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Foundation Reactions

Foundation Reactions

Foundation Reactions

(OP)
I have a job where I am not responsible for the design of the foundation system.  I have all of my loads broken down into service dead, live, wind and seismic values.  I prepare my reactions this way so that the foundation engineer can properly design his foundations.  

The reactions have been submitted to the foundation engineer who is refusing to design the foundation until I give him worst case loading on his foundation system.  Basically he is asking me to interpret the load combinations of the code for him.  I avoid doing this because of the increased liability.  

What do you guys do with providing reactions to another engineer to design foundations?

I understand with computer programs today, it is easy to export the reactions after they have been run through the load combinations.
 

RE: Foundation Reactions

I find it surprising that he is asking you to do his work. The equations are not too many or too complicated. I agree that you are taking on liability because you may have different combinations controlling the design.

We are Virginia Tech
Go HOKIES

RE: Foundation Reactions

I agree with Slick here....
I worked for a firm at one time where we did very little foundation work so we were issuing foundation reactions all the time.
Your foundation guy should be figuring out his own load combo's.
We usually factored the loads up quite a bit and issued them as "not to exceed" since they were often required early on in the job. We had to leave ourselves some wiggle room since we were essentially promising "not to exceed" them.
So we factored them up and I am sure the foundation guy threw some extra safety in there too....all said and done we probably had foundations that were 2 times bigger than they needed to be.

RE: Foundation Reactions

...still baffled that he is asking you what the worst case load is on his system.
That would be like the building designer asking a code official or a equipment vendor what is the worst case load on the building.  

RE: Foundation Reactions

Looking at it from the foundation engineer's perspective, a lot of it has to do with how the reaction data is described and provided to the foundation engineer.  I've seen lots of metal building drawings with several wind load cases (up, down, left, right, wind on parapet or eave only) and different dead load cases (building framing, collateral).  If these are merely labeled W1, W2, W3, etc., how is the foundation engineer supposed to make sense of it?  In cases like these, where interpretation of the submitted reactions is difficult or likely to result in error, it is completely understandable for the foundation engineer to ask for the worst case reactions.

RE: Foundation Reactions

nutte-
I can see your point there, but it sounds like he has his reactions pretty solidly labeled.  

RE: Foundation Reactions

(OP)
Nutte,

I have designed metal building foundation systems many times and I hear your frustration with the way the reactions are presented.  However, it is very easy to cut through the BS and figure out what loads are applied to the foundation system.   To ask the metal building mfr. to make this interpretation for you is risky.  How do you know they are even giving you the proper reactions for your foundation design?  Do you back check them, or just say that it is not your responsibility and place a not on your drawings stating the reactions you designed your foundation for were provided by (fill in the blank)?
 

RE: Foundation Reactions

Write the engineer a letter:

Dear Mr. Engineer:

My building reaction loads are as follows:
DL = xx lbs
RLL = xx lbs
RSL = xx lbs
RWL1 = xx lbs
(etc.)

The building code outlines several load combinations that are required.  You, as the foundation engineer, can simply take the above load cases and combine them in accordance with the code using simple high school algebraic methods.

Should you require assistance with these basic procedures, we would be happy to assist you in developing a comprehensive plan and solution to your calculation needs.  

Attached are our standard hourly rates that would be applicable should you choose to utilize our services.

As an estimate, we would expect the fees for these services to be between $3,000 and $4,000.  Please advise us if you desire to pursue this and we will quickly get you a fee proposal contract form.  A retainer fee in the amount of 50% of the above estimated fee will be required.

Sincerely,

xxxx


So my thoughts - shame them into doing their own work.

 

RE: Foundation Reactions

off subject, but we actually had a utility company tell us once that we had to show them, on a drawing, where THEIR utilities were running underground before we could get a permit. The reason we were calling this utility was to get them to mark their lines.
I was totally baffled....

RE: Foundation Reactions

He might only ask for a favour from you.

By the way, it may not be conservative to have one 'worst' case to check. I did have combo reaction forces from my colleague who worked on superstructures. But I have to make my judgement to figure out the worst cases, e.g. a smaller vertical reaction is likely to a worse case when checking stabilities (overturning or sliding). Section 1605 of ASCE7-05 should be followed in your case.
 

RE: Foundation Reactions

(OP)
Oversee,

I completely agree.  How am I to know what is going to control the design of the foundation system.  I have a general idea, but since I am not the designer it is not my responsibility.

RE: Foundation Reactions

The engineer should be able to do it himself, but I see some metal building suppliers print the reaction envelopes for each column.  I always appreciate it.  The code dictates the load combinations.  The building supplier is designing the building for these combinations.  Why not just print the results?  That being said, I would never ask if they weren't provided, I'd just figure it out.

RE: Foundation Reactions

Toadjones,

We were doing an excavation bracing job about 40' deep requiring tiebacks.  The phone company told us that their west coast trunkline was under the street behind the cut somewhere, but for security reasons they couldn't tell us where.....  

RE: Foundation Reactions

For bridge foundations, I generally provide a table of loads in all 6 DOF for each of the required load combinations. I clearly label the point at which the loads apply, and I state whether the loads are service or factored.  All of this is necessary to design the foundations and should be developed by the structural engineer.  There is no way I would or could choose one of the load combinations and say it is the worst case.

