Analyzing a 1950 concrete frame
Analyzing a 1950 concrete frame
(OP)
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).
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).






RE: Analyzing a 1950 concrete frame
With regard to your design, I would run 2D frame analyses of the existing structure in both directions as required (i.e. Typ. interior and exterior frames). This will give you existing member and foundation loads. I would not worry about design. I did not start working until 1973, but even then frames were done with 2D Moment Distribution or the Portal Frame method. Unless you request testing, I would assume f'c=3000psi and fy=33,000 to 40,000psi (and probably does not matter for analysis only).
With the reactions and the existing footing sizes you can back-calculate for the probable allowable soil pressures. Remember that design was done with actual loads (Working Stress) so don't bother with LRFD load factors, etc.
Then rerun with the new proposed Live Loads and the added loads from the 3-story addition. You can easily compare new vs. existing member loads. Your boss's intuition will probably be proven correct.
Lateral loads present additional problems, but again you can compare expected original forces with the proposed new forces. Depending on location both wind and/or seismic will have drastically changed from 1950.
If snow loads are a concern, they also may have changed dramatically.
Where your schematic shows a new wall landing on mid-span of an existing beam is somewhat disconcerting. Something special may need to be done there.
Can't help you with the hours, etc. as it depends on your tools and efficiency. Good luck.
gjc
RE: Analyzing a 1950 concrete frame
I would estimate it as if you're designing a new structure within the envelope of the existing structure. In other words, you're adding new frames within the boundary of the existing. That way you can sleep at night and as a bonus, they're unlikely to build it. If anyone asks why the design is so complicated/expensive/impossible to construct, just tell them that's what you get when you don't have any information.
RE: Analyzing a 1950 concrete frame
I have been told that we can provide additional lateral frames at exterior walls to supplement what we do not feel great about. Also, as you mentioned the new walls bearing on the slabs and beams, I have been told we can provide a framing system above the existing roof level if required to take all the loads to the columns. So, the good thing is that it appears we have a decent amount of leeway to strengthen the existing as required; I think that is trade-off to a large-scale testing program. If they (the owner/contractor) doesn't like the strengthening costs, they can pay for testing.
Do you think it would be helpful to get a hold of the ACI 318-47? I feel like I would benefit from having reinforcement ratio min and max for all the existing members at the time of construction. Something tells me that. Also, I imagine that I apply the same drift limitations as completely new construction?
No problem with the hours. As long as I have a clear plan-of-action with my analysis and design, I can figure that out.
Thank you, again!
RE: Analyzing a 1950 concrete frame
BA
RE: Analyzing a 1950 concrete frame
Well, I feel your sentiment, BA. But, this is the task I have been given with the limitations set before me. So, with the that given, I have no choice but to do what I need to do in order to make this structure safe and reliable. Maybe, one day when I have my own firm or am high enough up the ladder, I can say that I am not doing anything without a comprehensive testing and evaluation program.
RE: Analyzing a 1950 concrete frame
RE: Analyzing a 1950 concrete frame
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Analyzing a 1950 concrete frame
RE: Analyzing a 1950 concrete frame
RE: Analyzing a 1950 concrete frame
Trenno - I would love to x-ray; however my boss has convinced the owner that we don't need to do it. Once you tell a client that, you better be right, because they aren't going to want to renegotiate that.
Brad805, I agree.
Paddington, it won't be new columns on the existing floor slabs/beams, it will be light-framed wood or cold-formed steel walls. But, agree it still is going to cause an issue.
RE: Analyzing a 1950 concrete frame
I have added floors to existing buildings. Several times in fact. With one exception (a rather well built twelve storey beauty from the 1970s where we had impeccable drawings and AS-BUILTS - cue miracle sounds) I have always insisted on detailed field inspection, testing, followed by seismic upgrade for both existing and new loads.
It is often VERY easy to add new braced frames into these 1950s structures to give the building a proper seismic system. The concrete is usually of good quality and strength (get a Schmidtt hammer, as well as take some cylinders - Bear in mind it was very common practice to mix concrete stregths. Columns are often a better/stronger mix than beams, which may be another layer of quality and strength away from the floors). The issues are ensuring you have good load paths and connections... Something only field review will tell you. Even with existing drawings, you need to do detailed field reviews to confirm.
No, I don't get every job. Yes, I do quite well for myself, and I sleep very well too. Thanks for asking. *smiles*
RE: Analyzing a 1950 concrete frame
What I have learned over the years is that minimum shear reinforcing was not required until much later, so that may be a concern.
Once you have the member loads (Axial, Moment, and Shears) you can make the direct comparison of proposed new vs. reported existing and then determine where any additional analysis/design work is required.
I have worked on a lot of old reinforced concrete industrial/manufacturing facilities, but usually had access to any pertinent drawings, etc. That makes it a lot easier to evaluate.
gjc
RE: Analyzing a 1950 concrete frame
I don't think much of your boss for putting you in such a pickle.
RE: Analyzing a 1950 concrete frame
If I were going to go through with this assignment, I would write an office memo outlining all of the anticipated dangers of proceeding with construction. These would be developed over the time I was working on the proposal and presented to the boss before any construction actually took place.
It would be preferable to increase the width of the new floors and roof to match the width of the existing building; in that way, the new loads would bear directly over columns and avoid the concentrated load on the existing span.
BA
RE: Analyzing a 1950 concrete frame
Here's ACI 318-47: Link.
That's all I've got with respect to constructive comments. Now for some ranting...
There is some irony in the fact that, once you're a head honcho in your own right, you will almost certainly find yourself assigning a higher priority to client pleasing than you will to technical merit. And that's not meant to be a knock on your supervisor. It happens to us all. It's the circle of life in a profession that is expected to safeguard the public good while simultaneously competing for work as a commodity.
