substation bus structure using STAAD 2

[oengineer]

I am working on a single-phase bus support for a substation. I am trying to model the insulator in STAAD. The insulator is made of porcelain material and rest on top of a HSS column. What is the best the best way to model the insulator in STAAD so that the loads for the bus items sitting on top of the insulator are transferred to the HSS column. What type of member should be used for the insulator? Should the insulator be treated as a dummy member? Any suggestions/comments are appreciated.

 

[BUGGAR]

You caught me just a couple of years too late! I used to model insulators on substations in STAAD and I'm try to remember how we modeled them. This was for STAAD dynamic analysis. I think we used a fictitious material but my memory is vague. I remember that we modeled insulators as actual members. If I have a memory flash, I'll get back to you.

 

[oengineer]

@ BUGGAR That would be helpful if you remember. Thanks

 

[dvelarde4]

I treat it as a dumb/fictitious element. Then using vendor information verify that it can carry the required loads. I use Risa 3D.

 

[BUGGAR]

As I remember, you had to get the right EI in the insulator member to simulate the frequency? Develarde, are you doing the dynamics in Risa? I was using '97 UBC dynamic at the time.
I remember everybody that saw the STAAD files immediately saw the "weird" member and flagged it. I'm searching old STAAD files now.

 

[WARose]

I'm not sure if I get the picture of what is going on or not.....but if you don't need any stresses/other info on the "insulator", why not go ahead and do it's job and put the loads where they go into the HSS column?

 

[Lomarandil]

Agreed, unless you're doing a dynamic analysis, just model it as a rigid dummy member or bypass it entirely and apply the loads at the base of the insulator.

 

[oengineer]

My objective is to design the steel support structures, as far as insulators are concern I have already modeled them. However, I am trying to figure out a way by which I can transfer all my forces (dead load, ice load, short circuit load, insulator load, wind load) onto the HSS Column from 6” Aluminum pipe. Can I use insulator as a dummy member? If so, how can I depict that on STAAD. Also will the force be transferred correctly if I use a dummy member for insulator (REFER TO ATTACHED SKETCH).

http://files.engineering.com/getfil...edc-4d02-973b-406efe8e1202&file=Document2.pdf

 

[WARose]

If you aren't worried about how the insulator will perform stress wise (or otherwise) and all you need it to do in the model is transfer loads and design the steel....I'd just take a shot at the material properties of the insulator and make it a active member. I say that because I don't think you are going to be able to get the loads from that aluminum pipe to that HSS without an active member. (We may be mixing up our terminology here: some people call a "dummy" member in STAAD one that has a high E value but no density (for the purposes of constraint or load transfer).....but it is an active member IIRC.)

 

[BUGGAR]

Fuzzy memory says it is an active member with a special EI, or density, or?

 

[Lomarandil]

Absolutely, by dummy member I mean a geometrically correct rigid link -- still active in the model. Sorry, not familiar with STAAD to know if I'm getting the terminology right.

No offense, but substation steel design (IMO) isn't about getting the most precise answer to your problem. It's about developing a reliable, repeatable, constructable design. Way better to use a standard robust detail than it is to save 2lb/ft on a beam.

Besides, you are likely overestimating your knowledge/modeling of how each individual structure's foundation will behave anyway. For a bus support, you're probably on a bunch of individual drilled shaft foundations -- they're each going to settle and rotate differently from the next. That's likely going to affect your design as much as the properties of an insulator.

Is this really a single column bus support? Why bother with STAAD? Design it by hand and be done.

(All of the above comments assume that you don't need a specific dynamic analysis. And I mean really need -- it'd have to be a complicated substation retrofit or a high seismic zone with substantial space constraints before I'd get to the level of properly modeling insulator behavior.)

 

[BUGGAR]

For what it's worth, the last few California quakes damaged a lot of electrical equipment and so most of the SoCal utilities (SDG&E, PG&E) all do the dynamic dance. Loss of power to customers is a big deal with them so they are a little different from other industries. And they operate on our dollars so we need'nt be concerned with that.

 

[Lomarandil]

Buggar, it makes sense that SoCal utilities would be interested in seismic resiliency. What is the result of your dynamic analysis? Refined loading on manufactured components? Minimum clearances to prevent arcing during seismic events?

