Base Plate Design for Bus/Equipment Stands in HV Substation

[livewire9]

For three years I worked with a large utility that has a complete library of standard structures. Everything is overdesigned so as to provide fool-proof reproduction of substation structures. There are no managers or supervisors with enough gumption to question the use of costly designs which ultimately come out of the rate payers' pockets.

Now I'm with a company where my design will be implemented in the construction of brand new substations supporting renewable energy. By nature, I am meticulous and not wasteful of money or resources. I have no problem sizing the steel members to support bus work and equipment. But I could use a little feedback on base plate design. At the utility, 1" base plates were used on all structures. So far, I have done some fundamental calcs evaluating the yield strength of plate under a resolved moment. I found that 1/2" plate can be used for most structures and 3/4" plate used for the worst-case (i.e. tall, single-phase bus support). Any thoughts?

 

[ToadJones]

Check your designs with the many AISC base plate examples out there. The much maligned Design Guide 2 covers this.
For standard wide flange columns or posts with axial loads, the design is pretty straight forward.
Having said that, there is some savings in standardizing equipment & connections. No point in reinventing the wheel on a job-to-job basis.

 

[paddingtongreen]

Question, are you grouting the baseplates or are they supported by nuts on the anchor bolts?, it makes a difference.

With galvanized structures, we, and many others in the business, kept an air gap under the baseplate.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[SlideRuleEra]

Without going into detail, consider that components of the electrical generation / transmission system (such as the substations you are working on) are essential services and are expected (by the public) to survive predictable catastrophic events (e.g. hurricanes, earthquakes, heavy snow loads, etc.) with minimal damage. Having electrical power available is a primary requirement to begin recovery from a disaster. The bottom line, IMHO, be conservative in your designs - no one will care how much money was saved during construction if power cannot be delivered (by the utility) for weeks / months.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[livewire9]

I appreciate the responses so far. First, the base plate will be off the top of concrete with two nuts for each anchor bolt (galvanized steel structure). Second, I am aware of the critical nature of such facilities. Our substations are all within the same region where 90 mph wind and short circuit forces will govern. Seismicity is not a consideration (Seismic Zone 0). And lastly, our scope of work is such that I will be able to create a standard, efficient design for each type structure to be confidently installed as, again, all of the work is within the same region. Safety factors (LRFD and ASCE No.113) are built into the design from the top, down. I don't want to arbitrarily call out 1" base plates across the board to incorporate additional safety. From a preliminary drawing I have, we will need 283 base plates for equipment and bus stands. Consider this, if 199~ 1/2" and 84~ 3/4" plates are required for one large substation and 1" plates were instead ordered, how much unnecessary steel has been purchased? I estimate 5,410 pounds! Can anyone provide a dollar estimate for this excess steel?

 

[ToadJones]

very rough estimates, about $2750, or about ~30 engineering hrs.

 

[SlideRuleEra]

MasterSnake - Here is a thread from 2 years ago with estimates for fabricated steel from about $1.50 to $3.00 per pound:
thread507-219493
To make a fair comparison consider that the engineering time and fabrication process are very similar whether the thickness is 1/2", 3/4" or 1". I would say that an estimate of $1.00 to $1.50 per pound is reasonable, based on that thread. Say $8000 for 5410 pounds.

Please don't misinterpret my suggestion to be conservative as meaning to make a "mindless" decision to go with 1" thick material for all baseplates.

My idea of conservative for this application could be to use 3/4" thickness where 1/2" is calculated and 1" thickness for the 3/4" situations. By proportioning out your numbers this would appear to be about 3200 pounds of "unnecessary" steel that is "worth" about $4800.

To make a fair comparison, be sure to consider this cost (whatever you decide it to be) against the total value (not just the structural aspects) of the substation project.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[livewire9]

Thanks again for the feedback, fellas! SlideRuleEra, the link to the thread regarding steel pricing was informative. I should of done a little more homework before posting the question. But hey, we all learn as go along. Specifically, for substation design, I referred back to ASCE No. 113 and found formalae to determine required base plate thickness. I ran some calcs and it looks like, indeed, base plates will come in around 1". Maybe I was a little harsh on the big utility... at least my intentions are good in not trying to be wasteful in design.

 

[paddingtongreen]

I asked about the grout because those with airspace under them see bigger bending moments.

One of the things I quickly realized when I was a young, gung ho, designer was that the old guys were no slouches, If I found something I thought was over designed I learned to ask why; if I saw a short cut, I took a good look because if it really was shorter, it would be the well worn path, the people who came before would have checked it out.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[ToadJones]

Along the lines of what Pad is saying....
The first place I worked had some good "old guys". One in particular I remember was a very practical engineer. He had very basic approaches to his designs and was very good at explaining himself. This, even though there was a bit of a language barrier with his being from India.
I can recall him having me design base plates for a large industrial project. When I came back with 8 or so different thicknesses, he patiently sat me down and explained how my calc's were correct but my engineering was all wrong and how much more simple I could make our schedules and fabrications if I grouped those plates into one or two thicknesses.
He also used to explain that "engineering is not an exact science. We have to make guided decisions based on the best research available. If we get a member size that we think is on the edge of that research and science, there is nothing wrong with going to the next biggest size."
I've tried to carry his practical advice as it came from real number crunching, not an FEA program that spit out a stress ratio of 0.95.

 

[oldrunner]

Even though you are in Seismic Zone 0, according to the IEEE 693, your equipment (and structures) is expected to have acceptable performance at 0.1g and less.

Refer to Section 8.5.2.

 

[goncho]

Take a look to RAM Connection software and this paper for more accurate design including biaxial analysis

http://communities.bentley.com/prod...l-analysis-of-general-shaped-base-plates.aspx