Philosophically, I've always been troubled by renovation strategies that assign capacity to exiting structures based on the assumption that:
1) The existing structure was designed properly and;
2) The original designer saw the building's load paths as I see them.
There's definitely a logical thread to it. It just strikes me as a very fallible strategy relative to the amount of robustness and certainty that we, as professionals, are expected to incorporate into our designs. Frankly, one would be hard pressed to satisfy assumptions one and two above in a structure designed last month, yet alone in 1950.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Analyzing a 1950 concrete frame
Is there an architect, mech and elec engineers on board? Structure aside, the architectural realities to complete such a project can easily derail this type of project as this could initiate upgrading a lot of other items to meet current code requirements. If there is any asbestos that is another project killer. OSHA and the AHJ will have something to say about both those aspects.
I assume your boss is an engineer and you working at a consulting engineering firm.
Did you visit town hall for old construction drawings? Owners never keep this stuff, and half of the time they answer the question without thinking about the value of the information to us. I would be surprised if you didn't find some drawings there.
RE: Analyzing a 1950 concrete frame
RE: Analyzing a 1950 concrete frame
And even if I had drawings, I'd probably want a test or two to be sure what type of corrosion has taken place with the re-bar for a structure that old.
RE: Analyzing a 1950 concrete frame
mtu: shear reinforcement is definitely a worry. I have seen many concrete beams deteriorated in old firehouses that had a sad amount of stirrups/bent-up bars.
Jed: No worries! I can handle it. Today I picked my boss's brain a bit more and found out that this garage was not from 1950, rather it is pre-WWII. If you look at slideruleera's reference for garage LL per city, my city was listed as 90 psf in the 1930's. I think that is where the difference is. Now we have three existing stories of 90 psf = 270 psf, minus three new stories of (15 DL + say 60 LL to account for corridors) = a balance of 55 psf. although, I don't really feel much better knowing that. Never mind the possibility (as you mention) of design or construction mistakes (cause' they NEVER happen).
BA: correct, my sign and seal is NOT going on these drawings. Actually, the new floors are going to have a smaller foot print than the lower garage floors.
KootK: Inevitable is correct. I will be certainly updating as I go along. I am curious what your opinions will be once the design is complete - hopefully they will be more comforting and optimistic by that point. I mean, all we have from the architect is a historical society approval set, and we just send our fee in. So, inevitably, a great deal of more concrete information will come out of the closet as it move along. Thanks for trying to be constructive - it is very appreciated! I agree with your ranting too - I hate all of those assumptions, and try to use them very very sparingly on non-critical or very redundant structures.
Brad: This is the whole kit and kaboodle. Schematic phase through construction documents and construction admin. Yup, no geotech (yet). I can find a way to work around the lack of testing for the superstructure, but as you all know soils are a dangerous realm to assume (or presume if you are in the IBC chapters). Besides, I want geotech's to get a job too - I am sick and tired of clients, owners, etc. not wanting to hire geotechs or other disciplines we structurals need for support (pun!). I feel like we end up being the jack (asses) of all trades in the name of $$$. Feww...I started ranting there.
There is an architect - we work for them. And, yes, it could cause a problem. For instance, the preliminary set shows two stair towers, but they aren't continuous and move around the footprint as you go vertical!! So much for using them for shear cores. My boss is an engineer (not much older than I) and I work at a consulting engineering firm. Good idea on the dwgs.
Robbiee: I agree that the scope does not allow for reduced seismic demand as a retrofit. All ASCE 7-10 and IBC 2012 loads and requirements.
WARose: This thread may account for some of that documentation!
RE: Analyzing a 1950 concrete frame
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.
RE: Analyzing a 1950 concrete frame
RE: Analyzing a 1950 concrete frame
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.
RE: Analyzing a 1950 concrete frame
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).
RE: Analyzing a 1950 concrete frame
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?
RE: Analyzing a 1950 concrete frame
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Analyzing a 1950 concrete frame
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.
RE: Analyzing a 1950 concrete frame
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?
RE: Analyzing a 1950 concrete frame
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Analyzing a 1950 concrete frame
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).
RE: Analyzing a 1950 concrete 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.
RE: Analyzing a 1950 concrete frame
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).
RE: Analyzing a 1950 concrete frame
RE: Analyzing a 1950 concrete frame
RE: Analyzing a 1950 concrete frame
RE: Analyzing a 1950 concrete frame
RE: Analyzing a 1950 concrete frame
I'll keep you in mind, Ingenuity, if I ever get to that point.
RE: Analyzing a 1950 concrete frame
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?
RE: Analyzing a 1950 concrete frame
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.
RE: Analyzing a 1950 concrete frame
That said, I don't know what the courts hold as the standard of care for this kind of retrofit.
RE: Analyzing a 1950 concrete frame
RE: Analyzing a 1950 concrete frame
If the vertical column loads are known to be equal to or less than the original design values then it seems reasonable to accept the columns and their foundations without doing further analysis; if there is even the slightest doubt, columns can be externally reinforced to carry additional load without much trouble and some underpinning can be done under the foundations.
It is still necessary to inspect the existing building to see if there are any obvious deficiencies but with the above assumptions, there seems little point in carrying out any calculations on the existing structure at all.
BA
RE: Analyzing a 1950 concrete frame
Field service (a la CBD-230 and similar "tricks") is only valid when you know that the loading has been encountered.
The gravity case analysis is a must. Wind I'll take as field tested. Seismic needs to start with a "what do we have now" and go from there, nearly always to a formal upgrade program.