 

[BUGGAR]

I just did the structures that hold everything up. The electrical guys give me all the clearances from busses and similar that they need and I make sure everything stays that far apart during an earthquake. I recall all the manufactured things were seismically qualified and we had the masses and frequencies of the assemblies that we could plug into the appropriate node of STAAD.

 

[Lomarandil]

Hmm.. That's what I was guessing. I would have guessed it would be more intuitive to just space your conductors further apart and not worry about the nitty gritty of dynamic deflections. But most of the substations I worked on were in Kansas and Oklahoma... land is slightly cheaper there.

 

[BUGGAR]

And remember, this is California where it is wise to cover your shareholders legally by carefully following Codes. Our documentation of design was treated like a holy chalice and carefully filed away in their vault.

 

[transmissiontowers]

If you are worried about the deflection in the 6" aluminum bus tube, you need to model the insulator as a pipe but change the E to whatever the manufacturer of the insulator says it should be to get the deflection right. I use GTSTRUDL to model the bus supports and I am not very familiar with STAAD, but when you model the aluminum bus, you place the linear loads on the bus and you can do your load combinations.

I answered this question in the Transmission Structure Engineering forum but it seems like I am about the only one that posts there. If you have a copy of ASCE 113, read over the design sections. IEEE 605 also has info on bus design. We are revising ASCE 113 and it should be published in a year or 2. It is tough to get 30 structural engineers from all over the country to agree on anything.

http://www.eng-tips.com/viewthread.cfm?qid=402289

If you are trying to do a full dynamic analysis of the bus support for a short circuit event, you will need the forcing function from the EE's. From what I have seen and done for many years in the business, you use IEEE 605 or ASCE 113 to get the SC loading as a static equivalent load and combine it with the wind or ice load or whatever the substation owner wants you to consider, then design the column and base plate. If all 3 phases are on the same column, the SC forces cancel out on the support beam. The phase spacing on voltages above 345kV is so large, you get individual columns for each phase.


_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[oengineer]

Thank you all for your comments and suggestions. The information provided has been very helpful.

I am also working on base plate design for bus support structures. I am designing moment resisting base plates for HSS square columns. I am having some issues. I am aware of using 0.95 times the depth and width to determine m and n. The issue that I am having is with designing the base plate without it being supported by grout/concrete. The base plate is on leveling nuts. Based on everything I find in AISC design guide examples, all of the base plates are resting on concrete and all of the formulas are based on the base plate resisting on concrete to determine bearing pressure.

Would any one happen to know how to design the base plate without it being supported on concrete? I have designed base plates in the past for other structures in another industry, but all of them rested on concrete/grout. ASCE 113 mentions the design on base plates and the formulas given do not mention concrete strengths.The main issue I am having with the ASCE 113 formulas is the calculation of beff value. My base plate is a square with 4 anchor bolts. The center to center bolt distances are equal. Does AISC have any provisions for base plate that are not supported by concrete? What is the substation industry standard method for designing base plates? See link

http://files.engineering.com/getfil...c84-404b-9bdf-cbb025d6c9ed&file=Document1.pdf

for example sketch. Any suggestions/comments are appreciated.

 

[BUGGAR]

Yes there are specific guidelines for baseplates without grout, as is common in the electrical industry. My memory is bad but search for Substation Design Guides, or ASCE Design guide 8? Can someone else jog my memory here?

Don't forget the diagonal cuts through the baseplate at the 4 bolt holes in the corners.

 

[Lomarandil]

Ha, you're absolutely right about the documentation Buggar. We would send binders and binders of finite element output to the client. My pity on anyone who had to decipher that later (especially if they were using another FE software package).

I also recall that ASCE 113 is the right reference for baseplates without grout, but I left my references drive at home today. In short, you'll solve for a force distribution among your bolts using statics, then check the steel baseplate for point loads at bolt locations along straight failure planes (similar to a concrete footing). Often times a 45* failure plane that captures the most heavily loaded bolt (usually based on lateral load at a 45* angle) will govern.

Also, if you're using oversized holes to allow for some anchor bolt placement tolerance (typically a good idea), take that into consideration for distribution of base shear in your bolts. You probably can't count on all of the bolts bearing against the base plate simultaneously unless you tell the contractor to go back and field weld an oversized washer over each bolt. Usually it is easier to just assume the base shear only gets carried by some proportion (half?) of your anchor bolts.