Live Load (LL) reduction 7

[pelelo]

Hello,

I was reviewing a geotechnical report for a 40 story tower. One of the sections included settlement analysis.

The engineer, stated in one specific paragraph that for settlement analysis purposes, he considered 100% of the dead load and "a reduced live load (LL"). He did not go into more details about how much was the LL reduced and why.

Anyone one has any idea about this Live load reduction factor for settlement analysis purposes? and why?. TO my knowledge, it would make more sense to consider the full LL as we need to design for the worst case scenario.

 

[JAE]

I'm not a geotechnical engineer but I do know that some soils will deflect only under sustained loading....generally I think these are clay type soils where the pressure needs time to essentially squeeze the water out of the soil material...thus, only the sustained portion of the live load would really have any effect on the settlements.

Perhaps some "real" geotechs here could chime in as well.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[hokie66]

Just as live load on columns in multi-storey buildings is normally reduceable, the footing live load would also be reduceable. It does not make sense to include full live load on all elements, as it never occurs.

 

[hawkaz]

See ASCE7-10 section 4.7

 

[msquared48]

For a high rise structure, live load reduction to major columns beams and girders makes sense for the economy of the structure, and as Hokie points, logically, considering the periodic live loading patterns of the structure over time. The inclusion of this reduction gives a more realistic estimate of the settling of the structure over time.

However, for a two or three story structure, the difference is minimal.

Usually, the live load on the roof structure is not reduced, only the floors.

Mike McCann, PE, SE (WA)

 

[Retrograde]

It may be that what the geotech has called a "reduced" live load is a serviceability load case which does not include 100% of the live load.

DL + 0.4 LL is typical for calculating long term settlements.

 

[msquared48]

60% is the maximum column reduction. .4 fits with that factor. Nothing changes...

Mike McCann, PE, SE (WA)

 

[Terratek]

I've been asked recently about different soil parameters (bearing capacity in my case) for live loads and dead loads. The assumption seems to be that the live load is temporary, so you can use a lower factor of safety (2 vs 3). I disagree with this approach because live loads could include things like furniture which are in place long term. Furthermore, even if the load from people is temporary, it could still be there for half a day, every day, for the life of the structure. I don't see why the geotechnical engineer should assume the risk of using a lower factor of safety for live loads. I feel it is more appropriate for the structural engineer to make such decisions since he/she would likely be much more familiar with the intended building use.

I do agree with a reduction in FS from 3 to 2 for transient loads, but not live loads. Most of the time structural engineers don't even split the loads out for us. We are usually just provided with a maximum column load - most time we have to make assumptions about column loads based on rough rules of thumb for the building type.

 

[Okiryu]

Interesting. I was taught that live loads have more uncertainty than dead loads and therefore have larger load factors (although settlement calculations use non factored loads). Perhaps live loads were well known and they got reduced or as Terratek mentioned they were referring to transient loads.

 

[JohnnnyBoy]

I disagree with this approach because live loads could include things like furniture which are in place long term. Furthermore, even if the load from people is temporary, it could still be there for half a day, every day, for the life of the structure.

Just on another note about live loads, if you are designing the structure assuming apartment building, you would be looking at 40 psf design live load or at least in my location. Building are designed with full live load not to occur very often which is also one of the assumption why fatigue isn't fully designed for most cases. I would say an apartment including furniture and that a family is there at least half a day everyday, would hardly ever reach its full design load other than parties and such.