Soil Factored Resistance? LRFD

[COEngineeer]

So I received a soil report and this is the recommendations they wrote:

"Based upon bearing capacity calculations, we recommend a Factored Resistance (Rr) of 2900 psf for 3' wide footing, 320 psf for 4' wide footing and 3500 psf for 5' wide footing could be used for the proposed footing design of the culvert."

English isnt my first language but I think there is a grammar error in that sentence. But anyway, back to my real question. What do the numbers mean? I just factor my loads and my soil pressure cant exceed that? I have not done a lot of LRFD for foundation.

Never, but never question engineer's judgment

 

[Lion06]

I've never used factored loads for foundation design before (for the soil bearing part), but I have read a few articles that that is where they want to head.

I can't say I've ever seen different values for different strip footing widths either.

Have you tried giving the geotech a call?

As for the grammar, the word "could" should be replaced by "to", or just scrap the would "could" completely.

 

[PEinc]

A quick, condensed explanation: In allowable stress design, you use service loads and an allowable bearing pressure to size the footing. With LRFD (Load & Resistance Factor Design), you use factored-up loads (include various different load ("safety") factors greater than or equal to 1.0 for different load tyoes and then you make sure that your bearing pressure using the factored loads is less than or equal to the factored resistance. The factored resistance is the ultimate bearing multiplied by a resistance factor less than 1.0 to account for variability in the soil.

http://www.PeirceEngineering.com

 

[PEinc]

Also, in the original post, I assume the correct valuse for the 4' wide strip footing is 3200 psf, not 320 psf.

Bearing capacity is a function of soil properties, footing width, and overburden pressure. Therefore, all things being equal, a 5'wide strip footing should have a greater bearing capacity that a 3' or 4' wide strip footing.

http://www.PeirceEngineering.com

 

[Lion06]

Presumably it is what the allowable bearing pressure would be /1.5 or 1.6 (or something close to it).

 

[COEngineeer]

Thanks, I forgot one zero there. Vertical components I have are DL, LL and Impact load (HL-93). I wonder if I need to include impact for foundation design.

Never, but never question engineer's judgment

 

[BlastResistant]

You are designing a culvert and you reference HL-93 loads, so I am assuming this design is governed by the AASHTO LRFD code. For AASHTO LRFD projects, geotechnical engineers give "service," "strength," and "extreme event" limit parameters. Service is unfactored, strength is factored, and extreme event is factored per "extreme event" parameters in Ch. 3 of the AASHTO LRFD code. For example, I am looking at a geotechnical report for a bridge project I am currently working on. For "Box Culvert Foundations" it gives ultimate bearing capacity (i.e., "extreme event") of 13,500 psf, strength limit bearing pressure of 6,700 psf, and service limit state bearing capacity of 4,500. You will notice that ultimate capacity is 3.0 times service limit state, and strength level is 1.5 times service limit state (varies by project and region, but usually pretty close to same multiples on different projects/locations). Service limit state should be used for design of foundation for bearing, while strength limit state and extreme event limit states should be used for structural design of the concrete components.

The factored resistance you are referring to is probably strength or extreme event. You would use this to design your footings structurally. You should ask your geotech to clarify. Also, you should ask your geotech for the service level capacity so you can properly design for bearing. Your geotech should probably be using proper AASHTO terminology so as not to confuse the issue, but probably not a good idea to say that to him.

Also, just to clarify, HL-93 loads are considered live loads and you would use the vertical component as you mentioned to design your footings and culvert walls and slab. But impact is governed by TL loads, TL-1 through 5 based on the speed limit.

You should check out (if you haven't already) Ch.3 for load combos and load conditions and Ch. 10 for geotech info of the AASHTO LRFD code.

If the project is not AASHTO LRFD, then forget everything I said...

 

[BlastResistant]

COE,

If the "impact" load you are referring to is actually the "dynamic load allowance" aka "IM" in the Service Level load combinations in CH. 3 of the AASHTO LRFD, then yes, you do have to account for it in foundation and bearing design. Please see table Sec. 3.6.2 for explanation of dynamic load allowance and Table 3.4.1-1 for the applicable Service, Strength, and Extreme Event load combinations. Also, if you googe Minnesota DOT LRFD Bridge Design Manual, download it. It's a excellent reference and in Chapter 12 there's a perfect concrete box culvert design example.

Good Luck...

 

[BlastResistant]

COE,

In my first post, I said you should ask your geotech to clarify if the factored load bearing capacity is strength or extreme event. Let me correct myself. It is probably strength level. Extreme event is usually governed by seismic events, and for "buried structures" which culverts are classified as, AASHTO code does not require seismic analysis.

 

[BigH]

One comment - service load has been implied for "bearing" - rather, I think, one should think that it is for "settlement". You might have quite reasonable bearing but, due to various factors such as maximum settlements that are less than "normal", extreme size of footing (perhaps with the pressure bulb extending down into a more compressible layer), the service pressures would drop to accommodate the settlement issue.