Existing Steel Column Analysis

[USFEngineer]

I have somewhat of a problem with an existing WF steel building column. Some backgroung info about the column.

The existing building is 5 stories located in Miami. The floor system consists of bar joists, metal deck and 3-4" of concrete. The columns are roughly spaced in a 25'-0" grid. The floor to floor height is 15'-0". In between the drop ceiling and the floor above, the tenant wants to add a mechanical maintenance platform for new air conditioning units. The platform is mainly WF beams and metal grating.

Now for the problem. Currently, I am an EI. I work under the supervision of the principal at our firm. When the project came in I raised the question about the existing steel columns that would be supporting this platform. I was told not to worry about them because they were probably fine. So I said O.K. and designed the platfrom and sent it out a few months ago.

Last week I get a call from the tenant saying that the owner of the building has hired a consultant structural engineer to review the platform etc. The consultant has requested structural calculations of the platform design and the existing steel columns. The platform no problem. The existing WF column is another story.

This brings me to my question (sorry for the long post)? Without the existing plans, which to my knowledge can not be obtained (do not exist), how can I analize the existing column and show that the additional load will not affect or will affect the existing column? I was thinking of just showing that the dead and live load added to the column works in compression. Then again I have no idea of what and how the roof is framed. Nevermind the existing footing. I have no clue what is there. It just seems that there are to many assumptions being made. I do not want the consultant to laugh at my "rough" analysis and open up a can of worms.

Any help would be appreciated

 

[civilperson]

Make a site visit to see the roof framing system and the size of the columns. Estimate the dead loads and use current code for live loads. Analyze the column. If OK, list assumptions and sent back to reviewer with calculations; if over stressed, offer to design a fix for a additional fee.

 

[UcfSE]

You should ask your boss this. It's on his shoulders should there be an issue with the consultant. Also, I wouldn't just give anything to any consultant without consulting your boss as well. You don't work for the consultant, so who is paying for all this? If written-up calculations were not included in the original proposal to the client, then someone needs to pay for the time to do all that.

With the existing columns, you need to make some assumptions, go to the site if neecessary, find some old plans or as-builts, do whatever you need to do to estimate an existing load for the columns. then you can find the new load. One method for checking existing structures is to check the percent change in load. If the load changes only a few percent, it may be reasonable to assume that is negligible, column/footing check done.

You could go over this with your boss and provide him with your calcs and ask him to add his for the columns, since you didn't do the check since he said not to.

The other thing is take this as a lesson learned. Don't take someone's word that "it" works if you feel you need to check it, or ask that person to handle the check.

 

[lkjh345]

FWIW I will be the one to state the unspoken: I agree with you bosses. An open grate maintnetance platform, if it is just for someone occasionally going up to work on some piece of mechanical equipment probably adds a very minimal % of load to the column.

What is yuour actual dead load for the platform? I am picturing something that when you add everything up, is likely less than 10 psf. For columns at 25 feet O.c that is only 6 kips to the column. For columns in a 5 story building, that is a rounding error.

For an occasionally used platform, I would have no hesitation of using a live load of say 50 psf for individual beams, but only 10 or 15 psf at the columns. For columns at 25 o.c. each way, this is 6,000 to 9,000 lbs on the platform. It takes a lot of maintenance workers and hand held tools to get to that load.

Also, In my mind, when you are checking the column capacity, if you are adding on the platform live load to the load on the column, you also get to use this increase in area in your live load reduction formulas. Unless you are already down the code minimum, the decrease in the live load to the orginal floor will probably offset the increase in loading from the plattform.

I also agree that you need to make clear who will be paying for producing these calculatins if they were not in your orginal scope of work. If someone is willing to pay for them, I think you will find its not very hard to figure out that the columns work.

 

[haynewp]

At what story was the platform added?

 

[SlideRuleEra]

USFEngineer - In addition to the excellent advice given above, in this type situation I like to look at the problem "backwards" (using ASD methods).

Since you have quite a bit of information on the column, compute the maximum allowable load on the column and see how that compares with the suggested direct calculations. This won't give you any new information, but it should not take long and is a good way provide a "reality" check. This also will give you some "hard" numbers (not estimates) to present (thru your boss) to the Owner's Representative. (This will help convince him/her that you know what you are doing).

Also, consider that the design of your platform may reduce the unbraced length of the column, increasing it's theoretical capacity.

http://www.SlideRuleEra.net

 

[USFEngineer]

First, thanks for the great advice everyone.I think I may have solved the problem with everyones advice.

