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Lr (Roof Live Load) Reductions
2

Lr (Roof Live Load) Reductions

Lr (Roof Live Load) Reductions

(OP)
I'm not sure if I understand the roof live load reduction method completely. I understand how it might apply to a (flat) roof where there are purlins, beams and columns. Basically take the tributary area of the member and then check Atrib to see if it is greater than 200 ft2. However, I am wondering if it might apply to a a bearing wall as well? I have an interior bearing wall that is resting on a 6" concrete slab. My roof load and wall loads are:

Lr: 20 psf
DL: 15 psf
Wall DL: 10 psf

I have a 12' high wall with 20' of tributary length of roof joists. My load approximately on this bearing wall is: 120 plf + 700 plf = 820 plf ----> NG

The wall is approx. 51 ft. long. The roof tributary to this wall is 1020 ft2.

Can I apply a reduction of R1 = 0.6? which would then bring my load on my bearing wall to 120 plf + 540 plf = 660 plf ----> OK

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RE: Lr (Roof Live Load) Reductions

(OP)
Forgot to add that for reference I am using the allowable for a 6" slab from Table 3-2 of TM 5-809-1/AFM 88-3, Chapter 15

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RE: Lr (Roof Live Load) Reductions

I'm not sure if your wall is failing due to foundation issues or the wall itself. I'm also not sure whether or not you can apply the LL reduction. But I do know it's not a good idea to have your supporting wall/foundation as the weak link in any design. My approach is to be more conservative with the larger elements of the design, like walls or columns.
I remember where I used to work we used to add a 10,000 lb load on the top of every concrete column presumably for future stories but more likely to take them out of the equation of being overstressed.

RE: Lr (Roof Live Load) Reductions

(OP)
Or perhaps in this situation one needs to consider the spacing of the roof joists and what their trib. area is and consider that since that is what is directly point loading the wall. The tributary area of each roof/ceiling joist is 20ft x 2 ft = 40 ft2 which would not qualify for any live load reductions.

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RE: Lr (Roof Live Load) Reductions

(OP)
The 2x6 wall itself is fine. My stud wall calcs for that show in only minimally stress. The issue here is the 6" slab with only minimal reinforcement (6x6x10x10 WWM). I am trying to make this work without having to chop the slab all to pieces and pour new footings, unfortunately I may have no choice or so it is looking.

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RE: Lr (Roof Live Load) Reductions

Quote:

I remember where I used to work we used to add a 10,000 lb load on the top of every concrete column presumably for future stories but more likely to take them out of the equation of being overstressed.

Jed - I wonder about your method. I can see that adding load is typically a way to increase capacity in members, but for concrete columns the added Pu value sometimes decreases the required reinforcement in the columns.

In many past concrete frame projects, the counter-intuitive results showed that upper, corner columns required more reinforcement than the lower interior columns due to the moments in them.

This can be seen by looking at the column P-M interaction diagrams. If you were originally under the "bulge" in the curve, your column was failing. But if you added axial load the point moved up within the interaction curve and was OK.

So just adding 10,000 lbs. to a concrete column doesn't necessarily add capacity and might actually reduce your required column rebar.

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RE: Lr (Roof Live Load) Reductions

No it can not be applied to walls. Unless each stud supports more than 200 sf.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

RE: Lr (Roof Live Load) Reductions

(OP)
Thank-you Woodman, that was the answer I was looking for. Confirmed what I suspected, but I didn't want to miss something in case the reduction was available in this case.

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