Analyzing a 1950 concrete frame 8

[MacGruber22]

I feel a bit over my head in this..

So, I have 3-stories of existing RC-frame garage from 1950. Developer wants 3-stories of apartments above. See attached sketch.

This is what I am being told we are doing by my boss:
1. Ignoring gravity loads on existing RC columns because of the reduction in live load.
2. No geotechincal or other laboratory testing will be performed.

So...how am I supposed to analyze and design the lateral system of a structure like this with no material or reinforcement information?

Also, I am supposed to provide the number of hours for analysis and design (excluding detailing and drafting).

 

[CELinOttawa]

I wouldn't suggest quitting... But I have quit when this type of behaviour proved to be the norm. I would, in your case, simply insist on doing it right. If you are on record as saying this should be a seismic/wind upgrade job, and thereafter your supervisor (who is the responsible P.Eng. sealing the set) insist otherwise, you just do the best you can.

I would, at an absolute minimum, show every assumption you need for the system to work as you have designed and ask for contractor to verify.

 

[Brad805]

I do not predict this will go very well, but do your best. Projects that start poorly never get better and this seems to be a case of a design team trying to keep people busy when they should be up front with the developer and come up with a plan that shares the project risk appropriately. A fixed fee on a project like this is nuts and it would not surprise me if there have talks about budgets. Let us know how it goes.

 

[Ron]

With the availability of a variety of nondestructive test methods (radiography is only one....and by the way, X-ray won't work on thick concrete sections. It takes a gamma ray source. Semantic but important difference). GPR, magnetic, ultrasound....etc are available to plot rebar locations.
Further, you can load test the structure and backcalculate from its response.

My point here is that doing only a visual assessment and a lot of guessing does not meet the standard of care for an engineering evaluation such as this. Your boss is placing himself and the firm in a precarious liability position if something goes wrong.

 

[WARose]

I wouldn't suggest quitting... But I have quit when this type of behaviour proved to be the norm.

Same here. If this kind of stuff happens all the time, eventually it will come down to you stamping something like that. At some point I would make it clear to them what you will and won't stamp (this sort of stuff being in the latter category).

 

[MacGruber22]

Thanks, guys and gals - I hear you loud and clear. So...may we talk specifics? Yes? Great - thanks!

So, since the feedback from my superior is that we are PROBABLY not going to retrofit the concrete frame for lateral, I have to figure out a proper way to analyze this thing. ASCE 7-10 12.2.3.2 allows for "Two Stage Analysis", that seems to be geared for podium-style structures. Does anyone have any detailed commentary on this procedure? The ASCE commentary doesn't seem to have anything. The boss wants to "see" what lateral loads we have at the base of the new stories (roof deck of existing) and go from there (ambiguous I know). Little does he know, that getting to that part is not as simple as he would like, particularly with the constraints put upon me and my analysis.

Anyways, the 10/1 stiffness requirement of the lower to upper levels seems like a pain to calculate just to find out if I *can* use Two Stage Analysis.

Thoughts?

 

[KootK]

Is podium lateral going to be moment frames or shear walls? Either way, I don't see you getting the 10:1.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[MacGruber22]

They are going to be wood or light-gauge bearing/shear walls.

That is what I was afraid of - it just doesn't seem realistic, since this is 3+3, which I don't think is even a typical podium configuration.

 

[bookowski]

I think you're correct that 3+3 will your systems will not fall into the two stage criteria. This may be of interest

What is the objection to doing some form of investigation on the existing structure? A simple probe/investigation program would not be hugely expensive relative to the size of the project. Pre WWII is is relatively old and there was some funky stuff going on in concrete design at that time. Can't you at least remove some cover on a few columns/beams/slabs to get a sense of the reinforcing?

 

[KootK]

I was wondering about the lateral system for the base building MacGruber. That's going to be moment frames rather than shear walls, right? Either way, you're probably right that you're barking up the wrong tree.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[MacGruber22]

Bookowski: Yeah, I downloaded that slideshow already. The objection is money, but I am going to push for some cover to be remove at typical beams, slabs, and columns. The building is currently unoccupied, so that is a plus.

KootK: Correct, the lower 3 is a concrete moment frame.

I wonder if I can pull this off with ELF analysis, or whether I have to switch to modal analysis. I am in SDC C, so I am generally eligible to use ELF regardless of my vertical irregularities. But, I am still concerned with the drastic change in the structure period, and whether ELF can handle such a drastic change in stiffness and mass. Or, even how to go about analytically calculating the period for this building, rather than through computer modeling (I have never done that before).

