Foundation Reactions 1

[SteelPE]

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.

 

[ToadJones]

Can you find a new foundation guy?

 

[SteelPE]

Toad,

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

 

[audeuce02]

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.

 

[weab]

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?

 

[hokie66]

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.

 

[LowLax]

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.

 

[Bernied]

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.

 

[mijowe]

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?

 

[nutte]

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.

 

[msquared48]

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