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

 

[tweedledee]

If the original retail loads are higher that what you assumed,. this will mean that your % increase in load will be even smaller.

 

[AggieYank]

Everything I've read sounds reasonable. I have two questions / comments.

1) You come up with an allowable load of 348.5 kips for a W12x72 with Lu = 17.5 feet. This is the correct allowable load for a FY = 36 ksi column. If the steel grade is 50 ksi, the allowable load jumps to around 450 kips. Looking through AISC guide 15 (Rehab and Retrofit Guide), it appears that using higher strength steel, Fy = 50 ksi, became common in 1963. Are you assuming the building is older than that because you don't have existing drawings? Maybe something worth looking at if you need the higher strength. (Please note that I'm not an expert on what grade steel was used in what years)

2) I see people saying the extra 5% load is ok. This is very common. However, how do you know the building was designed correctly in the first place? Aren't you putting your faith into some random engineer who you have never met and never will? Maybe I'm just too cautious.

 

[JLNJ]

I believe that the IBC specifically allows a 5% increase in the chapter on Existing Buildings.

 

[haynewp]

The 5% increase from the IBC should not apply if the building does not work to begin with.

 

[JLNJ]

I was just pointing out a resource/reference for the 5%. The IBC is quite liberal in this regard.

 

[USFEngineer]

Update:

I submitted the calc's with the assumption that the floors above would also have the 40 psf live load.

Yesterday, I received my calculations marked up for revisions from the consulting engineer. Apparently, they have the original drawings. Absolutely unbelievable!! I have tried to get drawings for weeks. The only answer I would get is that the drawings didn't exist.

Despite the "redlines" on my calc's the consulting engineer gave the approval to the GC to start the work.

First of all let me ask this. Is is common practice for another engineer to mark up calculations and request for them to be revised for his file only? This is the first time I have ran across anything like this. Seems a little odd.

In the email to the GC the consultant explained that my calc's were incorrect due to the fact I did not have the original drawings and some of the assumptions were incorrect.

Some of my incorrect assumptions were the first level floor to floor height. Second, at the first level the column is a W12x99 in lieu of a W12x72. Yes, 12x99. The building was apparently built prior to 1958. As the consultant refers to the 1958 ASD code in his comments.

Some other items that were incorrect were the floor slab thickness 2 1/2" in lieu of 3-4" (assumed). Also that the 3rd, 4th and 5th floors should be checked with an 80 psf LL and that the 2nd floor should be 40 psf LL with 20 psf LL for partitions.

Should I just make the corrections or should I contact the consulting engineer so that I can obtain a copy of the drawings?

This whole situation is very odd. What should I do next?

 

[dik]

Obtain a set of the drawings, first, then review your calcs in light of this and then make the changes as reqd.

Dik

 

[USFEngineer]

What is the capacity of a W12x99 with an unbraced length of 18'-0" according to the 1958 Manual of Steel Construction? I think it is the 4th Edition of ASD.

 

[USFEngineer]

Correction. 5th edition Manual of steel construction - ASD

 

[JAE]

USFEngineer - why would you want to know a capacity based on an old code? You should always use the current code provisions when checking structures for load capacity.

 

[haynewp]

It was tabulated as 426 kips. I would also calculate the capacity based on the section properties and using your current AISC.

 

[haynewp]

JAE just barely beat me to it again.

 

[JAE]

two seconds ahead - I attribute it to the Espresso Cafe Mocha I'm now drinking.

 

[haynewp]

I'm laying off the hard stuff.

 

[dik]

What did you base your assumptions of construction on? I occasionally come across old drawings that do not reflect the manner of construction.

Dik

 

[USFEngineer]

The reason I was asking about the capacity of the column based on 1958 code is the consultant has requested that my "revised" calcs are based on 1958 code. I too have questioned the 1958 code issue. But, he is pretty specific in his notes that he wants it to show 1958 standards.

 

[dik]

Generally new work should be done in accordance with the current code... many jurisdictions permit the non-renovated parts to remain with the code in effect at the time the original was constructed.

Dik

 

[Lion06]

I haven't seen this mentioned in the post, but what type of lateral system does the building have? If it has shear walls or braced frames, then your K value of 1 is valid. I believe for a moment frame (fully rigid or flexible wind connections) the 9th edition green book considers this a sway frame and K will likely be greater than 1.