[AJ54]

Hey is there any formulas that find tension and compression on an object? Or any hints on how to find compression and tension on a deflecting beam?
Thanks heaps! :)

[smearcase]

Try googling beam bridges. There most definitely are formulas but a lot of it is done with software. Maybe just some basic physics texts.

I worked in bridge construction but all of that legwork was done by the designers. But it is a basic, everyday task for a designer. Your transportation agency might provide some advice.

[galactus]

I worked in bridge construction as well. As a layout/surveyor.

[ebaines]

The stress for a point in a beam is given by the basic formula:

S = Mc/I

where M = the moment (or torque) due to the forces applied to the beam, c = the distance from the centroid of the beam to the point in question, and I = moment of inertia for the beam cross-section at the point in question. The value of M depends on how the load is applied and how the beam is supported (simple support versus cantilevered), and c and I depend on the geometry of the beam cross-section.

For example, for a square beam that is 2 inches wide and 6 inches high, 10 ft long, simply supported at both ends, with a 1000 pound point load at the center:
The maximum value of M is 5 ft * 1000/2 pounds = 2500 ft lb = 30,000 in-lb. This maximum load is directly below the point load at the center of the beam.
The maximum value of c is 3 inches (the distance from the center of the beam cross-section to the top or bottom edge).
I for a square cross-section beam is calculated from bh^3/12 = 2*6^3/12 = 36 in^4

So the maximum stress as determined by Mc/I is 30000*3/36 = 2500 psi. This stress is the value of the compression at the top edge of the beam and tension along the bottom edge at the point directly under the load. Stresses along other parts of the beam are less, and taper off to zero directly above the end supports. This is true only for simply-supported beams - if the beam is cantilevered (ie. fixed ends) then the moment is actually maximum at the support, not minimum.

OK, you just learned everything you ever wanted to know about civil engineering in 1 minute!