Some other info to answers some of your questions.

1. I have already spoke with the tenant about the additional service fees for the calculations. This will be paid for by the tenant or the GC doing the work.

2. The platform is located just above the ceiling of the 2nd floor. The floor to floor height is 17'-6" from what I can tell from the drawings that I do have. The drawings that I do have are from a renovation in the 90's. I did notice on there drawings they show a pile foundation with grade beams. I do not have the existing drawings. Should I be worried about the pile foundation?

3. The platform is for servicing two additional air handling units. AHU-1 weight = 2400lbs. & AHU-2 weight = 2200lbs. Basically I am adding 16 kips to (4) W12x72 columns.

I ran some numbers making assumptions that the floor live load is 40 psf. I assumed that the floor use is School - Classrooms. Second, since I know that the existing floor system is bar joists metal deck and concrete I assumed a total dead load of 65 psf. I used 30 psf live load for the roof and 15 psf dead load for the roof. Also assuming that the roof is framed in the same manner the floor system is. (This is where I am a little skeptical with the asumption of the roof framing). I came up with a total load of roughly 300 kips on each column. I then assumed that the columns are sidesway inhibited in both directions (X-X, Y-Y) at the 17'-6" height. In addition, that the column is pin connected at the top and bottom. So my Lu=17'-6"

I then compared the alowable concentric loads on columns (ASD) (348.5K) to my actual of 300k + 16k and the column checks.

Am I in the right ballpark or way out of it.

Also, discussing this matter with my boss will do very little. His philosophy is to "give the client what they want". Meaning that he wants very little to do with the whole situation. Basically, he is leaving it up to me to take care of.

 

[JAE]

Your calculations and design assumptions appear to be valid. You should definitely use the applicable live loads for each floor and for the roof...don't just assume some value that works...you should use the appropriate live load that matches what the spaces currently are used for.

Also, don't just blow off the piling. You should fully investigate what was installed (if you can) and short of that, re-evaluate the percent increase in your loads from the original condition and see what sort of bite you may be taking out of the original piling safety factor of 2 to 3. My guess is that for this sized building its not a problem.

>....now for my final point - your boss isn't acting in a professional manner based on your description. Finish the project and get out of there.

 

[USFEngineer]

JAE,

It's not that I want to dismiss the pilings, but up until today I did not even know about the pilings. I do not know anything about the foundation. I have requested record drawings of the original structure but I am told by my contacts (contractor and architect) that they are unavailable-do not exist. This is really the problem that I am having. Trying to determine what is there and what is not. It seems the harder I try to dig deeper the less anyone seems willing to help. I really think that the contractor and other just want this to go away and view this as a minor blip. I just want to make sure I do not overlook anything that could potentially be a problem later.

As far as my boss is concerned. It is more of a matter that he views me as the project manager and wants me to handle it. Maybe he doesn't view this as a big deal to him.

Just trying to do the right thing...

 

[lkjh345]

USFEngineer:

You loads seem high. Did you take into account allowable live laod reductions?

Maybe I am missing something, but using your numbers:

- 5 story building with columns at 25 feet o.c. each way. Using 4 elevated floors in the calculaion (I am assuming the lowest floor is slab on grade). Area of influence to the column is 4 stories x 4 K(LL) x 25 feet x 25 feet = 10,000 SF. Reduced floor live load = .4 (40 psf) = 16 psf per Equation 16-24 of 2006 IBC. I personally might use a 25 psf reduced live load to the column.

Using a 65 psf dead load and say 25 psf reduced live laod over a tributary area of 4 stories x 25' x 25' = 225 kips from floors.

Using you 30 psf LL and 15 psf DL for the roof gives (30+15) x 25 x 25 = 28 kips from roof.

Total column load = 253 kips. Well withing the limits of a W12x72 with Lu=17 feet.

As for the piling, I personally wouldn't worry about them. I think its a relativley safe assumption to assume the piles were designed to roughly match the column. Therefore, the 16 kip additional load (which seems high to me for a open grate maintenance platform with with two small mechanical units) is only about 5% of the column capacity. You really don't know your true loads within 5% anyway. You have likely rounded up more than this in your calculations.

IMHO, your boss is not acting irresponibily. From expereince he/she knows that adding a fairly small load to a column in a muti-story building is just not a big deal.

