Piperack - design beams with diaphragm forces? 3

[Kom3]

Hello all,

Please see the attached photo. I am analyzing an existing multistory pipe rack. My question is; should the beam (pointed at with the arrow) be designed with diaphragm forces (assuming forces acting in the longitudinal Z direction)? These piperack do not have a diaphragm or horizontal bracings and those beams are not chords in the traditional sense since they don't carry axial forces. Any guidance would be appreciated.

 

[phamENG]

Since there's no diaphragm, I'd say no. But based on the visible framing, I'd say at least a few of them are moment connected - correct? If so, there will be minimal wind loading on the columns that will be transferred through the frame. Also, you'll have whatever lateral loads your pipes transfer back to the bearing points, which I assume are on these beams.

So while you won't have diaphragm forces, you will have several discreet forces from the various members connected to it.

 

[Kom3]

Hi phamEng, thanks for the feedback. My question is essentially what load factor should I apply to the weight that these beams supports (yes they support the pipes). Should I apply the same seismic coefficient used for the computation of the base shear? Right now, I am only looking for the forces for the design of these beams. Diaphragm forces seems reasonable to me since these beams and the pipes they support are similar to floors except there is actually no diaphragm. Thanks!

 

[HTURKAK]

Apparently the beams shown with arrows are transversal bent beams and they should be designed for pipelines vertical loads (in -Y direction)+ pipeline friction and guide forces (in Z direction ) and axial load (due to wind in X direction ) ....

Please look to the doc .for PIP STC01015 Structural Design Criteria at the following link

https://www.pip.org/docs/default-so...5a262f789edff00008ddc6a.pdf?sfvrsn=30e9cb9e_0

and Design of Structural Steel Pipe Racks =

https://www.aisc.org/Design-of-Structural-Steel-Pipe-Racks

 

[JLNJ]

It looks like you might be trying to use some sort of area load. I'd recommend against this.

Apply the pipe loads as point loads. This will make the beams bend in both their strong and weak axis like they do in real life.

 

[r13]

To be proper, for this case, the term "diaphragm force" shall be changed to "horizontal (longitudinal) force". HTURKAK and JLNJ both got it right.

 

[dhengr]

Kom3:
You should normally get these loads and forces from the piping analysis, stress and design people. They know how they have designed the entire piping system. The links HTURKAK provided are certainly a good place for us regular structural engineers to start. I believe that you will find that most (many) pipe rack bents are designed to lean a bit, either way, in the Z direction, within some P-delta range. Then, every so often there are two bents which are braced together into a four legged frame in the Z direction to take longitudinal loads. Otherwise, many of the pipe support saddles or devices will have low friction shoes at their bearing points at the cross beams. Of course, in the X direction the individual bents are expected to take their respective lateral loads.

 

[Kom3]

Hi HTURKAK,

I am familiar with both documents but please let me be a bit more clear with my problem.

For sure, I should be adding vertical loads, friction, axial and seismic. But my question is; how should I vertically distribute the lateral seismic forces on a pipe rack when designing the beams shown. I intend on using ASCE 7 12.8.4 (Horizontal Distribution of Forces) to design the vertical lateral system; and I intend on using ASCE 7 12.10.1.1 (Diaphragm design forces) to design the beams. Granted, there are no diaphragms in these pipe racks but nonetheless, the beams I pointed at serves the same purpose as the diaphragm in the sense that they are the "lateral" lateral system since they are the members that transfers the lateral forces to the vertical system.

dhengr,

I have received the piping loads from the piping department. However, this is an existing fully loaded pipe rack and we only have information on the new pipe installed. Are you suggesting that I use the same seismic coefficient for all the existing pipes when designing the beam? I believe they use chapter 13 for computing the pipe CS. I am also leaning towards that direction. Please correct me if I am wrong, but for sure we should not be using the piper's seismic coefficient to design the "vertical" lateral system, i.e. braces, moment frames, etc. However, since these beams are not part of the Z-direction lateral system, it should be permissible.

Thanks all.

 

[HTURKAK]

Hi Kom3 (Structural),

The approach for seismic design of pipe racks different from the building type structures which have floor diaphragm.

Pipe rack structures are typically (OMRF)ordinary moment resisting frames or (IMRF) intermediate moment resisting frames.

You are expected to perform two separate analysis . Analyzing in the short direction ( Transverse Direction (Moment frame) and analyzing in the long direction (with braced frame).

Pls look to the worked example 17.2 at the following link=https://www.fema.gov/media-library-data/1474320077368-125c7a1d1a3b864648554198526d671f/FEMA_P-1051.pdf

 

[Rabbit12]

I've always applied piping loads in your global Z direction to the transverse beams. If you don't know much about the existing piping, a good rule of thumb is 20 psf for the DL. Calculated the seismic force based on that. If you have large diameter pipes you should account for those individually.

One thing to consider is if the piping is only shoe supported, you can't develop any lateral force above what friction can carry into your beam.

 

[r13]

Very good assertion, Rabbit12.