Foundation Design

[ikram]

we are in the process of evaluating the existing foundation of a new heat exchanger with increased load.

Our vendor has asked to reduce the load by factor of 1.4 as per ASD approach. Upto my knowledge LRFD method is used in case of concrete. Also, ACI-318 also use LRFD method in its calcualations.

Can any one advice, what will happen if we allow the vendor to reduce the earthquake load by the factor of 1.4 i.e E/1.4 in load combination

Regards

Ikram Ali Khan

 

[IL-SE]

You should be using ASD to check the soil bearing, overturning, and sliding (you can use the ASD equation with 0.7E or the "alternate basic equation" using E/1.4).

The concrete needs to be checked using the LRFD equations (1.0E).

 

[ikram]

You should be using ASD to check the soil bearing, overturning, and sliding (you can use the ASD equation with 0.7E or the "alternate basic equation" using E/1.4).
The concrete needs to be checked using the LRFD equations (1.0E).

 

[ikram]

Our existing foundation with new loading has been checked as per UBC 1997 code. The earth quake load is one of the mjor component. So the vendor asked to reduce the load by the factor of 1.4 by using ASD load combonation.

By doing so there will be needed no modification in our exisiting foundation. On the contrary, using the combinaiton based on LRFD approach the load will exceed the limits and following modification is proposed.

A change in fixed foundation plinth, you have to remove the soil around it and provide teh new casting to tie the actual plinths with the two lateral foundation.

Now in this case can we allow the vendor to use the ASD code combination to bypass any modification in the foundation.

Best Regards

Ikram Ali Khan

 

[dastuff]

Concrete is LRFD.. Soil is ASD..

More importantly the current code is the IBC '06 (not the UBC 97). Aside from that i don't fully grasp your question.

 

[IL-SE]

Are you checking the soil bearing pressure or the concrete strength? That should tell you your answer. You cannot use E/1.4 in an LRFD combination, just as you can't compare nominal strength (phiMn) to an ASD combination.

You need to compare apples to apples. If you have an allowable capacity, then use ASD, if you have a nominal capacity, use LRFD.

What is still unclear?

 

[ikram]

Are you checking the soil bearing pressure or the concrete strength? That should tell you your answer. You cannot use E/1.4 in an LRFD combination, just as you can't compare nominal strength (phiMn) to an ASD combination.
You need to compare apples to apples. If you have an allowable capacity, then use ASD, if you have a nominal capacity, use LRFD.

What is still unclear?

Dear IL-SE

We are analyzing the concrete strength of existing foundation with increased load of exchanger. UBC 1997 is our design code. For the analysis purpose, we have to use code combinaiton as per ASD or LRFD approach.

Now i think the scope of my question is clear.

Now from above discussion i have got two understandings.

1. If concrete of existing foundation is to be checked with increased load then load combinaiton is to be based upon LRFD approach.

2. If foundation allowable soil bearing capacity is the prime concern in term of settelement then load combination is to be based on ASD approach.

Now, in case of our foundation both of the above criteria is to be checked. So, in this case which combinaiton (ASD or LRFD) is to followed.

Also, can you please provide the basis for, If you have an allowable capacity, then use ASD, if you have a nominal capacity, use LRFD.

Best Regards

Ikram Ali Khan

 

[IL-SE]

You have to calculate both combinations and use each where it is appliable. Nominal refers to LRFD, where you take the capacity and multiply it by a phi-factor (typically 0.75-0.9 depending on the check), allowable refers to ASD when you have the capacity and divide it by a safety factor (typically 1.5-2.0 and up to 3.0-4.0 for soil)

For soil, you should have an allowable bearing capacity, which is checked against the ASD combination.

For concrete you have a nominal strength (phi-Mn, phi-Vn, etc.) that is checked against the LRFD combination.

You have to do BOTH ASD AND LRFD COMBINATIONS and apply them to the different materials. ASD for soil, LRFD for concrete. You cannot use E/1.4 in an LRFD combination.