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StructuralEng
StructuralEng, Consultant
Category: Structural Engineering
Satisfied Customers: 6939
Experience:  Structural Engineer
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I have a 3.5 by 9.5 gang lam with posts every 7 feet. If I

Customer Question

I have a 3.5 by 9.5 gang lam with posts every 7 feet. If I want to reduce the height of the lam to 4.5 inches between two posts would 1/4 x 4 inch Steel plate bolted on both sides of the lam restore the structural integrity of the lam?
Submitted: 1 year ago.
Category: Structural Engineering
Expert:  StructuralEng replied 1 year ago.

Hello

Expert:  StructuralEng replied 1 year ago.

Cutting off the top or bottom?

Customer: replied 1 year ago.
What's up?
What is the answer to the question?
Customer: replied 1 year ago.
Removing from the bottom to increase headroom for a transom window
Expert:  StructuralEng replied 1 year ago.

You would need two 0.75" x 4" plates

Customer: replied 1 year ago.
you are saying that I would need an inch and a half of steel by 4 inches tall to replace half of the wood that I would remove? Seems pretty excessive.
Customer: replied 1 year ago.
What you are basically saying here is that steel is only twice as strong as wood
Expert:  StructuralEng replied 1 year ago.

Steel is about 10 times as strong as wood

Expert:  StructuralEng replied 1 year ago.

A 9.5" deep beam is 5.6 times as strong as a 4" deep beam

Expert:  StructuralEng replied 1 year ago.

A 3.5" wide deep beam is 2.3 times as strong as a 1.5" wide beam.

Expert:  StructuralEng replied 1 year ago.

2.3*5.6 is about 13, so the steel is right on the money

Customer: replied 1 year ago.
I would not be reducing the thickness of the beam only the height of it, if it is 5.6 times stronger to be at 9.5 than at 4.5 and you add 1/2 inch of Steel which is ten times stronger so the overall thickness of the beam is now 4 inches wouldn't that math work out if you say that 3.5 is 2.3 stronger than 1.5?
Expert:  StructuralEng replied 1 year ago.

But the wood will fail before the steel reaches its capacity because of the load share. They don't share load according to strength, they share load according to stiffness.

Expert:  StructuralEng replied 1 year ago.

And I apologize on the ratio, A36 steel is 7 times as strong as an LVL with Fb of 3100 psi

Customer: replied 1 year ago.
I would be using GRK lag type screws and Pl premium polyurethane construction adhesive to secure it, can't that be factored in?
Expert:  StructuralEng replied 1 year ago.

Screws transfer toad they don't carry load

Customer: replied 1 year ago.
They transfer both ways don't they, the steel to the wood and vise versa?
Expert:  StructuralEng replied 1 year ago.

Right. But they only transfer, they don't carry. The steel and wood carry

Customer: replied 1 year ago.
Sounds like there might be an I beam in my future if I don't want to spoil my clean lines, would a 4 inch tall by 3.5 wide I beam work for me 7 feet long?
Expert:  StructuralEng replied 1 year ago.

Not quite. Two MC4x13.8 will match the strength and stiffness

Expert:  StructuralEng replied 1 year ago.

But that's 5" wide

Customer: replied 1 year ago.
A local steel supplier here has a "W" style I beam that is 4x4 inches approximately, not good enough?
Expert:  StructuralEng replied 1 year ago.

It doesn't match the strength and stiffness. It's close to the strength, but stiffness is only 60%

Customer: replied 1 year ago.
There isn't any load bearing walls above this beam and the roof system above is gable ended on both ends.
Expert:  StructuralEng replied 1 year ago.

I'm trying to match what is there

Customer: replied 1 year ago.
thank you. I put this lam in 7 years ago, 42 feet long with posts every 7 feet to support a third floor 2x8 floor. I don't want compromise it, just want to keep my wife happy with her transom.
Expert:  StructuralEng replied 1 year ago.

I would sister the channel on one side and a plate on the outside

Expert:  StructuralEng replied 1 year ago.

Or a custom shape. I can design a custom shape as an additional service if you're interested

Customer: replied 1 year ago.
The problem with sistering is that it would create a soffit. This lam has the third floor 2x8s sitting on it and is the plane of the wall on both sides of it. So my goal was to keep close to the same profile,
Expert:  StructuralEng replied 1 year ago.

I see. I think you need a custom shape then. It's the only way to match the strength and stiffness of what is there now

Expert:  StructuralEng replied 1 year ago.

Do you have any other questions for me on this topic? If you could rate my answer, I would appreciate it.

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