Reduce live load factor for column or foundation design of multi-storey buildings to ASCE7-10 1

[tatadj]

Dear all,
I have no idea how to get moment for column or foundation design of multi-storey building to ASCE7-10.
I can get the reduced axial force by hand calculation for ground floor column (for example), but I cannot calculate or analysis to get reduced moment.
It's because the reduced method of ASCE7-10 is base on influenced area. So the interior column and edged column have reduced factor themselves.
From this, i cannot apply these factors for each floor live load with different area on one floor in my modelling(Robot structural analysis professional).

I would appreciate your answers as well as your comments.
I would like to say thank for your help.

 

[slickdeals]

You don't reduce moments.

 

[tatadj]

Yes, I'm really thank for your comment.
I still not sure about this. normally, the column have axial force and moments at the same time.
so you mean we take the moment from live load without reduce factor and take axial force with reduce factor to design the column.
If so, I think this two internal forces are not get simultaneously.
Could you explain me more clearly about this?
Thank for your help.

 

[slickdeals]

You need to tell us more about your building. You should have very little moments from live load. Typically, if you are a moment frame, the moments come from lateral loads.

However, if you have a very asymmetric building, you could possibly have gravity moments.

 

[JAE]

Well you could have some significant moments in columns with live load assuming you alternate the live loads between bays per code.

The reduction should be applied to the live load overall and this results in reduced load effects like shear, axial and moment.

You are correct that each element of the structure (corner column, interior column, members from different floors, different beams) each have their own influence or tributary area and therefore have individual live load reductions.

So in a sense, you have to be careful to apply the LL reduction in separate analyses to get the appropriate reduction for each piece.

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[KootK]

The reduction should be applied to the live load overall and this results in reduced load effects like shear, axial and moment.

Is this stated explicitly in ASCE 7 someplace? I've bee gone from the US a while. Logically, I don't think that this approach is consistent with the probabilistic "spirit" of the live load reduction provisions.

Consider a first floor column in a 100 story building. Obviously, the accrued live, axial load at the first floor qualifies for full live load reduction. However, the moment delivered to the column by adjacent, moment connected beams will be based on a much smaller influence area. In this case, I would be hesitant to apply the live load reduction to those bending moments based on the influence area associated with the axial load.

Matters get even more complicated when one considers that sometimes a column with applied bending moments is actually better off with more axial load.

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[JAE]

KootK - I think my sentence was perhaps too global and misunderstood.

Each piece of the structure, of course, has its own influence area associated with it.

This area (for the design of that particular member) would determine a live load reduction factor.

You would apply that factor to all the live loads that produce any influence on that member. For columns it would include all the live loads above it, for example.

The analysis would be run and the resulting effects (axial, shear, moment) would be determined for that member and the member designed.

For a different member, that has a different influence area, you would have a different LL reduction factor and thus effects that would be affected by the reduction.

This is very tedious of course and I'm sure not many engineers go through this process. We typically use the LL reduction somewhat sparingly and lump members together to minimize the numbers of analyses that we have to run.

Not sure of your column example with affected moments (from live loads) vs. multi-story axial loads from above. I'll have to think about that but the IBC is pretty straight-forward in its application of LL Reductions. I don't believe you separate axial influence areas from moment influence areas.



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[haynewp]

I don't see anything in ASCE 7 that does not allow for a reduction in moments based on tributary area. It doesn't make sense why axial loads would be permitted to be reduced but not moments. I thought live load reductions had to do with probability of areas being loaded and that would be independent of the effect the loaded area produced (axial versus shear versus moment).

 

[KootK]

@Haynewp: certainly, my reading of the ASCE7 provision is the same as yours:

1) One is allowed to reduce all live load actions on a member based on an influence area that includes all of the live load that affects the member appreciably.

2) Moment effects due to live load can be reduced.

3) The crux of it is the probability that influence areas will be fully loaded.

As I described in my "100 story building columns" post above, the concern for me that different load actions experienced by a member have different influence areas associated with them. Logically, I'd rather see a column moment delivered by a floor beam be reduced based on the influence area of that floor beam, not the influence area that includes all of the axial load delivered to the column by floors framing in up the whole height of the building. In my mind, that fits the probability of loading model much better.

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 that I want to either change it or adopt it.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[hokie66]

KootK,
You are correct, but your example is a poor one. 100 storey building columns don't worry about moments.

 

[haynewp]

Kootk, my post was just attempting to clarify that the code is not more stringent with moments and allows moment reduction based on area the same way as other effects. I agree with your concept as well.

I think this analysis could be further complicated if you look at alternating live load patterns between floors on a low rise building with moment connected beams at every floor(and not just alternating the load on adjacent moment connected beams on any single floor). You could even combine that pattern with lateral as well. But I doubt most engineers consider that and the probability must be low of such cases actually happening.

 

[KootK]

@Hokie: fine... it's six story building now. So much for hyperbole.

@Haynewp: in practice, I actually do as Slick suggested at the top. I leave the moments at full intensity.



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 that I want to either change it or adopt it.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[haynewp]

Using unreduced live loads may simplify the effort, but it doesn't address the case of patterning of live loads vertically...i.e. alternating loaded sides of a column from one floor to the next. As an example; if you consider public assembly on one side of a column that exists at every floor in a low rise building (such as in a courthouse), and the other side of the column has a secure hallway but I know that hallway will see minimal use while a court on the SAME floor is in session, why shouldn't some type of full height patterning be considered to find a worst case moment on that column?

 

[KootK]

No argument here Haynewp. I consider live load reduction and unbalanced loading to be two independent mechanisms. I wouldn't ever suggest that one ignore pattern loading because they chose not to take advantage of live load reduction. The one cannot be traded against the other.

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 that I want to either change it or adopt it.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.