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Tributary Area Calculations and Live Load Reductions

Tributary Area Calculations and Live Load Reductions

Tributary Area Calculations and Live Load Reductions

(OP)
I have a continuous beam with two equal spans. Consequently the bearing load is considerably more at the center post than the two end posts. I'm wanting to calculate the tributary area for this post. If the total beam length is 2L am I correct in saying that the trib. length for this post would be 5L/4? Then KLLAT = 5L/4 x W x 4 = 5L x W

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Tributary Area Calculations and Live Load Reductions

I think that is stretching it a bit. I have always just used the area of the half span for the purpose of live load reduction.

RE: Tributary Area Calculations and Live Load Reductions

The answer should be 4LW. The code definitions of KLLxAt are clumsy. The best way to think of it is as follows: every square foot of floor that, if loaded, would add load to your column, contributes a square foot to KLLxAt.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Tributary Area Calculations and Live Load Reductions

(OP)
I agree with both of you if this were two simply supported beams connecting the three posts, the center post load would then be twice the load of either of the end posts however with a continuous span my center post load is 3.33 of the end post loads so in reality its tributary area must be logically larger than a center post with two simply supported spans.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Tributary Area Calculations and Live Load Reductions

I know what you are saying, but I just don't do it that way. I don't really think of it in KootK's way either, as load anywhere along the span adds load to your column. Of course, the live load is what it is on the continuous span, but the reduction factor is what we are talking about, and I would rather be a bit conservative in that respect.

RE: Tributary Area Calculations and Live Load Reductions

(OP)
Perhaps this picture will help illustrate what I am trying to describe:

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Tributary Area Calculations and Live Load Reductions

Remember, this is a probability thing. And the probability of full load with continuous spans is no higher than it is with two simple spans. The parameter of interest is not "tributary area". Rather, it is "influence area" which is as I described it above.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Tributary Area Calculations and Live Load Reductions

(OP)
So calculate the tributary area as if all beams were simply supported. I get the probability angle on this.

However, with the actual load to the center post, calculate that based on the mechanics of the situation? In the case where the joists and beam are continuous the center post will see considerably more load than if everything was simply supported.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Tributary Area Calculations and Live Load Reductions

If you are designing the column for a Live Load of 1.25wL2, you should be entitled to a live load reduction factor based on the area 1.25L2. Like hokie66, I might not do it that way but I believe it could be justified in court when defending your design.

BA

RE: Tributary Area Calculations and Live Load Reductions

(OP)
56.3% more load.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Tributary Area Calculations and Live Load Reductions

I believe that the same probability argument applies in both directions
Medeek.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Tributary Area Calculations and Live Load Reductions

I should probably not have participated in this discussion, as "KllxAt" is not in my vocabulary, and neither is "influence area" in this sense. We obviously do it differently in Australia.

RE: Tributary Area Calculations and Live Load Reductions

It won't be as much as 56.3% more load because the central beams deflect. Each continuous joist settles a variable amount at the central beam and very little at the stud wall, so the negative moment of each joist is somewhat less than wL2/8 due to support settlement. If the central column is deemed to carry 56.3% more load, its design is conservative so a live load reduction factor based on the larger tributary area seems reasonable.

I'm not convinced that making the glulam members continuous is a good idea but that is a separate issue.

BA

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