I deal with mostly 3 to 5 story buildings. Adding 16 kips to a column in one of these types of buildings just doesn't bother me too much. Its much more likely that the platform itself will fail through undersized members, bad connections, improper construction etc, than column or piling overload.

Don't worry about the mouse in the room while ingnoring the elephant in there with him.

 

[USFEngineer]

lkjh345,

When running my calc's I did check it with the alowable reduction and without. My point I guess without reduction was just to make sure that it worked for my piece of mind. My actual numbers without any reduction was about 290.6 kips. With allowable live load reduction (assuming all floors have same occupancy) I came up with 230.6 kips.

I still have not verified the occupant loads for the other floors. I just tried to contact the GC to verify the occupants just to make sure.

 

[UcfSE]

As the project manager you should handle it, until the decisions go over your head. If I really need drawings, or need to know what is there, I would tell my boss that I need a site visit, or whatever else you need to do your job, since that what you were told.

Another way to look at it: 16k out of 300k is about a 5% increase. That's on the border of negligible for some and very negligible for others. I think your approach is sound. Just double check your assumptions and make sure they are all reasonable.

 

[bkgd]

A couple of comments, if you braced the platform for lateral loading then you in affect braced the columns thus cutting down their KL/r and increased thier capacity. Typically bracing of 2% or less is typically considred enought to brace the column. Taking this into account along with all the other previous advice the columns are a none issue.

In terms of the piling you generally will have a column load that is less than the pile capacity. The reason being is that your column loads never (rarely) match the capacity of the piling. For example you might have a column load of 670 kips, and you have a pile capacity of 200kips per pile so you would install 4 piles for a total capacity of 800kips which exceeds the column capacity. Otherwise you are trying to make 3.8 piles work, obvioulsy not going to happen.


In terms of your boss he sees this as not to big an issue based on the loading and is trying to allow you to spread your wings and grow, if you hand solutions to younger engineers all the time they become dependant and never gain the experince of "free thinking". Obviously there is a point when he needs to step in if you are spinning your wheels and burning budget, but that is his call.

One last comment becareful when a GC says he will pay for some work like this, it has been my experince they are quick to say ok, but when the bill comes thru it is, oh a different story.

Regards,

Blake

 

[haynewp]

The platform is above the second floor so it does not help the unbraced length between the first and second floors. But overall, usually 5% will not hurt the system. As long as 5% hasn't already been added somewhere else that you aren't aware of.

I also noticed you didn't mention any allowance for partitions and used the 40psf live. Are you sure all these floors will remain open use spaces?

 

[USFEngineer]

Speaking with the contractor late yesterday, The only floors that are occupied are the first and second floors. The other three floors are vacant. The original occupancy for this building was a five story retail mall in Miami. So is my assumption of 40 psf still valid? How can you determine future use of unoccupied space?

 

[haynewp]

40 psf is a little low in my opinion for spaces that are now available for new tenants. Even if they are converted to typical office spaces, that is 50 psf without partitions. Why did you use classroom live loads?


From FBC:

1607.5 Partition loads.
In office buildings and in other buildings where partition locations are subject to change, provision for partition weight shall be made, whether or not partitions are shown on the construction documents, unless the specified live load exceeds 80 psf (3.83 kN/m 2 ). Such partition load shall not be less than a uniformly distributed live load of 20 psf (0.96 kN/m 2 ).

 

[USFEngineer]

I used a 40 psf live load for classrooms beacause the first two floors are occupied by a technical/art school. Maybe a live load in excess of 40 psf would be warranted. But not really knowing anything else about the occupancy has me scratching my head.

 

[lkjh345]

If the orginal building was designed as a retail store, the floors/columns/footings should have been designed for 'Retail' Live Load.

Do not know what the Florida Building Code specifies for this, but IBC 2006 has 100 psf Live Load for First Floor Retail space and 75 psf Live Load for 'Upper' Floor Retail space. The IBC ( and before that the UBC) have used these fvalue for quite a while. So this should be a fairly stout building, with some fairly heavy floor structures. Certainly heavier than school or office space would dictate currently dictate.

 

[haynewp]

FBC has retail at 75psf, office with partitions is 70psf.

I missed where you said it was originally retail, but either retail or office doesn't make much difference. I would use the 70 or 75psf instead of the 40psf.

 

[UcfSE]

You should get exactly what needs to be there from the architect. "I don't know" isn't going to work. Technical and art schools may have equipment or storage that will be in excess of 40 psf. I would check also for some minimum point load requirement in addition to the uniform load.