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StructuralEng
StructuralEng, Consultant
Category: Structural Engineering
Satisfied Customers: 6681
Experience:  Structural Engineer
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I plan to knock out an existing CMU column to install a

Customer Question

I plan to knock out an existing CMU column to install a patio door that is 155in wide. A 3.5in wide by 7in high (actual dimensions) beam (I'm pretty sure that the beam is made out of Douglas Fir) holds the roof on a relatively flat pitch. The beam supports 2x6 trusses that is about 24in on center, which holds a plywood/asphalt sheet roof with silicon coating and has solar panels on top of the roof. I made some calculations myself and came up with the following:
Shear: 645 lbs
Max Moment:0.562 in
Section Modulus of 25 in
Am I safe to knock out the CMU column to install the large patio door.
Submitted: 5 months ago.
Category: Structural Engineering
Expert:  StructuralEng replied 5 months ago.

Hello

Expert:  StructuralEng replied 5 months ago.

You want the 3.5" x 7" wood beam to span 155"?

Expert:  StructuralEng replied 5 months ago.

Is this a gable roof?

Customer: replied 5 months ago.
Nope, just a flat built up roof.
Expert:  StructuralEng replied 5 months ago.

What is the city and state so I can look at snow loading?

Customer: replied 5 months ago.
No snow, Mililani, HI...Honolulu county
Expert:  StructuralEng replied 5 months ago.

What is the rafter length?

Customer: replied 5 months ago.
From the horizontal beam where the patio door will below and the next support over is 13.5 ft (which is a large 4x16) beam is 13.5ft
Expert:  StructuralEng replied 5 months ago.

That 3.5" x 7" beam is really not even close to the correct size.

Customer: replied 5 months ago.
Really....what should it be? I calculated that the Max Moment is 0.5625 in where the allowable is 0.646in; Section Modulus calculates 25 in cubed, allowable should be 28.5in cubed or less.
Expert:  StructuralEng replied 5 months ago.

What are you calling the max moment. That is not the correct magnitude nor units for the bending moment.

Expert:  StructuralEng replied 5 months ago.

I calculate a maximum moment of 5,250 #-ft

Customer: replied 5 months ago.
My calcs are max deflection at center.
Expert:  StructuralEng replied 5 months ago.

Deflection is not the same at moment.

The deflection under a full design load in the 3.5" x 7" beam is over 1"

Customer: replied 5 months ago.
Also, my assumption for distributed weight on the beam was 100 lb/ft
Expert:  StructuralEng replied 5 months ago.

About 1.3"

Expert:  StructuralEng replied 5 months ago.

100 plf means about 15 psf. That is DL only. That does not account for the required roof minimum live load of 20 psf. It also doesn't account for the creep deflection.

Customer: replied 5 months ago.
Wow...I just have done something wrong or using the wrong equations... are we sure that I've framed the problem correctly?
Customer: replied 5 months ago.
You're correct, I didn't factor in live load or creep deflection.
Expert:  StructuralEng replied 5 months ago.

I don't think so. I think you just didn't account for all of the loading and all of the deflection. These are not things you would necessarily know if you don't do this every day.

Customer: replied 5 months ago.
I guess that is it...3.7in W x 7in H beam under a distributed load of about 35 psf (dead + live load) with a span of 155 in, exceeds the beam's capacity to hold the roof?
Expert:  StructuralEng replied 5 months ago.

It does. By a factor of almost 3.

Customer: replied 5 months ago.
OK...thanks.
Expert:  StructuralEng replied 5 months ago.

No problem. I'm happy to help. Good luck with your project.

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