ASCE Substation Structure Design Guide (Manual No. 113) 2

[structural-eng]

The ASCE substation structure design manual has its own load factors and combinations which are lower than the ASCE7/IBC. I understand why they are lower but I'm wondering what legal grounds an engineer has to use them since the document is a guide and not a code. We work for an electrical engineer and design the steel support structures and foundations for their substations. We just want to make sure that if there damage from a weather event that we're not drawings additional liability by using the reduced loads. I have tried reading both the IBC and a few state codes as well as the NEC and NESC and it's not clear to me which documents apply to substations.

 

[transmissiontowers]

First, I am not a lawyer and with today's society, someone will sue anytime something goes wrong (just my personal observation).

The NESC states that it covers only structures that support conductor wire that goes past the substation fence. Many designers use NESC for the other structures inside the fence just because they wanted to, but it is not required by NESC. You are correct that ASCE 113 is a guide and we are currently revising the 1st edition to add a few things like a foundation chapter.

I was Vice-Chair of the 1st edition and I am the Chair of the revision committee.

I've been designing substation structures for 40+ years and we created 113 to put into words what the trends were in the industry for the new Engineers coming into the industry. As a "guide" you are free to use your own factors and combinations. I don't remember the page and chapter, but somewhere in ASCE 7 there is a disclaimer that it does not cover our T-Line and Substation structures. I have been told by some really smart guys, we have ASCE 74 (also in revision) for our loads. In 113, we give some guidance on which loads to use and some suggested load factors, and they are different from ASCE 7.

My own philosophy is, the building and bridge engineers have to consider human life in their design loads and combinations and ASCE 7 reflects this with large overload factors. In our substations, the public or our workers will not be climbing our structures during an ice storm or hurricane or other extreme wind event. If our structures fall over in an earthquake or hurricane, the chance of loss of life is very small. It will knock out power for a few hours/days/weeks and your kids will not be able to update their Facebook page.

ASCE 113 is a consensus document and several Engineers got together and put in what we thought was a "Best-Practice" manual to guide the less experienced designer. It has gone through a peer-review and it is the best effort of some experienced engineers. If the Substation owner is a big company, they may have an idea of what they want the structures designed for or they may have some load combinations and factors that they want followed. The smaller owners may have to rely on a consultant to give them some guidance. ASCE 113 attempts to give that guidance.

The owner is free to specify that the structures remain standing after a 3000 year return period storm if they want a very strong structure (and are willing to pass the extra cost to the consumer), but we felt a 50 year MRI (like used in ASCE 7-05) was appropriate for our structures. The new edition of 113 will use the wind maps from ASCE 7-2016 with a few modifications.

Sorry for the long post, Comment back if you have other questions.

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

 

[structural-eng]

Thank you for the response. I know that ASCE 113 is THE document to use for substations. I was just having a hard time finding clear direction on whether substations fall under the IBC which most states adopt. I did some online searching of state codes in a couple states we're working in and was able to find that in those states utilities were exempt from the building code.

 

[transmissiontowers]

We try to stay away from the building code folks. We have gone to the extent of calling the "Control House" as a Control Enclosure when everyone I know calls it a house because it holds all the electronic stuff I know little or nothing about. I have heard that some utilities have fought with the building officials to keep from making the control house ADA compliant with ramps and all that goes with it.

My personal opinion is the building officials are more interested in collecting the fee from a big rich company for the building permit.

It is the opinion of the ASCE 113 committee that the IBC does not cover our structures inside the fence. We do not allow the general public inside the fence where only qualified and trained personnel are allowed. In most jurisdictions, the utilities operate as a regulated monopoly under a franchise agreement and are exempt from following the rules that regulate other businesses. As a public utility, we have the right of eminent domain (although these days it is harder to take property for the good of the public at a fair price) and can condemn land for projects.

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

 

[structural-eng]

A little follow up...

We designed our latest substation (located in South Carolina) per the ASCE Substation design guide and the owners 3rd party reviewer has come back and said that our approach to seismic connection design and anchorage is philosophically different than their recommendation which is to design all connections and anchorage per ACI 318, AISC 360 and AISC Seismic Design Manual.

Our analysis consisted of evaluating the structures for both seismic and wind loads per the Substation Design guide and then designing the members, connections and anchorage for the governing case per ACI and AISC. Since wind governed in all cases (we are located in coastal region) we didn't follow any seismic connection or anchorage requirements. If we had used ASCE 7 we would have had to determine a seismic design category and we would have been in Cateogry D for this site which according to AISC would have required the consideration of the Seismic Design Manual and using an R greater than 3. Based on the seismic design manual we used a max R of 2. I'm assuming the difference in R between the documents and the extra detailing requirements for seismic all goes back to the fact that these are unoccupied spaces.

My response was going to be that we are exempt from the ACI & AISC documents per South Carolina law.

Any thoughts would be appreciated.

 

[transmissiontowers]

Thankfully, I don't have to deal with seismic because my part of south Texas does not have them. The 100-140 mph hurricane winds generally control our substation structures. We do use load combinations of wind and short circuit (SCF) or NESC and SCF.

AFA your question, can you directly talk to the 3rd party reviewer and work out the differences? If they are extremely experienced in Substation Structures, you may lean to their way. If they are very inexperienced in our structures and have building or bridge experience, then you might be able to reason with them that their assumptions on seismic do not apply in your area because the high wind controls the design.

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