LRFD Foundation Design 3

[vmirat]

I'm attempting to design a building using only LRFD (OK, old ASD guy here). I'm down to the foundation design at this point. The problem is that the 2006 IBC allows either LRFD or ASD load combinations (1805.4.1.1). It seems unfair to use the LRFD column loads since the ASD loads would be much less. Is there a compensating factor for LRFD that equals things out or do I have to go back and recalculate everything using ASD load combinations?

 

[steellion]

Footing sizes would be determined using "ASD" load combinations compared to the allowable bearing pressure of soil. The concrete and reinf that comprise the footings would be designed using "LRFD" combinations (actually USD combinations because LRFD is a steel term, but they are the same). You can also get conservative footing sizes in CRSI.

 

[Stillerz]

Am I missing something here???
I woould think the ASD/LRFD design/debate would only be for the steel structure and have virtually nothing to do with designing the foundation. Allowable soil bearing pressures for sizing the footings would be based on service loads, not factored loads. From there, you essentially have to design the footings per ACI ...which is an "LRFD" approach.

 

[PSlem]

I'm of the ASD school also but I wondered the same so today I picked up Resistance Factors for Use in Shallow Foundation LRFD by K.C. Hoye, R. Salgado and B. Scott. It says factors are needed for reliability index. e.g. CPT did better than SPT and they decrease the soil resistance factor as LL/DL increases due to uncertainty. Resistance factor ran from .17 to .32 for sand and .6 to .75 for clay. I'm still studying it, but looks like improvement over ASD.

 

[Stillerz]

"...improvement over ASD. "
Again, I am confused here. "ASD" is a term used in steel design.

 

[steellion]

You're right, Stillerz. Even though ASD and LRFD are strictly steel terms, some are using ASD to refer to unfactored load combos and LRFD to refer to factored load combos.

In the ASD vs. LRFD argument, I side with ASD because there is no need to switch load combos when it comes to footing design for bearing capacity and sizes beams for serviceability conditions.

 

[271828]

Then how do you design the concrete footing? Do you not need 1.2D+1.6L for that?

 

[Stillerz]

For thickness and reinforcing, yes, use your factored loads. For the footing footprint, you use service loads because you are working with an "allowable" bearing pressure.

steellion: 100% totally agree with the ASD vs. LRFD thing although for steel desing I use STAAD alot so swithing load combos is not really an issue.

 

[271828]

Right. My point is that everybody, on practically every project has to deal with D+L and 1.2D+1.6L because every bldg has a foundation.

 

[vmirat]

This brings me back to my original post. Instead of running two sets of calcs (service loads and factored loads), has anyone ever used a factor that can be applied to the factored loads (LRFD or ASD) to approximate service load conditions?

 

[hokie66]

Yes.

 

[vmirat]

HA! Message received, hokie66. I guess I'm back to doing it the long way.

 

[BAretired]

Not exactly. What I have done upon occasion is the other way around. If I know the total load (D + L) but I'm too lazy to determine the exact breakdown, I use a load factor somewhere between 1.25 and 1.5 in Canada (or between 1.2 and 1.6 in the U.S.A.).

To be conservative, use 1.5 (Canada) or 1.6 (U.S.A.).

If you estimate that Dead Load = 20% of total load, then the load factor would be 0.2 * 1.25 + 0.8 * 1.5 = 1.45 in Canada or 1.52 in the U.S.A.

For concrete structures where Dead Load is much higher than 20% it is probably better to run the dead and live loads separately.

BA

 

[hokie66]

I agree with BA's way as being more typical of what I have done. My other flippant reply just means that somewhere along the way I am sure to have done that too. I don't think it is lazy, just some cases allow for simplification. As long as you note what you have done in your calculations, it is all good.

 

[vmirat]

BAretired,
Thanks for providing an answer to my question. I've actually used 1.5 on really small projects before, although I would never recommend this to anyone. This is a small (3,000 s.f.) single story addition to an existing large building. Sometimes I wonder about the amount of effort required to design things.

 

[BAretired]

I consider it good engineering practice to make approximations of the sort we are talking about when more exact calculations are unlikely to affect the result in any significant way.

Let's face it...we don't know the magnitude of live loads precisely. The live loads listed in our code already contain a factor so they are conservative, particularly for floors. Sometimes the dead load is modified in the life of the building...more insulation or gyproc is added, more gravel is placed on the roof or some equipment is added.

Nothing about this work is precise and those who calculate with double precision on the computer are kidding themselves if they believe they are any closer to the truth than calculations produced on the good old slide rule.

End of rant.

BA

 

[Stillerz]

BAretired-
Spot on man!!!
I love it when engineers knit-pick the hell out of code requirements, wildly scientific factors and design procedures in order to obtain some "perfectly designed" structure all the while using educated guesses for the structures loading....especially for wind & seismic. Engineering design is an applied science, it is not a science itself. None of us here is named Castigliano or Timenshenko.

 

[miecz]

The problem, as I see it, is that we're only going half way with LRFD. If we're going to design with factored loads, then we should design the whole structure to factored loads, including the foundation, the drift and the deflections. That means the geotech needs to issue bearing capacity, not allowable bearing pressure. If we're not going to do the most important part of the design to factored loads, then forget about 'em.

 

[broekie]

For the bridge work that I do, our geotechnical engineers do give us factored bearing capacities.

 

[271828]

LOL, I already use LRFD for drift checks, beam deflection checks, floor vibration checks, and bearing capacity checks. The load combinations just have 1.0 factors.

 

[miecz]

A factor of 1.0, or 1, or 1.00000 is no factor at all. It's a check at service loads.