2-story extension of exist. steel building 5

[Heldbaum]

Hello, I have a question to you guys. I work on a project in NYC, extension of 9-story steel building. We are adding 2 additional floors, it's a commercial space so LL has to be at least 100 psf. The building was built 100 years ago. It's got dimensions in plan 45'x100'.Since it's a commercial space, the owner cannot get rid of people during the construction time. We had a meeting today, my boss who is a PE was there with me, and he told them to check only a few columns every two stories and we will be ready to evaluate the building to establish if columns have to be reinforced or not..I know that some of the columns are simply not accessible but most of them can be checked. Furthermore, it looks like the 9th floor was added later on because the columns are different ( they did one probe already). We could find a two double angles (6x4) for the internal column at the 9th floor.
First I am wondering if columns like that would have enough capacity to carry additional two floors, but that can be easily checked. Second, what would be your approach in case like that, what would you start from, would you check all columns one by one or would you check 2-3 columns at each floor and assume that all of them are the same..? What about the beam to column connection ? Should be checked to I guess. I am not sure if we should include the column schedule on our drawings and show what load we are adding to the existing footings or not necessarily..? I would appreciate any thoughts on this subject. Thanks!

 

[BAretired]

I would want to check all of the columns as well as all of the foundations before adding two commercial floors to a nine story building, or possibly an eight story building which has already had one floor added.

I would want to check the taller building for wind and seismic loads if applicable. Hopefully, there are some existing drawings available to make the work a little easier.

BA

 

[Heldbaum]

BA, thank you for your response. That's the main problem that there are no structural plans for this building whatsoever. Neither owner nor architect have any old plans. We also checked the department of building's website, no structural plans were ever filled.There have been plenty of alterations filled but no existing structural plans.

 

[JedClampett]

I'm not a high rise designer, but this makes me nervous. You're buying the design of this building, lock stock and barrel. Who knows if it was formerly a six story building? Or bracing was removed or cut in the 100 year lifetime? Refresh your memory on the Minnesota I35 Bridge. The bridge stood for 50 years. But there was a mistake in the original design that didn't become a problem until they did some work on the bridge.
This is a tough problem. If your boss is overly confident, you have very little wiggle room. If you quit, someone else will do it.

 

[Heldbaum]

JedClampett I feel likewise. And I am pretty sure my boss will share this position too after we will go into this issue in greater depth.And yes, it might have been a six or seven story bldg. formerly.
Like BA said, we should check the columns and footings and its capacity. In order to check wind and seismic loads we would have to know the beams and connections too - basically we should get familiar with a framing as a whole right?It sounds reasonable.

 

[dhengr]

Heldbaum:
The phrase ‘get intimate with the existing structure and details’ may be quite a bit more appropriate the your phrase “get familiar with a framing as a whole.” It seems to me that the owner should be willing to devote a fair amount of time and money to start to understand the two story addition potential of his bldg. If they just want the whole thing blessed for a few bucks, they should probably get in touch with their favorite church/minister, they are in the business of blessing things for a small donation. And, their obligations and liability are slightly different too, for that small donation.

 

[XR250]

I can't imagine the existing lateral system would be adequate. How would that be fixed?

 

[calvinandhobbes10]

This is a very tough dilemma.

What you are probably being expected to do is:
---Check some gravity sizing.
---Not think about lateral.
---Get a building permit.

You should NOT go this route.
You are hired to assess the structural ramifications of the proposed renovation. All of it.
8 stories original up to 11 doubles your overturning lateral moment.
The 45' weak dimension is pretty skinny. You may be introducing uplift where there was none previously.

This early in the project, your job is to educate. Educate the owner by emphasizing the information you have to somehow obtain. This is a tough conversation to have because owners are generally better at negotiating and controlling meetings than we engineers are. But it's very important.

If he is willing to play ball and listen to your needs, keep pursuing. If not, I'd just reiterate that information to him and let him decide if he wants to re-shop.

You need, as a start:
---As XR implied, to mention EARLY ON that his expansion plan may require a new lateral system for the whole structure. Bring some numbers or sketches to help. This is an unknown. It's not a yes, not a no.
---To walk each floor of a building and sketch out some column layouts, unless the renovation drawings show them well enough.
---As BA said, to determine what the foundations are (corbeled brick? Concrete? Deep?)
---To determine what the lateral system is (it could just be the limestone/masonry skin). Also check the elevator core and stair walls, which too may have been added as a renovation.
---To determine floor construction/weight. Find an electrical room without a ceiling and start there.
---Spot-checking the columns may be OK for starters, but if you are getting say 0.99 stress ratio you may want to do more than spot-check.
---This is without getting into any of the potential corrosion/water issues that may have transpired in 100 years with multiple renovations.

 

[dik]

You need to start with a detailed investigation of the existing structure; from there you can analyse the building, both existing and with future add-ons. If built 100 years ago, you also need to check the materials of construction. Original was likely riveted construction and material should be checked for weldability and strength.

Dik

 

[Heldbaum]

Thank you gentlemen for your comprehensive replies. I feel likewise.

 

[JLNJ]

Here's the question to ask yourself: If something were to happen, who would be at fault? Could you stand before the judge and with a clear conscience say that due diligence was performed? It seems to me that the standard of care for such a project would be a very high standard indeed. The is the USA where the individual is important and has the right to a safe public building.

