How far to apply non-structural loads vs structural loads (ASCE 7-16)

[Cpw628]

I am looking at a pipe rack that is going to add a new pipe with ASCE 7-16 loads. There has been discussion in my office about how to apply the non-structural pipe loads to the rack. Because it is ASME piping, the R factor per ASCE 7 Chapter 13 is 12. This compares to the R factor of 1 for the frame per Chapter 15. This means that the Ch 15 loads are over 3 times greater than Ch 13.

The question is, can we use the smaller loads to capture the local stresses on the frame? Below is an image with the new pipe in red and the blue column is the member who's stresses are of concern. With the smaller Ch 13 loads, the blue beam is under a 10% increase in weak axis bending and torsion, but with Ch 15 loads, it increases to about 20%.

 

[mikek396]

Take this with a grain of salt as I have very little knowledge of pipe stresses, and I havent looked at ASCE 7 before writing...

I think it would be prudent to design for the higher loads. If the column doesnt need to be reinforced for the higher loads, great. If it needs to be reinforced for the 10% additional stress, then reinforcing for the 20% additional shouldn't be too much more difficult. And if it doesnt need to be reinforced at 10%, but does need it at 20%, it seems to me youre splitting hairs if this is for 1 rack. Unless of course this is repeated many times, and could significantly increase costs, at which point you may want to consider the risk of failure of the rack and/or piping system.

 

[JLNJ]

No, I don't think it's appropriate to use the Chapter 13 component forces instead of the Chapter 15 non-building forces for design of the frame.

Logic would say that the anchorage forces from Chapter 13 would usually be higher than the frame-as-a-whole forces from Chapter 15.

Do you have moment frames or cantilevered columns? I'm a little surprised you have torsion of any significance. What is causing the torsion?

 

[Cpw628]

Mikek36, both your points are spot on, which is why I am asking. The higher forces would require a reinforcement, while the smaller ones would not. Additionally, the rack is also repeated many times, so that is correct that it wouldn’t be cheap to reinforce.

JLNJ, the pipe rack is a moment frame and the torsion is on the columns because of the pipe forces in and out of the page. The columns are also wide flanges which makes the little bit of torsion worse.

The thought process to justify using the smaller loads is that Chapter 13 typically captures the local forces on members while Chapter 15 is typically the forces on the main wind force resisting system. I am leaning towards going on the conservative side and reinforcing them all, but I may get out voted in the office. I couldn’t find anything in ASCE defining this line, but if someone else knows of a section please let me know.

 

[dauwerda]

Take a look at the ASCE document, "Seismic Evaluation and Design of Petrochemical and Other Industrial Facilities"

https://ascelibrary.org/doi/book/10.1061/9780784415481

 

[JLNJ]

If it's a moment frame, isn't your R value greater than 1?

 

[Cpw628]

It uses Chapter 15 SOMF with permitted height increase. The frame was built well before AISC 341 was a thing, so it does not meet any special detailing requirements.

 

[JLNJ]

Can you use the exception i in Table 15.4-1? I think it is intended for situations like yours (ASCE 7-16). You might be able to bump your R to 3.5.

 

[Cpw628]

I think that exception only applies if you also meet the detailing requirements of AISC 341.

 

[JLNJ]

You are correct. Which SDC are you in? The Commentary gives you a little wiggle room if you are in SDC C.

 

[TLHS]

You're worried about into the page forces here?

I wouldn't resolve piping seismic forces on any reasonable sized pipe purely in column torsion unless that torsional stress if completely insignificant and is resolved very very locally. If it's a long run of piping I wouldn't be transfering the pipe's axial seismic load at every support point. I'd be specifically identifying periodic locations where I'm expecting to carry the axial pipe load into the frame, and make sure these align with the piping engineer's restraints. I'd probably want one of these on each major rack segment? Depends.

In those locations I would try my hardest to do one of these things:

Run a beam into the page aligned with the column with moment fixity on at least one end to pull a second column in as a force couple
Brace into the page to the next column or a vertical member between longitudinal pipe rack struts
Horizontal moment frame doing the same thing as the brace
Move the new bracket up so that it aligns with an existing beam inside the rack and resolve the moment into that somehow

Other support points that aren't detailed like this will be significantly less stiff and can reasonably be designed as vertical supports with a small nominal lateral load.

If you really can't do the above, then push that pipe as close to the column as you can and be really careful. Remember that seismic deflections aren't reduced by the R value, so torsional instability issues may be worse than you think if you're not mindful.