Soil Factored Resistance? LRFD
Soil Factored Resistance? LRFD
(OP)
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.
"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






RE: Soil Factored Resistance? LRFD
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.
RE: Soil Factored Resistance? LRFD
www.PeirceEngineering.com
RE: Soil Factored Resistance? LRFD
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.
www.PeirceEngineering.com
RE: Soil Factored Resistance? LRFD
RE: Soil Factored Resistance? LRFD
Never, but never question engineer's judgment
RE: Soil Factored Resistance? LRFD
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...
RE: Soil Factored Resistance? LRFD
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...
RE: Soil Factored Resistance? LRFD
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.
RE: Soil Factored Resistance? LRFD