Expert: Stephen Major - 8/1/2007

Question
QUESTION: I'm looking for a general idea about a basement support beams before I talk
to contractors so that I'll know if they know what they're doing so to speak.
The house is in east tennessee and has some uncommon construction
features in the basement. The basement is 24 x 48 and lengthwise has a 6x6
hemlock beam running down the middle where the floor joists meet on top of
it. The beam is supported every 12 feet by a 6x6 locust post. I now want to
put a basement slab in and get rid of the 6x6 beam and replace it with  two
24' steel beams or glulam type beams supported on the ends. The load above
the beam is just open floor space with furniture with no load bearing walls
etc.  My questions are;

- What size S beams or W beams would be adequate ?

- What size gluman beams would be adequate ?

-What is a ballpark figure for the load being carried by each of the posts (4)
that would be supporting the two beams ?

Thanks very much for you time

Larry


ANSWER: Larry,

Unfortunately it is somewhat unrealistic to support two such long beams at their ends only.  The beams would need to be very deep (and heavy) and the post footings would need to be very large and specially designed as well.  This load and span combination is at the extreme of most beam charts, and some charts don't show it at all.

I highly recommend that you support each 24 foot long beam at a minimum of three points: both ends plus center.  

Let's assume a floor load of 50 pounds per square foot (10 dead and 40 live).  Your beam is carrying 6 feet of floor on each side (the remainder is carried by the perimeter foundation) for a total of 12 feet supported.  At 50 pounds per square foot, each foot of beam is carrying 12x50 = 600 pounds. This is a design load; actual loading is typically much less unless you have, for example, a grand piano or a very crowded party.  Each span of beam is 12' between supports (with our center support scenario) and this carries 12x600 =  7200 pounds.  Many steel I-beams will work here:
W6x25
W8x10
W8x15
W8x18
7I15.3

For LVL (laminated veneer lumber) beams you would need a minimum of (two)1-3/4 x 11-1/4 or (three)1-3/4 x 9-1/2.

If you really want to span 24 feet with the steel, you would need a W12x22 or W12x26.

The end posts will be carrying 3600 pounds (six feet supported), while the center  posts will be carrying 7200 pounds (twelve feet supported).

The easiest way to make the post footings is to dig a circular recess 3' across and 10-12 inches deep, with fairly vertical sides.  Fill this with 3000 psi concrete, and lay a rebar grid of #5 bar wired together in a 3 pcs. x 3 pcs. tic/tac/toe arrangement, into the wet concrete, at a depth of 1/3 from the bottom.  Pour the concrete floor over the post footings and around the post bases.

These recommendations are rough guidelines only.  I highly recommend that you consult a licensed architect or engineer to determine any structural loads and designs, in  consideration of your site particulars (soil, foundation integrity, etc.)

Best of luck, and let me know if you have additional questions.

--Steve Major

---------- FOLLOW-UP ----------

QUESTION: Thanks alot Steve, I'll go with the 3 supports per column. Concerning the acceptable I-beams, does it sound logical to you to pick the W8x10 if for no other reason it would weigh 240lbs for a 24' section versus 600lbs for 24' of W6x25, thereby being safer to work with not to mention cheaper in price.Was the post footing design you described for the 3-support or 2  support beam?  Thanks again, Larry

Answer
If I was doing this, I would certainly choose the lighter/deeper beam, unless headroom under the beam was a critical issue.

The post footing description would be for the three-support option, and is our typical design in this region.  Again, if you have poor soil conditions or other issues, you may need a different design.  

--Steve Major