 

[KootK]

To jump on the band wagon, I really think that you're screwed without doing some field testing. Given the building's age, there's a high probability that it wasn't designed for lateral loads in any rigorous fashion. It's unlikely that you'll have the developed positive moment beam bottom steel at the columns that is required to make a go of things in a concrete moment frame.

I have a question for those that are familiar with specifying rigorous testing programs. Do you somehow get rebar lengths / anchorage details out of these testing programs? When I've done this in the past, I usually get information of this sort: 15M @ 200 one way; 20 M @ 250 the other. Without very detailed information, how does one know if bars make it out past inflection points and are are hooked where they need to be etc? My gut feel is that, even with testing, a lot of assumptions are still required. So how does it work?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[MacGruber22]

Yeah, bar development is a problem for sure. I am ignoring that for the time being - I will have to fight that fight once I show the boss the loads at new level 3 (old roof deck). Maybe I will just inflate the loads to such a degree that even he feels inclined to add a separate lower lateral system. He obviously isn't familiar with this type of problem, so I can just blame it on the weird configuration that amplifies the loads. Ugh...I hate having to talk like that.

I have a word document that I am starting to compile all of these concerns. When I finally get an answer for the shear at level 3, my boss will get a pdf listing the concerns as well as an inflated level 3 base shear. I am going to pause on this thread until I get further into the work. Meanwhile, I may start another thread asking some specific analysis questions (try not to beat me up about it applying to this building).

 

[Ingenuity]

I did site testing, sampling and reporting on a mid-1930's RC building for the US government (historic structure) and the rebar was 1" square (non-twisted) WITH small rib deformations and we did some bond testing (in a lab setting) to verify bar development etc.

 

[SAIL3]

I would also email/copy all individuals involved in the project and their superiors your conclusions/concerns as the project moves forward. I have found that if an individual is in the information loop that they suddenly become alot more reasonable.

 

[CELinOttawa]

Ingenuity: Did you ever get someone to identify the specific type of old bar? Sounds like Corrugated bar or Universal bar, more likely corrugated if it was quite square....

 

[Ingenuity]

CELinOttawa: "Corrugated", Type D, I think. See attached, 2nd page.

 

[MacGruber22]

I started out in bridge rehab and analysis, and almost every pre-WWII culvert and bridge had square bar. I wouldn't be surprised if my garage has the same.

I'll keep you in mind, Ingenuity, if I ever get to that point.

 

[CELinOttawa]

Kootk: My experience has always been in the context of a seismic upgrade programme. Where we did not have good drawings we flat out adviseethe owner that a comprehensive upgrade programme was a must right at the outset.

The details you're talking about are of preeminent importance when doing an existing capacity evaluation... Those reports always read the same way: "Your structure has X% latteral load bearing capacity with respect to the current code. This cannot be interpreted as a competence of X%, however, due to the existing detailling not meeting current code requirements, thus not providing the robustness required of a modern structure, and as such a full seismic upgrade will be required to permit change of use."

We own and do our own Schmidt hammer surveys. We rent a Profoscope, and are currently considering buying one. GPR can give you better information about the laps, etc, as can some of the radiographic methods. GPR and radiographic are both more expensive, and I have never believed them to be worth the additional cost when the answers are invariably known - The detailling simply doesn't meet current requirements.

Why spend the client's money looking for the miracle of a pre-late 1950s structure with long laps and continuity steel?

 

[KootK]

Why spend the client's money looking for the miracle of a pre-late 1950s structure with long laps and continuity steel?

Thanks for your comments CEL. This is precisely my concern. I don't understand the emphasis on testing unless:

1) We're willing to make dubious assumptions about the detailing OR;
2) The testing is comprehensive enough to shed light on the important details AND we're willing to tolerate a lot of detailing that would not satisfy modern code requirements.

Without going down one of these paths, it truly does seem like wasting the owner's money because, as you said, we generally know the answer before we start the testing program.

As an example, I once looked at refurbishing a large light well in a heritage building to accommodate street traffic including fire trucks. We had some testing done and determined that the simple span concrete beams in question were generously reinforced. Several 25M for bottom steel. Yay! During a subsequent site visit, I got a glimpse of some of these 25M near a foundation wall support where the bars were exposed. All of the bars terminated about 3" shy of the wall! Fail! I made a go of it by claiming partial fixity and relying on plain concrete capacity for a stretch. Not too proud of that though.



The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[JLNJ]

If something were to happen, I can't see the "we assumed it was designed correctly to start with" argument holding much water with the judge.

That said, I don't know what the courts hold as the standard of care for this kind of retrofit.