Check Calcs for New Lintel 3

[richburton]

I am a commercial plan reviewer trying to maintain my mathematical skills which helps me do a better job while reviewing proposed structural projects submitted by engineers.
The attached problem illustrates a new overhead door cut into an existing masonry wall.
The criteria includes a deflection limit of L/600, 36 ksi steel, and a ground snow load of 30 psf. Rather than calculate the live and dead load for the roof, I simply assigned a value of 40 psf. The total load is then 20,819 lbs of masonry and roof being supported by the new steel lintel.

Please confirm or correct the following:

1) Max. moment = 31.23 kip-ft

2) Bending stress = 23,641 psi

3) required section modulus to resist failure due to bending = 15.85 in^3

4) required moment of inertia to resist deflection = 116.3^4

Thank you for volunteering your time.

ICC Certified Plan Reviewer
NFPA Certified Fire Plan Examiner

 

[jayrod12]

We would also need to know the span of the lintel, span of the roof members, height of wall above lintel. Generally I've found that it is deflection that ends up governing for masonry supports at any reasonable span.

Your approximation for roof live (snow) and dead is probably not incredibly far off however if you find your analysis needs some more precision you should calc the actual snow load on the roof. And depending on your roofing type the load strictly due to dead could be upwards of 25 psf so it's something to consider.

 

[richburton]

Thank you Jayrod.

As noted in the illustration...
The span of the lintel is 12 feet with 8 inches of bearing on each end.
The span of the roof truss is 50 feet.
The height of the masonry wall above the lintel is 4 feet.

 

[jayrod12]

My bad, didn't look at the attachment.

 

[hawkaz]

I see a span of 12' in the attachment and a T.A of 288 SF. This would seem to indicate 48' long trusses- is that correct?
Using 40 psf D+L, your uniform load for the roof would be 960 plf. By reverse engineering the moment- your equivalent uniform load appears to be 1735 plf. This would put the load from the masonry at 775 plf. Seems high for 3'-4" of block.

Base on your numbers- Ir looks appropriate. Without checking to see if the section is compact, I might be more inclined to use .6Fy instead of .66Fy for bending.

I have not used channels in this configuration as lintels before. I would be more inclined to use a W section or double angles.

 

[jayrod12]

My thought at first is I have never seen a lintel built-up in this way, however I'm assuming it is to allow for site matching of the block thickness.

I got that your load takedown onto the beam was conservative as well.

 

[bookowski]

Lintels like this are usually used to avoid shoring - they cut in one channel, then then next, then remove the wall below in sections welding on a closure plate. A little sketchy but pretty common (in my area anyway).

 

[richburton]

hawkaz,

Terrific feedback.
For my CMU blocks, I am using 150 pcf. Should I be using 100 pcf or 120 pcf instead?

 

[EngineeringEric]

Fully grouted 12" CMU is about 125 pcf (normal) most situations are not fully grouted. even still 3'@125 = 375 plf