Lateral Earth Pressure Question 2

[Rabbit12]

I'm doing a preliminary analysis on a buried structure from the early 1960's.

Reviewing a Geotech report from the same site and they are recommending a lateral at-rest pressure of 60 pcf above the GWT and 100 pcf below the groundwater table. Then they have this tidbit: "The values calculated for the above parameters would provide ultimate values. We recommend a minimum factor of safety of at least 1.5 be applied to the calculated lateral values."

Help me interpret that. If I apply the loads (60 and 100 pcf) to a wall and I'm interested in the forces in the concrete would I need to add the 1.6 load factor per ASCE 7 or would that be double dipping?

 

[r13]

Both show the concrete failing, but I'm assuming rebar strength (40 ksi), concrete strength (3 ksi), and...

Weren't the rebar and concrete strength indicated on the drawing (I assume you've the drawing otherwise you wouldn't know the As and pattern)? I'll make the same assumptions as you did. But now, since all methods indicate falling, it is time to dig a little deep into it, because if I am the owner, I might buy into that logic that says "it has been there for...", but then turn my head and say "gee...!".
Since the cover is to be removed eventually, I suggest to take this opportunity to take a core sample, and have a lab analyze the rebar and the concrete.

and the loading is likely not what was used for original design.

Why? What you think likely had been used?

The unity levels are over 1.5.

I don't understand where you get the "utility level", can you elaborate further?

 

[Rabbit12]

Retired13, the rebar and concrete strength was not in the drawings hence the assumption. I'm not terribly surprised that information isn't shown. I've done lots and lots of existing modifications and it's a crap shoot whether that information is shown. It's also possible that sheet is missing. Thankfully I had section width and rebar layout to use.

I have no idea what loading they used in 1962. The drawings don't show anything so it's another variable in my analysis. Likely leading to my trouble making the structure check out on paper.

Not sure what's confusing on the unity. Mu/Mallow.

I don't agree that we need to dig into this deeper. As you stated earlier, the structure has worked for almost 50 years. I've analyzed the changes we are making and they are acceptable when compared to the original in place structure per an adopted code (2018 IEBC) why would I take this any further? Asking the owner to spend several thousand dollars just to prove something on paper seems like a waste.

 

[r13]

I wouldn't think it (core sample) is a waste, if I am expecting to do more works in the same yard and for the same owner. But it's your project. I drop my case.

 

[r13]

I got the word "utility" now, thanks. But here is my little note, historically ACI USD load factors were:

DL - 1.5 (early 60s), and 1.4 (since 70s)
LL - 1.8 (early 60s), and 1.7 (since 70s)

I consider lateral earth pressure and hydrostatic pressure both are live load. Since no dead load is involved in the design of the wall, the utility number should be greater than 1.5. Agreed?

 

[r13]

I retract my last post. Your finding makes sense.

 

[MTNClimber]

I don't usually pay too much attention to what most geotechs say

How dare you... just kidding.

Some of the best geotechs I've worked with could do both (relatively basic) structural design as well as geotechnical work. Actually, my university didn't offer enough graduate classes to only take geotechnical courses so most of us took 1/2 geotech and 1/2 structural graduate courses. Geotechs that could do structural design seemed normal to me until I moved west. And now I'm very grateful for my "geo-structural" background. It helps me to get more work but also helps me translate concepts to either my boss or to the structural engineer on the project. I personally wouldn't want it any other way.

 

[BridgeSmith]

Some of the best geotechs I've worked with could do both (relatively basic) structural design ...Geotechs that could do structural design seemed normal to me until I moved west. And now I'm very grateful for my "geo-structural" background.

I'm on the other side of that - I'm a structural trying to muddle my way through doing half of what a geotech would normally do (without any formal training in it), because our DOT only employs geologists and structurals.

Rod Smith, P.E., The artist formerly known as HotRod10