maximum weight held by 4x8 beam

[ddesign]

Can anyone tell me what is the maximum weight a 4x8 douglas fir beam can typically hold?

[Rich]

It will depend greatly on how long of a span you are dealing with.

[mjpliv]

It would depend on the length of the beam and if it is holding up roof or floor (or other) loads. It also depends on the type of loading (ie - uniform, uniform partial, trapizoidal, concentrated). It also depends on the amount of bearing you have available at each end and what that material is.

It also depends on how the beam is carrying the load. Is the load all carried on the top of the beam? Is the load carried on both side faces of the beam equally? Is the load all carried on one face of the beam? Are there any holes or notches in the beam (size and location).

What are the design criteria required in your area? This would be maximum deflection/length ratio, required live loads and required dead loads.

Your best bet for reliable beam design is an engineer. If this is a critical structural member (most beams are) then spend a few bucks and have it designed by a professional.

[ddesign]

the beam spans 6 feet

[ddesign]

my apologies for these sporadic clarifications.
the beam spans 6 feet and is currently holding about 750 lbs, evenly distributed. it does not support anything else.
my concern is whether this is too much weight, and if and how to go about reinforcing it if so.

[mjpliv]

Can you clarify a bit more - 750 pounds of what?

I know it sounds like a redundant question but it does make a difference.

Also, is this 750 pounds the actual force of the object on the beam or is the 750 pounds the total weight of an object that has one edge/end/side sitting on the beam and the other edge/end/side sitting on something else.

It would be very helpfull if you could give us as much information as possible.

[ddesign]

my apologies again, as i am researching these facts for my boss, who is currently on site (a remodel of an older building, c. 1910) and updates me with replies.
the weight consists entirely of 8-inch cinder blocks and, while there is some displacement on either side of the beam, it is very slight and, according to him, subsequently irrelevant.
He's specifically wanting to know how much this beam will support, and if the estimated weight of the cinder blocks (750 lbs) will fall within that range.
The wall was building entirely of cinder blocks, with 2 arches that appear to have originally held windows. In an apparent remodel, these were bricked in, and the current work has been to restore the windows. There is, however, no proof that this was the original construction, so the concern is whether the "fill-in" cinders were in fact acting as an additional support of the blocks above, or if the beam itself should be sufficient. The beam is in reasonable condition, and nothing has moved in the process of removal. Should it still be stable, or has the support of the upper cinder blocks been compromised?

[mjpliv]

Now we are getting somewhere. Are these 4x12's used as headers to support an opening in a block wall?

I ran the beam through my software with a dead load only of 125 pounds per foot (750 total load over 6 feet) with 3" bearing at either end. The design deflection was negligable. My only concern would be lateral support.

The wall above the timbers, what do they support? Or are they just non-load bearing partitions?

[Rich]

A thought just came to mind.. hopefully the blocks aren't full filled. Then that 17 pounds a block is substantially higher.

[ddesign]

actually, after measuring, the 8-inch blocks are 8x12. don't believe they are full filled, but we estimated the weight at 35 lbs in case they were, just to be safe, and used that to get the 750 lbs figure.
nonload-bearing partitions.
by "4x12's", did you mean the blocks? not sure on the answer to that, but just assuring that you did run 4x8 as the measurement for the beam.

[mjpliv]

I am sorry, I did mean 4x8.

If there are no other loads, just the 125 pounds per foot, then your beam is fine. In fact, being filled helps the performance of them beam because it provide a bit of lateral restraint because a beam with no lateral support has a tendancy to "roll" under load. Lateral (side-to-side) support works against this rolling action.