# Maximum I-beam girder S12x31.8 span

Does 200lb/in of load on a S12x31.8 over 15 feet seem reasonable? I'm having a tough time trusting the numbers.

Two story house with an attic as below. Dotted lines show load bearing walls W1 and W2. There is currently a single wood 4x6 rough hewn girder running the entire 36 feet bowing 1-2.5 inches between lally columns spaced 8ft apart (see the red highlighted areas with greatest negative deflection (up to 3" from level). I've started raising slowly (1/4" every 4-6 days), and I'm thinking just replacing the wood girder with steel in the basement (unfinished). And using 24x24x18" poured, rebar reinforced supports over packed calcareous sand which should support 9000-12000PSF.

Right now, I am guessing that center girder needs to be rated to 160-190 lb/in. For S12x31.8, my numbers are showing I can span up to 15 feet (180in) and still maintain an acceptable L/360 deflection:

Unit Load - q : 200 (lb/in)

Length of Beam - L : 180 (in)

Moment of Inertia - I : 218 (in4)

Modulus of Elasticity - E : 29000000 (psi)

Distance of extreme point off neutral axis - y : 6 (in)

Support Force - R1 : 18000 (lb)

Support Force - R2 : 18000 (lb)

Maximum Stress - : 22294 (psi)

Maximum Deflection - : 0.432 (in)

This is attractive because I can use two supports and span the entire structure, perhaps using three separate sections. Does this seem right?

I'm considering running three evenly spaced girders using three sections each, because I'm worried there needs to be more support for the tired floor joists. See the image below option 2. This would seem to make 120lb/in more than adequate and even single center load @ 10 tons looks like it would be acceptable (especially near W1, which would fall middle of the girder). I'm having a hard time believing these numbers. Does this seem right?

Length of Beam - L : 144 (in)

Moment of Inertia - I : 218 (in4)

Modulus of Elasticity - E : 29000000 (psi)

Distance of extreme point off neutral axis - y : 6 (in)

Support Force - R1 : 10000 (lb)

Support Force - R2 : 10000 (lb)

Maximum Stress - : 19817 (psi)

Maximum Deflection - : 0.197 (in)