine pushing down in the middle of the board, how much pressure can the board take?

I know this is totally wrong but is it something like this? Spruce wood has a 10lbs per square inch load so the board is 3.5" by 1.5" by 96" so there are 504 square inches so the board can support 5,004 pounds?

Im just trying to understand the basics of loads or tensile strength or whateer its called so in your answer please hold things constant as in no wind blowing on the board, or no wood rot, lol

thanks

Answer 1

This is not a question that has a very simple answer. You need to know about understanding static and dynamic loads, free body analysis as well are shear loads, tensile and compressive loads as well as stress from moments.



Finally in designing a beam other factors can come into play like fatigue and deflection. Deflection is often over looked by the inexperienced. For ceiling joists, specially in public buildings, a beam may be sufficient to hold the design load but deflect sufficiently to cause cracks in the ceiling thereby can alarm or just an unsightly appearance.

Answer 2

I think for the structure and material that you are presenting the critical action on the beam may be the bending moment at the 2 points of anchor and the middle point where your load is applied. If the bending moment is the critical action, then clearly the beam should be able to withstand flexural stress.



the formula is: s = Mc/I. where s is max. moment and I is the beam cross section constant.



The computed maximum acting stress above is then compared with the published or tested tensile and compression strength of the material, with the appropriate safety factor applied. If the computed stress is less that the strength of the material, then the structure is safe, Otherwise, then redesign your structure to a bigger cross-section or to a stronger material.



In addition, you may also want to check for the effect of shearing stress at the 2 point of anchor.



This is a bit complicated to discuss in this forum. You may want to consult a structural engineer if you are actually designing a structural member.

Answer 3

You are basically on the right path, although the question is a bit complicated.



As to how much force a beam can take, this is basically done by experimentation or testing. The results of the tests are then incorporated in various engineering documents, for wood this would be the NDS (National Design Specification), for steel it would be the AISC ASD or LRFD manual (American Institute of Steel Construction, Allowable Stress Design, Loads Resistance Factor Design).



The job of the engineer is then to ensure that the applied loads on the structure or beam are always less the maximum value allowed based on the engineering manuals I referenced above.

Answer 4

there are building codes and engineering design hand books that give average values you need to know the span and the floor weight loading per SQ foot ceramic tile is heavy and has to be subtracted from the allowed weight the upper story load has to be supported by walls Not the floor span