A Structural Question

[Dan Schultz]

I just took an old barn beam out from under the old part of my house. Yeah... it's REALLY old! The house was built in the 30's and the beam was a used barn beam then. I've always been a little worried about it so I bit the bullet and swapped it out for a W8x18 steel beam. The beam is 25 feet long and supported on both ends by a concrete pier. The original wood beam was 8" square and probably oak. The floor in the center of the house has always been about 1 1/2" low and I suspect it has been that way since this part of the house was built mainly because the center of the beam was supported by another concrete pier.

I've removed the wood beam and installed the W8x18 steel beam. I have absolutely no idea what the load in the center is but the steel beam is deflected about 1 3/8". However... I think much of this deflection has to do with the fact that the center of the house was low in the first place. I've temporarily placed two jackposts 1/3rd the way from each end just as 'insurance'.

My question is.... is this 1 3/8" deflection anywhere near the yield of the W8x18 steel beam?

[Dan Schultz]

Too much cypherin'. I'm just being lazy and looking for an easy answer.

[Three Valves]

We had a device, like a round slide rule back when I was in the US Armored Cav.

It was supposed to help us determine the load capabilities of bridges before driving an M60 over it.

Most the time I just said, Hell no, I won't go!!

And took the long way.

[Dan Schultz]

We had a device, like a round slide rule back when I was in the US Armored Cav.

It was supposed to help us determine the load capabilities of bridges before driving an M60 over it.

Most the time I just said, Hell no, I won't go!!

And took the long way.

I'm sure there are 'fill in the blanks' things on The Internet where you can simply plug in a couple of numbers but I'll be danged if I can find them.

[Three Valves]

This reminds me of the old Ford I-beam suspension ads.

You never hear a car company call something a 2x4 suspension, do ya??

[opus37]

I somehow lost my engineering manual when I retired. From memory, the deflection you found sounds about right. If you support it in the center the deflection will be reduced. If you support at the 1/3 and 2/3 points, you'll get even less. The deflection is caused by the weight of the beam as well as the load placed on it by your house.

[Dan Schultz]

Welp... the house hasn't fallen down yet!

[iiipopes]

How much wood would a wood beam hold if I wood beam would hold wood ?

[opus37]

Here is the on line calculator http://www.engineersedge.com/beam_bendi ... tion_1.htm" target="_blank" target="_blank

The catch is, you have to know the weight of the house.

We had a device, like a round slide rule back when I was in the US Armored Cav.

It was supposed to help us determine the load capabilities of bridges before driving an M60 over it.

Most the time I just said, Hell no, I won't go!!

And took the long way.

Yes, and you knew the weight of the tank.

[opus37]

You know, you can try to work the deflection out by very slowly lifting the beam. Use the two jacks you have and raise them about 1/8 to 1/4 inch. Then wait a week or a month. Then do it again. See how things settle out. Likely about 3/4 of an inch total should level things out.

[iiipopes]

You know, you can try to work the deflection out by very slowly lifting the beam. Use the two jacks you have and raise them about 1/8 to 1/4 inch. Then wait a week or a month. Then do it again. See how things settle out. Likely about 3/4 of an inch total should level things out.

That's what we did for the main floor of the house I lived in as a young boy: the floor joists started sagging, and so we put two post jacks in the basement, which had a good, thick concrete floor, slowly and incrementally raising the jacks over time to level the main floor, a quarter turn at a time, making sure there was no adverse stress being redirected anywhere else, until the main floor was level again.

As far as I know, the post jacks are still there, @ 45 years later. Dad died not too long after we had the floor jacked, then Mom and I moved, so the final step, replacing the jacks with posts, was never completed to my knowledge. To this day I wonder about the integrity of the jack collars, or whether a subsequent owner of the house completed the job.

[opus37]

This is a very common technique. The jack posts are fine to stay there, but permanent cement pillars look nicer (if they are constructed properly). If you don't mind the look of the jack and no one messes with it, it can stay there for a very long time.

[Rick Denney]

The allowable deflection for a structural member in a home is L/240, unless it is supporting brittle finishes. For a 25' beam, that is 1-1/4", but for live loads only (meaning: not for the deflection of the beam from its own weight). You're at the limit, but some of that sag is from the weight of the beam. If it were my house, I would probably use a center post just to keep the supported floor from being too bouncy, and to give me a chance to level it up, but it doesn't sound like you are risking much.

Rick "who doesn't need to know loads if the deflection can be measured" Denney

[Dan Schultz]

The allowable deflection for a structural member in a home is L/240, unless it is supporting brittle finishes. For a 25' beam, that is 1-1/4", but for live loads only (meaning: not for the deflection of the beam from its own weight). You're at the limit, but some of that sag is from the weight of the beam. If it were my house, I would probably use a center post just to keep the supported floor from being too bouncy, and to give me a chance to level it up, but it doesn't sound like you are risking much.

Rick "who doesn't need to know loads if the deflection can be measured" Denney

There was always a center support even with the old wood barn beam. My main objective was to get rid of the center post but not necessarily go with no support. I've split the 25' length into thirds and installed TWO 6" x 6" posts. I can now add another cold air return where the old center post used to be. Mission accomplished and I now have a spanking new STEEL beam in place of the old barn beam and an absolutely solid floor above.

The really interesting thing is.... the W8 x 18# steel beam weighs almost exactly the same per foot as the oak barn beam I took out! This house is eight years old and the barn beam was used when the house was built. I'm just guessing the old oak beam was AT LEAST 200 years old. It was still solid as a rock except for about six feet of one end that had been bug eaten at one time.... probably BEFORE the barn was torn down. The floor joists in the old part of my house are 3" x 8" yellow poplar. No bugs there!

[Donn]

I'm just guessing the old oak beam was AT LEAST 200 years old.

Might have taken a century or two to grow, too. It's an interesting kind of long term business, planting oaks for forecasted need many generations later. I don't know if anyone does that in the US? but the English have been doing it since the middle ages, since they have uses for large oak beams.