Hello all,

For ASCE 7-10 Chap.13, there are 3 Fp values as below which gives you calculated value(by 13.3-1) and the max value(13.3-2), and lastly min value(13.3-3) as follow.

Fp = 0.4*ap*SDS*Wp/(Rp/Ip)*(1+2(z/h)) : 13.3-1
Fp = 1.6(Sds)IpWp : 13.3-2
Fp = 0.3(Sds)IpWp : 13.3-3

Here is the question. If calculated values from 13.3-1 is less than 13.3-3 value (my Rp is 12 according to table 13.6-1), would it be possible to use 13.3-3 values for the project seismic factors? Would it violate the ASCE 7 rules?

As I understand it, yes, 13.3-3 will set your Fp if 13.3-1 is too small.

I suggest you review B31E if you are performing a B31 analysis.
Paragraph 3.3 of that document states that, if using ASCE 13.3-1, Rp should not exceed 3.5 when using the stress limits found in B31E para. 3.4.

So now it is getting a little more complicated.

What is the B31E para. 3.4 stress limit? Basically, the stress equation is the sustained plus occasional stress that is typically used in, say, a B31.3 evaluation. But the allowed stress is higher in B31E. While B31.3 would have a stress limit of 1.33Sh (for sustained+occasional), B31E shows: MIN(2.4S, 1.5Sy, 60ksi). (S is a B31 allowable stress at operating temperature, Sy is specified minimum yield stress.) B31E also has you check the F/A<Sy, where F is the resultant force caused by seismic anchor motion.

So B31E uses a GREATER Fp (using Rp=3.5 rather than Rp=12; Rp is in the denominator) and also a greater allowed stress. You may ask, why?

ASCE 7 uses a strength approach (load resistance factor design or LRFD) while B31 uses an allowable stress approach (allowable stress design or ASD). These terms and their relationship are explained in ASCE 7. Here's my basic understanding - the full Fp from ASCE 7 can be used for structural evaluation (pipe position and system loads) but a reduced Fp would be used for stress evaluation (due to the LRFD versus ASD approach). What B31E does is use the full (but with an adjusted Rp) for both structural and stress evaluation but with an increased stress limit (e.g., 2.4Sh rather than 1.33Sh).

So why the changing Rp from 12 (as specified in ASCE 7) to 3.5? In my opinion, ASCE 7 does not address piping; it addresses structures. Piping is just a component attached to a structure. That Fp load is the calculated load on the structure used to evaluate the structure, not the piping. Rp indicates the energy absorption capability of the "component" (piping). Piping is very tough - it can take a large amount of load and distortion and not "fail". I guess that's why it gets such a large Rp (giving a smaller Fp).

B31E makes no mention of that 0.7 factor for seismic load that ASCE 7 uses in describing the relationship between LRFD and ASD. This, too, makes it confusing.

Again, this is how I see it, perhaps others can comment...

MK, I didn't mean to hijack your conversation To reiterate, 13.3-3 provides the lower limit for Fp. But consider your value for Rp; a lower Rp will will give you a larger Fp in 13.3-1.

Thanks for the insight, Sir

I've known the Rp limit in the B31E, but did not know the stress limit you've mentioned.

Looks like calculating Fp based on B31E or ASCE 7 are still on debate, right? Follwing your explanation, simply taking Rp 3.5 from B31E and use ASCE 7 13.3-1 could be a way to determine Fp, but not enough to calculate the proper seismic stresses/loads. I need to increase material allowables stress(not sure CAESAR II able to do that auto matically), and to do that manual adjustment will be needed.

Even on the owner's spec their aren't any specific limit or suggestion for the Rp values. Honestly, I ask this question to decrease our expanses on piping and supports BM not violate any code regulations. But changing Rp on my own is risk and not from any legitimate authorities (well, Rp=3.5 is specified but I think this is too conservative, I might use Rp=6 instead).

Any opinion is valuable to me, please leave any reply.

Strictly speaking, under today ASCE 7, a ratio Rp/ap=9/2.5=3.6 is valid under their requirements. They said this fact in the commentary sections, where they commented that starting 2005 edition, they changed the values- valid for piping- from Rp/ap=3.5/1=3.5 to Rp/ap=9/2.5=3.6. It is true that pre-2005 edition the ap value for piping was set to 1 (why?).

By the other hand, it is hard to guess an exact value of Rp.
The component response modification factor Rp represents the energy absorption capability of the component's structure and attachments.
For example, one can think that the piping may have a high capacity to absorb the energy but the supports (attachments to structural frames) may be stiff enough reducing the overall capacity to dissipate the seismic energy.

It is my understanding that B31E addenda solved an issue considering the conservative way and ultimately reducing the tentative value Rp/ap valid for piping by a factor of 2.57 (setting it to Rp/ap=3.5/2.5=1.4 vs Rp/ap=9/2.5=3.6), but in fact following the general rule of ASCE 7 for flexible components. I wouldn't say that following calculation based on B31E or ASCE 7 is on debate, I prefer to observe that they are aspects in ASCE 7 that seems to be not specifically developed so it's better to be conservative.