You also can get some help from the building code. Chapter 34 gives some guidance regarding which members need to be checked and/or upgraded.

 

[dik]

JLNJ: That's why he needs to start with a detailed investigation.

Dik

 

[bookowski]

2 things to add:

- NYC is unique in that the code allows quite a bit of leeway in terms of not upgrading lateral. The 2 on 9 would be nothing out of the ordinary. There is a little flowchart that determines if you need to upgrade but generally speaking I wouldn't be surprised if you aren't required to. Not saying it's right - but that's the code in nyc and it's common.

- Top column might be original, I've seen them switch at top floor if columns don't stack or I guess savings in steel... not sure but I just worked on one that had single angle columns at top floor and everything below was built up shapes.

 

[Heldbaum]

I have another question to you guys. Contractor made some probes where we could see the columns sizes.He revealed around 40% of the columns, but this is not the issue now. In the basement all columns were opened. It turns out that in the corner ( they opened only two corner columns) we have 8x8x1/2 angle as a column. It seemed weird for me from the beginning. Then I made a model in Robot and those corner columns look bad before adding extra two floors..they have around 24 ksi taking safety factor as 1 . So I am wondering if it's possible that 9 story building ( from around 1920's ) has an exterior brick bearing wall ? I did some research and found on the net that they used to build buildings like that up to 9 story. It's a steel frame but with exterior brick bearing walls going all the way up from basement to 9th floor. I would appreciate any comments and some references/materials on that subject.

Best,

Jacek

 

[dik]

That's way too low for SF or too high for loading... whichever way you look at it. Columns don't often redistribute loads well. As noted, you need to do a detailed investigation of the building first. What grade of steel did you assume? The real Fy could be 25-30 Ksi... have you taken any coupons for testing? Is the steel weldable? Just too many questions at this point.

Dik

 

[Heldbaum]

Dik, I assumed 30 ksi. I have not taken any coupons for testing. I know I should perform a detailed investigation of the building but I can't do much..and my boss is okay with that. So at least I am trying to figure out how the building was built..Isn't it weird that there are angles 8x8x1/2 in the corner of the building holding up 9 stories above, including walls..? Is it possible that exterior brick wall was built as continuous bearing wall all the way up ? And in that case the exterior columns wouldn't be holding the exterior walls what would make them less loaded..? Thanks!

 

[txeng91]

Not sure if you or your boss took the job but adding 2 stories to a 100 year old building seems pretty risky. Even if you were able too get all of the info needed for as-builts, I am pessimistic it would pass current engineering standards as code compliant structure. As other people have said you should have the steel tested, all of the column and brace sizes noted, as well as the connections. I have no idea how you analyze the foundation system or any braced frame connections to the foundation for 2 additional stories without any plans, which is problematic. I guess I can kind see where the city is coming from (lots of tall buildings = reduced wind loads, also low seismic regions) if they allow this sort of thing without engineering analysis, but they better know what the history of the building is and whether or not nay floor have been added to it previously. In conclusion I would say the addition should probably should be avoided but if not the owner should understand its going to be a lengthy process that could result in expensive testing and retrofits.

 

[MotorCity]

Even if you have the original drawings, there are usually two more significant items that you don't have: drawings or records of renovations and material properties from buildings constructed 100 years ago. Those two items alone would be enough for me to walk away from this one. Depending on the building geometry and seismic design category you may be required to perform a 3D and/or dynamic analysis. How could you even begin to construct a model without member sizes, orientation, properties, etc. This structure is too large and too significant in terms of public safety to make guesses.

 

[Heldbaum]

Unfortunately I do not have any original drawings or records of renovations. All we have been able to establish so far is column sizes (not all were available at upper floors though)and exterior brick wall thickness. And I made a model based on that information to start with checking the gravity loads. I found this book on the internet recently :

http://www.seedengr.com/Building Retrofits Breathing New Life Into Existing Building Structures.pdf

and I am trying to get as much info as I can from it. I agree with you that thorough investigation should be carried out - undoubtedly. But it seems like we are going to make some assumptions based on how they used to build at that time. I am not PE yet so basically I do what I am asked to do but I am trying to do my best to assess the building properly.

It looks like the gravity loads will not be an issue. Columns can withstand an extra load from two additional stories. When it comes to lateral stability - I want the client to open a few column - to - beam connections to make sure there are moment connections frames. Plus there are 3 core shafts - 2 staircases plus elevator.

Thank you for your answers. I really appreciate that.

Best

 

[Teguci]

Usually, contractors require a barricade around their construction site and they keep any public from entering their construction zone. Accidents do happen to buildings under construction and allowing the public to be directly under floors being cast or beams being raised seems like a walk away proposition to me. Key times to prevent the public from being below the construction - Steel framing, concrete casting, shoring removal. How would you even be able to verify that all occupants are out at those times?

As for the structure, I'd approach the problem assuming that everything needs to be supplemented to support the additional loads (and they probably built the thing wrong), prove me wrong. As Bookowski noted, NYC's code is unique (and a PITA) and may not require an upgrade to the lateral system. If that's the case, this sounds like a great project for your competition to put its name on. This causes my estimator to hate me, but, all early assumptions should add cost to this project. Only with assurances should those costs be removed. Optimism should be beaten down at every meeting with the owner. Save your optimism until the end.

So you checked the corner columns and they're good (you've already checked overturning)? OK, remove 4 columns from the "supplement columns on the ground floor" cost item.