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harleysilo 01-02-2007 08:57 AM

Need to build 10' beam, specs???
Hi everyone! In preparations for tile, I've finished several support walls in part of my basement, the ones under the kitchen/pantry part of a hallway. I've also finsihed sistering about 80% of the floor joists that I need to. I am now to an area in the basement where the two previous solutions I've used will either severly intrude on the dynamics of the basement (another wall) or be near impossible to install (sistering joists.) So I need to build a beam.

The beam shall be 10' in length, Height and width will be whatever they need to be. It will be supported with two posts (material yet to be determined) at the 1' and 9' marks more or less.

The purpose of the beam is to reduce the span of the joists from 12' to 9'.
Current joists system is 2x10's 12' long 16 O.C. It's also a bathroom floor....the area i want to help support.

So would something like 3 2x12's with 2 pieces of plywood, glued an nailed sufice?

Once in place, I'll probably jack the sucker up a tad to slide my posts in place.

Joe Carola 01-02-2007 09:30 AM

Why would you need a beam under 2x10's that span 12'?

harleysilo 01-02-2007 09:35 AM

Well, according to calculations to determine deflection in the floor, i'm currently at L558, but I'd like it to be higher, shortning the unsupported span would bring me up to L1083, good enough for natural stone. I don't want to lay over 700sq. feet of tile and then have a spot where it cracks.....

I don't have a picture, but the two post supporting the beams will double as corner posts for a future U shapped bar in basement....

jbob 01-03-2007 06:53 AM

Three 2x12 southern yellow pine, sandwiched with 1/2 inch ply
would be more than plenty. Nail from both sides (meaning nail
one sandwich together, flip the header over, nail the 3rd ply.)
using 16d galvanized ring shanks, a 4 nail vertical line per lineal foot.

Since yellow pine is very strong, but seldom very straight, I usually
need to compress the plys with a sledge hammer. Lay the header
flat on a hard surface and pound it with moderate blows until the
plys are tight:)

mighty anvil 01-04-2007 05:41 PM

see below

mighty anvil 01-04-2007 07:54 PM

You are not thinking about this the right way. The beams proposed are much larger than needed.

What criteria are you using for deflection?

mighty anvil 01-04-2007 09:58 PM

The most severe design criteria for design of flooring substrates is not the maximum uniform live loading required by code but a single point load at the center of the span of an individual framing member. The resulting deflection places the maximum stress on the flooring material not only from the curvature of the substrate along the length of the member but from the curvature of the substrate between the loaded member and the two adjacent unloaded members.

The Tile Council of America recommends a deflection limit of L/360 using a 300 lb. point load for ceramic and stone floor tiles.

For natural stone I recommend doubling both numbers for a point load of 600 lbs and a deflection limit of L/720 in order to achieve a floor that is four times stiffer.

Assuming S-P-F lumber and a 600 lb. point load:

The original floor joist spanning 12 ft. would have a deflection of .270 (L/534).

The new floor joist spanning 9 ft. would have a deflection of .114 (L/950).

A double 2x10 S-P-F beam spanning 8 ft. would have a deflection of .040 (L/2400) which is perfect because it also supports a 3 ft. joist span which can tolerate only a third of the deflection of the 9 ft joist span. (intermediate supports are best placed at the middle of a span)

This beam would also be able to support the code required floor loads and a stone flooring weight of 20 psf.

If the stone is heavier or the bathroom will have a very large bathtub it might be necessary to use three 2x10s or two 2x12s for the beam.

The triple 2x12 + plywood beam would be more than 10 times stiffer than the new 9 ft. floor joists with a 600 lb. point load deflection of .011 (L/9875). There is little point in making one part of a floor much stiffer than another unless you expect unusual loads at that location.

harleysilo 01-08-2007 08:11 AM

Hi, thanks for your responses. I built a 2x10 beam to use over the weekend prior to reading your response. I don't know how to look up, or where, load ratings etc. for beams, so I was just throwing out possibilites hoping someone would provide an answer. I was using a Defection calculator from another website to determine initially if my floors were "strong" enough. Unfortunately when I typed 12' it should have been 14' which might clarify why I want to increase the floor strength.

Unfortuantely it will be impossible to place the beam in the middle of the support, although I think I understand the point you were trying to convey. The beam will be placed underneath the doorway leading into the bathroom.

I am now trying to determine whether I need 4x4's or 6x6's to use as posts, they will be placed approx. 8' apart, so that shortens the span and increase the beam deflection i would imagine.

This is the current bathroom upstairs.

Original construction, tile over subfloor! You can't see it but it's got a crack running down the middle of the floor. It has a full tub, but I think we are going to install a larger, i.e. longer tub.

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