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?

 

[civilperson]

The soil capacity will be given by the geotech for either ASD loads or LRFD loads or both. Call the geotech and request your load type capacity.

 

[vmirat]

Yeah, I checked the soils report. It doesn't specify which load combination system to use for the allowable soil bearing capacity given. However, what if the soils report is based on ASD? That's my problem. If I have to do two separate sets of calcs to get to ASD for foundation design, then I have a real problem with LRFD as the preferred method in today's world.

 

[BAretired]

Soil reports normally provide allowable bearing pressures, not factored. Carry your live and dead load down separately. For footing size, use the sum of the two. For concrete and reinforcement design use factored loads. It is not an onerous requirement.

BA

 

[JAE]

vmirat, what BAretired says is true. Anytime you see the word "allowable" you've got an ASD situation. Your geotech most likely is giving you an allowable bearing pressure to be used with non-factored service loads. Usually they use grand safety factors such as 3.0 to arrive at their recommended pressures.

For LRFD design, you design the structure for strength using factored loads against your nominal strength [φ]R_n. You design your structure for servicability using non-factored loads (i.e. deflection control, vibrations, etc.).

When you get to your footings, you use LRFD for the strength of the footing. Then use service loads for checking soil strength capacity for sizing the footing.

 

[vmirat]

JAE,
Thanks for the response. As I said in my post, I'm an old ASD guy. I was hoping to use LRFD only for this design, but it seems that's not possible. It seems to me that, if I did this design using ASD, I would only have to do one set of calcs.

 

[271828]

LOL, you're going to give yourself high blood pressure if you don't let it go...

 

[Lion06]

Even if you could do the foundations in LRFD, you still need service loads for deflections and drifts.

 

[miecz]

Why doesn't the code specify deflection and drift limits for factored loads, thereby saving us all a lot of work?

 

[hokie66]

Heh, heh. Because, miecz, deflections and drift are real, and factored loads are not.

 

[271828]

The code doesn't usually specify any deflection or drift limits in factored or unfactored loads.

LOL, why not make your own deflection and drift limits that can be compared to factored loads? For wind, you could convert H/400 for unfactored, 10 year wind to H/whatever for factored, 50 year wind. It's a simple ratio. Same for 1.2D+1.6L, assuming some ratio of L/D. Knock yourself out!

The same could be done for bearing pressure also, obviously.

Honestly, I'll never understand this whole issue. At the last two firms I worked for, they adopted AISC's first LRFD Manual as soon as it came out and used LRFD ever since. I'm sure they go just as fast and make just as much money as anybody whose green book has fingernail marks on it from the desperate attempts to hang onto it as others pull it away. People just get used to carrying D, L, etc. separately and combine as needed. It's thoughtless after a while.

 

[JAE]

ok....deep breath...here we go again....the old LRFD vs. ASD challenge/debate and general fisticuffs marquee.

In this corner....

 

[msquared48]

WSD rules...

Mike McCann
MMC Engineering

 

[BigH]

In 90% of the cases, the bearing capacity is meaningless - as JAE and others have pointed out it is the structure's ability to withstand settlement and distortion. The big problem with LRFD is you spend a lot of time on the computations - to dismiss them anyway.

 

[asixth]

I have been raised in the LRFD era and I can't understand why there has been such a large shift in the way that foundation elements are designed.

For example, bearing elements have been designed for decades using a factor of safety (FOS) of 3. This is accepted as good engineering principles. Now with the introduction of LRFD, the loads are factor 1.2*D+1.6*L, lets say this averages to give us design actions 1.4 times greater than ASD. But instead of applying a FOS of 3 on the bearing capacity, we apply a reduction factor of 0.5 to the ultimate capacity. When these numbers crunch out, our total factor of safety when designing to LRFD is 2.8, making the element less safe than designing to ASD.

To me, the design should be the same if designing to LRFD as ASD. I know there is the argument that live loads are more variable then dead loads and so should receive a higher load factor, I just don't like hearing "Design to LRFD and you may get more capacity", I don't think the design philosophy of these elements should change.

 

[271828]

"ok....deep breath...here we go again....the old LRFD vs. ASD challenge/debate and general fisticuffs marquee. In this corner...."

LOL!!! No, I'm in too mellow of a mood for that today. Besides, vmirat already said in another thread a couple of months ago that he didn't have any respect for my opinion anyway.

I will say that I'm starting to wonder about folks who get so worked up over this issue. Is there nothing else to think about?! I mean, come on, there are real issues in the world to think about: taxes, religion, Middle East peace, end of the world in 2012, what happened on a reality show last night, etc. Whether I have to do two calculations, one of them D+L and the other 1.2D+1.6L doesn't show up as a blip on my radar screen. I have to submit something today, though.

 

[asixth]

Is the world ending in 2012?

 

[271828]

Some are saying that it will. I don't have TV, so just caught wind of this the other day. Some kind of Mayan prediction or something. I think there's a moving coming out about it.

In any event, even if it's just a movie, that would seem to more rightfully command attention than D+L vs having to deal with both D+L and 1.2D+1.6L, LOL. After all, I might have to get money out of an ATM and THEN drive to the movie and pay for tickets. What a pain.

 

[miecz]

Because...deflections and drift are real, and factored loads are not.

You know...I think you're onto something.

 

[mtu1972]

My hat is off to you for doing a LRFD design. No matter how many times I say it's time to make the switch, I just never have.

Both the 8th edition (Red book printed on "Bible Paper" to make it thinner and more briefcase freindly) and the 13th edition are on my desk - but the 13th only gets used to look up sections that were not published in 1980.

GJC