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).

 

[mtu1972]

I have not done what you are being tasked with, but I have added 1 level onto an existing building, making the existing roof a floor and adding a new roof.

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

 

[JedClampett]

This sounds like a disaster in the making. How can you add to an existing design without drawings (I'm assuming you don't have drawings) or investigation?
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.

 

[MacGruber22]

Thanks, mtu. That gives me a nice overview.

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!

 

[BAretired]

I agree with JedClampett. Without the original drawings, I wouldn't even consider building the suggested addition.

BA

 

[MacGruber22]

Jed, no drawings. Investigation is visual only. I plan on being very very (very) conservative in my estimation of the strength of the existing lateral system, and to add steel moment frames along the perimeter of the building masonry walls if we don't feel good about the existing doing the lateral work. The good news is that seismic and wind are relatively low in this neck of the woods, so retrofit shouldn't result in insanely-sized members and connections.

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.

 

[Brad805]

It sounds like all you can do at this time is a basic load evaluation to determine if it is feasible to move forward. I suggest you determine the original loads and new loads as mtu suggests. Once you have a new/old load increase throw it back to your boss and tell him to sort it out. Loading the roof beams as shown in the sketch seems problematic.

 

[paddingtongreen]

Even if the total load is reduced to match the estimated original design loads, that will not account for those new columns landing on those old roof beams!

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[hokie66]

If this was a parking garage from 1950, it will have only been designed for about 50 psf live load. So you won't have a lot of live load to reduce. Also, lateral loading was probably not even considered, so you should probably ignore the existing structure in that regard, especially since you don't have drawings. There is not much lateral capacity in a concrete frame which is not detailed for lateral loading, but which is really just post and beam.

 

[Trenno]

Concrete X-Ray? Even atleast to get an idea of the density/spacing of the reinforcement?

 

[MacGruber22]

Hokie, I am being told this garage was some sort of a service garage that required at least 75 psf.

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.

 

[CELinOttawa]

Don't do this. It is worse than irresponsible...

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*

 

[mtu1972]

I don't have any ACI information from that long ago. It may be beneficial to find some design reference from that era.

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

 

[JedClampett]

Sorry MacGruber, you must feel like you're a punching bag. But without any records, how can you even believe that 75 psf assumption? Maybe they made a mistake, maybe they didn't know it was a service facility. Lots of risk just with that.
I don't think much of your boss for putting you in such a pickle.

 

[BAretired]

The person in a pickle is going to be the one who seals the drawings; hopefully that is going to be your boss, not you.

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

 

[KootK]

I think that you're actually in something of an enviable position here MacGruber. You stand to learn a ton from this assignment and, when all is said and done, the responsibility for all decisions rests with your supervisor. I'd be interested to hear more so please consider updating us as you go.

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.

 

[Brad805]

Are you to develop construction documents or is this a feasibility study? I see the biggest challenge being the foundations, so the comment about the geotech report is ridiculous if this is beyond the feasibility study. The costs to do this properly will easily pay for themselves by reduced construction costs. I never follow the logic when clients tie our hands behind our backs and then complain about the results we come back with.

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.

 

[Robbiee]

I don't know what code you use, but here where I am, changing the loading characteristics of the existing structure the way you intend to do, would require the existing structure to conform to the current code. You don't want to analyze a structure by making assumptions for all the unknowns that you have, do you?

 

[WARose]

No way would I seal something like that. And even if I wasn't (but was working on it), as others have said, I'd make sure I documented (in writing) the dangers associated with this.

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.

 

[MacGruber22]

CELinOttawa: So you suggest that I quit my job rather than try to make it work?? I would love to insist on testing, but I am the workhorse in this project, not the project manager. Again, I appreciate the sentiment, but you are preaching to the choir on this one - I am just trying to not go crazy during the 40 hours have have to perform this analysis/design.

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!