RE: Foundation Reactions

(OP)
The code requires use to use superposition to define the loads on the foundation system.  However, under the new AISC procedures, we are required to do some form of a P-Delta analysis on our structure.  With a P-Delta analysis, the theories of superposition do not apply because the loads due to a P-Delta analysis are non-linear.  This means giving the pure dead, live, wind and seismic loads would not be accurate?  

What effect does this have on the overall question above?
 

RE: Foundation Reactions

dcarr-

I think their security "reasons" looked a lot like donuts and coffee smile

RE: Foundation Reactions

SteelPE..

That may be true of the superstructure (and specifically internal stresses in the members), but the reactions are the reactions.. The foundation engineer can use the loads and decide whether he is to carry out a linear or non-linear analysis of the foundation system.


I'm with the majority on this one..  seems like the foundation engineer doesn't want to do his job.  

RE: Foundation Reactions

(OP)
Getting the engineer to do his job is proving to be very difficult.  It's like dealing with a little child.  He refuses to begin the process without the "minimum and maximum" loads on the foundation.  The owner just wants the job done and doesn't care who does the work.  Unfortunately, everyone wants me to resolve the problem at my own cost.

RE: Foundation Reactions

It actually scares me a little that he wants "minimum" and "maximum" loads.
Is he just going to use those loads only?

when doing my own buildings and foundations, I may have tons of load combos for the steel design.
When it comes to foundation time, I just use all my service level reactions and make up some new combinations...i have never just used the load envelop and designed the foundation for that!

RE: Foundation Reactions

(OP)
ToadJones,

And nobody realizes why I want no part of this (at least outside of this forum)!  You need strength level loads to properly design the concrete in the foundation system.  How are min and max loads going to help him with this.

RE: Foundation Reactions

Can you find a new foundation guy?  

RE: Foundation Reactions

(OP)
Toad,

I wish, actually, I wish I could dump the contract but I can't.

RE: Foundation Reactions

Am I thinking wrong, or does ACI 318-05 not require the use of factored service loads.  The maximum reaction should be used for a check only.  I agree with JAE, send him a fee letter stating what it will cost him for you to do his job.

RE: Foundation Reactions

I share this concern.  I have seen many different types of loads at a single joint such as dead, live, material, seismic, wind - vertical and lateral loads for each of these where some should be + and  others - concurrently.  Not knowing the structure geometry, it may be impossible to know if +ive or -ive loading applies in any particular load case.  Some loads don't apply at the same time as others.  It can be very confusing to determine the worst case.  And the worst case calculated by the foundation engineer can be way worse than reality causing overdesign.  

I have seen one engineer place a huge table on the design drawing in order to explain the load cases.  I don't see how clarifying one's own design load cases increases one's risk.  It would seem to me that clarity lowers risk.

Am I not understanding properly?

RE: Foundation Reactions

The only kind of footings which I ask someone else to design are piles.  Even then, I design the pile caps or mat myself.  I already have the loads, so why not give them to the engineer designing the piling.  I think there is too much possibility for confusion in asking someone else factor and combine loads for a bunch of loading cases.

RE: Foundation Reactions

Maybe it depends on what kind of foundation we are talking about.  If it is isolated footings under individual columns, I don't see how providing minimum and maximum reactions (load combinations) could be a problem.

I can see where if it is a mat foundation or something similar the maximum and minimum reactions might not reflect the maximum and minimum forces on the foundation system.

RE: Foundation Reactions

SteelPE, I agree that simply giving maximum and minimum loading combinations for each column is particularly dangerous when designing foundations against sliding for vertical bracing systems. It's often advisable to combine the two (or more) footings in order to pick up vertical load from one footing to help the adjacent footing with minimum or even negative loads. But it may not be true that the maximum case on one footing occurs simultaneously with the minimum load on the adjacent footing.
I think it's the foundation engineer's job to combine the individual service loads as he/she sees fit.  

RE: Foundation Reactions

I agree that he should just be able to use the laod combinations and figure them out himself but..

If you were  give him the solved equations for the 7 Strength Design and 8 Allowable strength combinations (referencing ASCE 7) essential giving him 15 loads max and min included, would he be happy then, or would he still require interpertation?

 

RE: Foundation Reactions

Quote (mijowe):

If you were  give him the solved equations for the 7 Strength Design and 8 Allowable strength combinations (referencing ASCE 7) essential giving him 15 loads max and min included, would he be happy then, or would he still require interpertation?
In the case I mentioned at the beginning of this thread, that's exactly what I would suggest: the building designer provide a list of each load combination with the corresponding reactions for that load combination.  Otherwise, the foundation engineer is forced to interpret the building designer's load cases, which may or may not be easy to do.  Although, with the various wind directions and collateral load variations, these 15 load cases might be closer to 100.  Nevertheless, that is still data that the foundation engineer can use with minimal risk of misinterpretation.

RE: Foundation Reactions

I have seen it both ways over the last 32 years or so of designing metal building foundations.  The thing is that to size the footing footprint, you need unfactored, working loads, but for the design of the concrete foundation itself, you need factored loads.

Personally, I like to see it presented both ways so that I can see what the metal building designer did, and manipulate the numbers to suit what I need to do.  I just feel more confortable with that.  Hard to get, but preferred.

No worries.  

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

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