# Comparing strength of channel versus flat steel

1. Dec 12, 2007

### OldHouseGuy

I am trying to repair my 1920s pier and beam house and have a situation where I need to slide under two butted beams on top of a post the thinnest steel I can that will support the two beams. What would help me determine this would be a general idea of how to compare the strength of a flat piece of steel with a force acting perpendicular to the flat width surface (that is, supporting a beam) to one of channel (or C shaped) steel.

I had strength of materials coursework thirty years ago, but do not remember how to calculate the relative strength for these two types of cross sections of steel and have looked everywhere for reference materials or a quick comparison with no success.

Assume the vertical sides of the channel steel are each about 25% of the width of the steel plate high, and that the flat and channel steel are the same thickness (relatively thin as compared to the overall dimensions) and the same width (say 4 inches wide and 1/8 inch thick with 1 inch high sides).

I am looking for an answer such as "with sides 25% as high as the steel is wide, channel steel is X times stronger than a flat piece of steel of equivalent thickness and width".

Thanks!

2. Dec 12, 2007

### FredGarvin

The initial figure of merit will be to compare the cross sectional moment of inertia for each section. Assuming a pure bending scenario, which is not a bad assumption, the section with the higher moment of inertia in the vertical direction will have the lower stresses and thus handle the higher loads.

If you need a refresher on area moment of inertia, take a look here:
http://www.physics.uoguelph.ca/tutorials/torque/Q.torque.inertia.html
http://en.wikipedia.org/wiki/Second_moment_of_area
http://www.globalsecurity.org/military/library/policy/army/fm/3-34-343/appd.htm

If you have a catalog for a structural steel supplier, they usually will have these tabulated for you.