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)

Total Load : 36000 (lb)

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?

Total Load : 20000 (lb)

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)

enter image description here

Options for girder replacement


1 Answer 1


You need an engineer to calculate your project and submit it to the authorities for approval.

The beam calculations seem correct but the supports and connections need to be designed and may call for the structural upgrade of the whole house.

Concentrated loads in the middle of a beam have different code and a load factor of 2.8.

Before you do anything talk to the permit authority. They have a free code and planning advisory service.

  • $\begingroup$ @SolarMike, apologies the correct word is, may! $\endgroup$
    – kamran
    Commented Jan 5, 2020 at 21:52
  • $\begingroup$ It’s clear now :) +1 $\endgroup$
    – Solar Mike
    Commented Jan 5, 2020 at 22:09
  • $\begingroup$ What do you mean by "may call for a structural upgrade of the whole house?" Hmm, looks like there is a 1.7 load factor for LL and 1.4 load factor for DL, so this beam may be undersized for a single beam. It would be more than adequate for a triple beam system, but you're saying I'd need a load factor 2.8 if I think W1 is actually a weight bearing wall. . . A footing of 24x24x18 should theoretically support, even at 6000PSF, up to 24k pounds if on appropriate substrate. 48k between two posts Yes, I plan on touching base, but I was hoping to make the process go as smooth as possible. $\endgroup$
    – Justin C
    Commented Jan 5, 2020 at 22:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.