Live Load reduction Factor (LLRF) for Sizing Foundation? 3

[NewbieInSE]

Hello Dear Engineers,

Is it allowed to apply LLRF while sizing footing or determining the number of piles required?

I thought it's only applicable (to reduce live load) in the strength design of structural members. Our office is doing an assessment of an already constructed structure. Some of its piles exceed their bearing capacity. My senior asked me to apply LLRF to see if any benefit may be achieved.

 

[MIStructE_IRE]

BS 6399 (withdrawn now I know but remains as relevant as ever for sound principles in my view!) states that reduction can be applied to foundations.

If columns can be designed for a reduced load then it absolutely makes sense that same can be followed through to foundation.

 

[NewbieInSE]

Thanks, MIStructE_IRE.

Another question, a column has a tributary area of 20 sq. meter. If the total number of stories is 10, then am I going to consider 20x10=200 sq. meter area while calculating the LLRF for that column at the lowest story?

 

[NewbieInSE]

 

[NewbieInSE]

Besides that,
What is this DL/LL ratio if anyone knows?

Thanks.

 

[phamENG]

Not sure about your codes, but in the States ASCE7 does it based on total influence area (related to tributary area) supported by the column...not individual floors. Then there's a cap on it of something like 0.4*L[sub]o[/sub]...you can't reduce it any lower than 40% of the original, full live load.

I imagine DL/LL ratio is the dead to live load ratio. Take your dead load and divide it by your live load. If it alters your curve, then I'd say it's probably supposed to be unreduced live load for calculating the ratio...but again...I work under a different code so I could be way off.

 

[NewbieInSE]

ASCE7 is ok for me to understand. I think the concept is the same in all the codes. Our code is similar to ASCE7, though uses older version of ASCE7.
So do you mean that, my second approach of calculating reduced live load is correct?

I don't understand what could be the relation between DL/LL ratio with the live load reduction. If you are aware of any texts in that regard, please share.

Thanks phamENG.

 

[jayrod12]

Locally here (Canada) it is applied per member, so your second method would be how we calculate it. The reduction factor on the top column would be 0.9, but for the foundation it would be 0.645. And appropriately modified for the columns in between.

Often, I don't do LLR on my first go through a project unless it is tall and relatively light (which rarely both happen). That's because on an average 4-5 storey steel structure with a concrete floor, it doesn't take long before the dead loads dwarf the live loads and the LLR no longer provides significant benefit. At the foundation level, if I'm right on the edge of needing to add an extra pile, that's when I'll look into using LLR. But if I'm healthily in the between a 3-pile and 4-pile cap, then I don't see the benefit to using live load reduction just to be at 99% of a 2-pile cap.

 

[phamENG]

Yep - Jayrod summed it up better than me. As for what the DL/LL ratio is used for...not sure.

 

[skeletron]

DL/LL is used for concrete deflection calcs

 

[phamENG]

Ah...that's Etabs, isn't it? Haven't seen that logo in a while. Makes sense. Odd that it would be lumped in with LL reduction, though?

 

[NewbieInSE]

Thanks Jayrod for your helpful comment.
Also thank you, phamENG for your help.