Some questions for the evaluation of exsting buildings

[dcceecy]

The building was built in 1960's and we did not find any drawings for this building. It is a one story building about 20' tall, 84' wide and 165' long.
The roof includes ballasted roof (ply felt and gravel, fiber board and rigid insulations), metal deck and open bar web joists (span is about 33’).
The end CMU walls (84’ long) are load bearing walls supporting the joists. The side CMU walls are not load bearing and are not reinforced except the bond beam at the top of the wall. I have some questions when I check the buildings.

1. Should I use Component and Cladding wind loads to check the CMU walls?
2. The side CMU wall (165’ long) has a 12” thick strip footing, which is 3’-6” below grades. So should I use 20’+3’-6”-12” =22’-6” to check the wall? The wall is not connected to the interior slab-on-grade. Can I use the passive soil pressure as a support and use a span of 20’ to check the wall?
3. The roof joists are supported by some continuous small W shape beam. So there are negative moments near the columns. If the length of negative moment area is longer than Lp , then the nominal moment capacity of the beam is less than Fy*Zx. Is that true?
4. There is section modulus Sx and plastic section modulus Zx for each W beam. I understand the Sx*Fy is the moment causing the extreme fiber to yield. So for Zx*Fy, the total flange will yield or total W section yields? Any reference I can study? (some parts of steel beam has been repaired with steel plates at the flanges of W beam, so I have to find the Zx of repaired sections)
5. The geotechnical engineer checked the soil bearing capcities. For 4000 psf, the estimated settlement is 1” and for 3000 psf , ½” with the expected column loads (we are going to build a roof above the existing roof). So which one should I use? Or the PE who is going to sign the drawing will have the final say.

Thanks

 

[JAE]

1. yes - C&C used to check wall designs
2. Yes - top of footing to upper lateral support point.
3. True - but the unbraced length of the beam is not taken from a column to an inflection point. The unbraced length is the full span adjusted for the moment curvature per AISC - (Cb ratio calculation).
4. Zx represents total section in plastic.
5. The PE who is going to be the engineer of record should have the final say. If you are not that PE, then the actual responsible PE in charge should be guiding these types of decisions.

 

[MiketheEngineer]

If you have a PE signing off on this and I am glad you are - take your concerns to him and let him work out the details. That is his job!! And he needs to answer those questions for himself anyway!!

 

[dcceecy]

Thank you for your replies. My first impression is that a 20 ft tall wall with 12" CMU hollow block should not be good. But the walls have been there for almost 50 years.

I am a new PE but not in that state. No offense to PE. some PE just close their eyes and stamp anything you give them. They may be too busy to take a carefull look or just do not understand the issues (I bet many will pick the wrong anwsers or do not understand the issues for the first 4 questions I listed here)

About the unbraced length for the negative moment, I found an example in Chapter 7 Design of Building Members in Structural Steel Designers Handbook. It seems they use the distance between the support and inflection points. I may do some more research. It's good if I can find some test results.

Thank you for your input.

 

[JAE]

Inflection points are NOT brace points. This was assumed to work some time ago but has recently been found to not be good practice.

 

[MiketheEngineer]

If you find a PE that will "stamp anything" - turn him/her into the state board.

Those people are a danger to society and on a lighter note - ruin the reputations of engineers who work HARD to make things right!!!!!!!!!!!!!!!!!!!!!!!

 

[STRBeth]

I agree with JAE - IP's are not brace points. There's a paper out there that discusses this (maybe by Yura) that I can't seem to put my hands on.

 

[JedClampett]

Whenever I have to check an existing structure, I go into the task with the thought that whomever designed this was competent and knew their stuff. There were codes in the 1960s. If must of worked to his/her satisfaction. If anything, designs were more conservative then because more approximations were used instead of computer analysis.
So what's not clear to me and my question is; Why are you checking this building? Is there a change in use? Has the code changed drastically (probably not)? If the building has stood for 50 years, it's been exposed to a lot. They did something right. As far as 12 inch hollow CMU, there's an excellent chance they do work. Have you tried checking them using the section on unreinforced masonry in ACI 530?