hello guys , this may be a dumb question but I'm going to ask anyway. The engineer of record on a house I'm building wanted a steel beam spliced on middle post with a concrete pad of 24x24" . I built it with a 34x34 concrete pad and just left the post solid because it came that way and seames to make more sense not to cut it , I still supported it on the middle post . Now he is telling me I have to redo it because the pad needed to be 36x36 for a solid beam . This makes zero sense to me but I guess I could be missing something. Beam is 50' Thanks
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Engineer wanted splice on middle post 9
- Thread starter hgcdesign
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OP sounds like a great contractor, and the type of contractor I truly enjoy working with. However....
Foundation issues aside, just because it is continuous now, doesn't automatically make it better. If the beam is set below the floor joist, the compression flange is continuously braced by the joists - if you now make the beam continuous over the post, the compression flange is now on the bottom flange of the beam (un-braced) over the post. If you're taking (2) equal simple spans and turning it into a (2) span continuous beam, then your bending stresses are the same for uniform load, but your un-braced length goes up considerably.
If your beam is set flush with the floor joists, less of a concern.
Foundation issues aside, just because it is continuous now, doesn't automatically make it better. If the beam is set below the floor joist, the compression flange is continuously braced by the joists - if you now make the beam continuous over the post, the compression flange is now on the bottom flange of the beam (un-braced) over the post. If you're taking (2) equal simple spans and turning it into a (2) span continuous beam, then your bending stresses are the same for uniform load, but your un-braced length goes up considerably.
If your beam is set flush with the floor joists, less of a concern.
- Thread starter
- #62
Canpro
I really appreciate you taking the time to explain that makes more sense now for sure. Final out come is we are splicing the post . Wether he is proving a point, being overly cautious, or simply doesn't know/care is not for me to sit and ponder . It's already cut and splice in progress. Him, the client, and I had a very civil conversation and that's what he wants , so that's what he gets. Thanks again for the interesting observations and information. I will definitely be more articulate the next time around, as trying to post on the fly with two boys around my neck didn't work out to well.
I really appreciate you taking the time to explain that makes more sense now for sure. Final out come is we are splicing the post . Wether he is proving a point, being overly cautious, or simply doesn't know/care is not for me to sit and ponder . It's already cut and splice in progress. Him, the client, and I had a very civil conversation and that's what he wants , so that's what he gets. Thanks again for the interesting observations and information. I will definitely be more articulate the next time around, as trying to post on the fly with two boys around my neck didn't work out to well.
kissymoose
Structural
Looks like OP just wanted an explanation that he wasn't getting from his engineer. It doesn't look like its been thoroughly explained yet, so I'll take a stab at it.
hgcdesign, in structures, the loads are bigger in elements that are stiffer, the loads follow the stiffness. Stiffness is a pretty intuitive concept. A rod of steel is obviously stiffer than a rod of balsa wood. If you took both rods in your hands and bent them, it would be the steel that would be providing the vast majority of the resistance. How much the steel and wood bent together would be pretty much the same as how much the steel bent alone because the steel is "taking the load" due to its much higher stiffness.
In our beam situation, two separate beams sitting on a post is not as stiff as one beam continuous across it. That's because the ends of the two beams can rotate freely and there won't be any flexural stress at the ends. So the continuous beam takes more load at the column then the two separate beams. In a simple span between two columns of a beam, the load on half the beam will go to one side and the other half will go to the other side. But if the beam over one of these two columns keeps going continuously, it ends up taking more than half the load of that span.
So in your case, you're going from (2) adjacent 2-span conditions to (1) 4-span condition. Our handy dandy steel manual shows us that our load on that middle footing will go from 6/8*wl to 26/28*wl, a 24% increase. Now your 34"x34" has 100% more area than a 24"x24" footing, so I don't understand your engineer's call, but he made the call and carries the liability. Looks like I still can't provide you an explanation for the call![[deadhorse]](/data/assets/smilies/deadhorse.gif "[deadhorse] [deadhorse]")
, sorry.
hgcdesign, in structures, the loads are bigger in elements that are stiffer, the loads follow the stiffness. Stiffness is a pretty intuitive concept. A rod of steel is obviously stiffer than a rod of balsa wood. If you took both rods in your hands and bent them, it would be the steel that would be providing the vast majority of the resistance. How much the steel and wood bent together would be pretty much the same as how much the steel bent alone because the steel is "taking the load" due to its much higher stiffness.
In our beam situation, two separate beams sitting on a post is not as stiff as one beam continuous across it. That's because the ends of the two beams can rotate freely and there won't be any flexural stress at the ends. So the continuous beam takes more load at the column then the two separate beams. In a simple span between two columns of a beam, the load on half the beam will go to one side and the other half will go to the other side. But if the beam over one of these two columns keeps going continuously, it ends up taking more than half the load of that span.
So in your case, you're going from (2) adjacent 2-span conditions to (1) 4-span condition. Our handy dandy steel manual shows us that our load on that middle footing will go from 6/8*wl to 26/28*wl, a 24% increase. Now your 34"x34" has 100% more area than a 24"x24" footing, so I don't understand your engineer's call, but he made the call and carries the liability. Looks like I still can't provide you an explanation for the call
, sorry.
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