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Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure
2

Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure

Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure

(OP)
I am 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 (from AISC Design Guide 1 & AISC design examples v13), 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 have information (an example calculation, reference, design guide, spread sheet etc.) on how to design the base plate without it being supported on grout/concrete? I have designed base plates in the past for other structures, but all of them rested on concrete/grout. This structure is located in a substation. ASCE 113 mentions the design of 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? If AISC has any information on non-grouted base plate design that would be preferred. What is the substation industry standard method/design procedure and steps for designing base plates? Any suggestions/comments are appreciated.

RE: Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure

The baseplate strenght is independent of the concrete strength if it is supported by leveling nuts. The anchor rods, however, will be dependent on the concrete strength. What do the equations look like? I've never had a look at ASCE 113.

RE: Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure

Oengineer:
Take a look at the design of the large formed steel transmission poles which are supported on leveling nuts and stand several inches above their conc. base. They should be pretty much the same, except for larger size and bigger loads. The base pl. is pretty well supported and stiffened from above by the HSS pole. Then the base pl. acts as a cantilever out to the leveling nuts and the A.B’s. You may think a bit about how much base pl. width has the max. canti. bending stress if the A.B’s. are widely spaced. Otherwise it should be pretty straight forward., a couple btwn. the T&C A.B’s. for the moment, plus the gravity loads. Of course the tension A.B’s. have to be sufficiently embedded in the conc. to take their load, and the found. and pier must take the loads. There may be a slight complication if loads are applied on a diagonal axis, but this might be resolved as Mx, plus My, plus gravity loads.

RE: Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure

(OP)
@mike20793

ASCE 113 just mentions two equations. One is for determining the force acting on the anchor bolts and the other to determine the thickness of the base plate (BP) steel. Here is the link to ASCE 113 discussion of BP design http://files.engineering.com/getfile.aspx?folder=b... . Since there is no bearing pressure from the concrete to the bottom on the BP I am trying to determine how to design the BP. All the material I have found on BP in AISC is dependent on f'c. Here is a sketch of the type of BP I trying to design http://files.engineering.com/getfile.aspx?folder=4... .

RE: Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure

(OP)
@dhengr

Would you happen to have any information (examples, references, design guide, etc.) regarding the design of large formed steel transmission poles which are supported on leveling nuts and stand several inches above their conc. base? Is there a link to where I could get this info?

RE: Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure

The information you need is in the document you referenced in response to my earlier post. Those are the equations you need to follow for your case.

RE: Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure

(OP)
@mike20793

In my OP, one of the issues I am having is in determining the beff value. This is mentioned in the document I referenced. As it states on page 103 in that document this value is rather difficult to obtain. I am trying to determine if anyone has any other information. Another way to approach this design.

RE: Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure

See attached. If you think about what is happening to the baseplate, this 'failure' line makes sense, although I'm sure it wouldn't be quite in that shape but it has worked well according to the document. The shortest effective width will result in the largest stress.

RE: Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure

Aren't we talking about this somewhere else? Where are your corner cuts?

RE: Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure

The anchor bolt WG of ASCE 113 is proposing options for the design of anchor bolts supported by nuts. For the time being, look at the ETAG method of designing the anchor bolts. Designing the base plate itself should be designed in a conventional method of a rod/bolt supported situation - ignoring the grout. (the bolts will be in compression and tension - make sure the bolts don't punch through the bottom of the footing)

RE: Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure

(OP)
@ Buggar

I mentioned it in my bus support structure post. The sketch is a template.

RE: Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure

Can someone explain why you would ever not grout beneath a base plate? Even if you could make it work without grout (which I'm sure you can), why would you ever want to do this? Wouldn't dirt and rubbish etc get underneath it? Must be much worse from a fatigue perspective too.

RE: Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure

(OP)
@buggar
I have been instructed that for the bus support structures, I will not have corner cuts. Do you have any examples of base plate designs that you have designed?

@mike20793
Thank you for the sketch.

@oldrunner
Right now I am just focus on the design of the base plate. My main goal is to determine away to calculate the req'd base plate thickness due to axial, shear,& moment forces. ASCE 113 appears to have a straight forward way of designing anchor bolts.

RE: Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure

(OP)
@BowlingDanish
Apparently, it is very common to design anchor bolts without grout for substation structures in the utility industry. I am new to this industry. In the past, when designing base plates I always had grout so I just based all my designs on AISC.

RE: Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure

I have never worked on any substation structures, but the lack of grout may be due to the fact that you can get really large baseplates and have large circular or polygonal shapes that don't allow for grout holes near the center of the plate. It may be too difficult for them to get the grout in, whereas for a wide flange column, we can easily provide grout holes near the web to get grout under the center of the plates.

RE: Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure

Mike has it why we don't grout the baseplates! There are flowable grouts now but it takes time for industry to catch up. And if it ain't broke...........
The 2016 AISC code is finally going to allow us to use larger hole sizes for the larger diameter anchor bolts so we can finally tell the fabricators what they have been whining for. Remember to keep your anchor bolt projection short enough that anchor bending is not an issue. No, I don't remember the maximum distance to the bottom of the lower nut (2 x anchor diameter?).

RE: Non-Grouted Moment Resisting Base Plate Design for Substation Bus Support Structure

I should mention a big issue in this industry is the welding of monopoles to baseplates to develop the full strength of the pole. For some reason, gussets are seldom used. Frequently, the plate has an opening for the pole which is fillet welded above and below the plate. Some poles are big enough for a cjp weld with person access to the inside. You will see other details for this joint also.
Did I mention they don't like field welding?

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