Topic: Structural Help

[sea-dawg]

My company asked me to draw a structure that will need to support three transformers that weigh 900 pounds each. Unfortunatly, although the drawing may look nice, I am not a structural engineer. Is there any one on here with their infinate wisdom that might be able to tell me wether or not the structure will be strong enough to support the transfomers.  I have all of the deminsions on the drawing, the only thing that I have not included is the type of steel, all of the steel will be galvanized and be ASTM A-36 grade. Any help would be greatly appreciated as well as any recommendations.

Thanks!

[Keith™]

The pieces of the drawing are not labelled and thus I cannot determine which material you are using for the angles the transformers will be set on. I am going to guess that given the 8' length of the structure it is the 5x3x1/4. Coming from my persepective it seems a little light, but then my calculations are probably based up a more rigid requirement than something of this nature.

[jnieman]

That pdf I printed you didn't preserve the color coding because I plotted it with my color table unintentionally... so the colored text got changed.

[Bob Garner]

In the U.s., angles and channels are produced to ASTM A36, Yield strength = 36 ksi.

Wideflange shapes are now produced to ASTM A992, Yield strength = 50 ksi.

In the weak direction you are using a braced frame.  In the long direction, you are using a moment frame (meaning it resists lateral loads by bending at the joints).  Are you in earthquake country?  If so, you have to do the seismic analysis dance.  IEEE has special requiremen st for electrical equipment in seismic zones.  You also may have to allow for wind loads.

If your not in seismic land and wind is not an issue, I can do a rough static structural check for gravity loads only.  Let me know.

Bob G.

[Bob Garner]

Some suggestions:

Angles are very weak in bending due to buckling of the outstanding legs.  I would suggest you make the runners at the top out of channels and cross brace with the angles.  These angles would also brace the channels for more bending capacity.

Connect the end braces to the tops and bottoms of the legs so they don't induce bending in the legs.  I think the FB 1/8" x 1" is not strong enough for the abuse such structures seem to be subject to.  If you see such braces you will notice they are usually bent all to he77.  Again, maybe use a different section for the legs since angles aren't that strong.  HSS (hollow structural sections) square in shape are common for legs, er. columns.

Can you cross-brace it in the long direction?  This gets you away from relying on the bending strength of the members, but it will induce axial loads.

Foundation?

I think between all of us we can wack this out for you.  Unless there's seismic.  Anyone game?

[Bob Garner]

s'more thoughts (I just finished a project and have time on my hands - can you tell?)

When I've done electrical like this, you have to allow for the weight of the electrical drops, the wires that drop down from the bus (or whatever) and fasten to the tops of the insulators.  You can safely allow 6 pounds per foot of wire for this.  What are those brackets on the sides of the transformers?  Is someone gonna hang a load off these?  Hopefully they're not dead end brackets that take a lot of wire tension - 6,000 pounds on the last substation I designed.

What is the configuration of the transformer "feet"?  This could dictate how you run your framng under the xformers.

Bob G.

[Bob Garner]

One more and I'll shut up.

If you can tell me where in the country you are located, I can tell you if you have seismic woes.  The new building code (IBC) now has most of the U.S. in some kind of seismic category.

Bo

[Kerry]

Sorry to spoil the fun. I'd be getting this designed and certified.

nevertheless,
I don't think you have sufficient lateral restraint on the unit
and I don't think the top support is strong enough, considering the point load in the center.
Are the transformers restrained in any way ?
Is the unit fixed in any way to the floor ?

My rule of thumb is "would I stand on it" ..

[Strucmad]

I agree with Kerry , that thing is going to need certification.
I don't think designed by 'swamp members' is going to cut it.

[Bob Garner]

I don't disagree with you, Kerry and PSmPort.  I would design this (not for wind or siesmic), but let it be understood I would not seal this design.

Actually I would provide a complete signed and sealed design.  But it's going to cost $2500 U.S. plus expenses. 

Bob G.

[Keith™]

I don't think designed by 'swamp members' is going to cut it.

Perhaps not, but those who are familiar with this type of structure should be able to determine with some degree of accuracy the suitability of the design.

This structure appears as if it will be placed on a foundation and will support 3 dead load weights of 900# each, if we consider 3 strands of wire, as per Bob's second post, depending upon the distance of the supporting structures, you could be experiencing an additional 3000#.

Using an evenly distributed load and a maximum deflection in the Z axis of 1/8", I find the top 2 angle members inadequate. By my calculations, an appropriate member would be HSS6x2x3/16 A500 Grade B (47ksi) loaded on the long axis of the member or alternatively a W4x13 A36.

I don't normally do seismic calculations, but in the long dimension of the structure I think you need to have additional bracing. I agree with Bob that the FB1x1/8 isn't adequate. Additionally, it is possible that twisting of the frame in even a moderate seismic event will likely cause collapse.

If we address the number of issues, I think you can have something that will withstand whatever codes and engineering requirements you need.

[sea-dawg]

I appreciate the feed back, I will get with my boss and discuss some changes that need to be made. Mr. Bob, as far as the siesmic studies, I am in S.E Georgia so that is not something that we really have to worry about. Since you know about sub-stations you will understand what I am referring to, the structure will be placed in the same bay as the regulators and it will have switches directly above, so the weight of the wire will be minimal (there is only going to be 5 foot or so). The brackets on the side of the transformers are the mounting brackets, although not shown, we plan on using those to brace and secure the transformers to the structure.

It was mentioned that there wasn't any description of what the steel was, I apologize for not mentioning, but I had the descriptions listed on the layout page. Also it was asked if the structure is going to be mounted to any thing, yes, there will be a concrete pad poured with bolts installed to secure the structure to the ground. I agree with the fact of the legs being made out of angle is not a good idea, I mentioned that to my boss, prior to drawing, that we should use square tubing and he wanted to go cheap on me!! The transformers are basically 75 kVa distribution transformers, it really doenst have any "feet" so to speak, it's just a round pot with a bunch of oil and copper inside.

Keith, let me make sure that I follow you, you are saying that the two top peices of angle are not adiquate because of the thickness? That if we go with a thicker angle it would be ok? Also if we use something heavier than the 1" x 1/8" cross bracing that it would be ok.  We are planning to use 6" channel to run long ways, what if we were to use the same thing on the short side. It was mentioned about the legs buckling
would the channel not help support that?

Like I said in my first post, I am not a structural engineer, and I really appreciate all of the feed back that I have recieved as it will be very helpful in making the modifications that we need.

Thanks again!

[Keith™]

Keith, let me make sure that I follow you, you are saying that the two top peices of angle are not adiquate because of the thickness? That if we go with a thicker angle it would be ok? Also if we use something heavier than the 1" x 1/8" cross bracing that it would be ok.  We are planning to use 6" channel to run long ways, what if we were to use the same thing on the short side. It was mentioned about the legs buckling
would the channel not help support that?

I believe the top angles that support the weight of the transformers will not be adequate and may buckle under the weight of the transformers.

Understanding that steel shapes are typically sold by weight, the key is to use the strongest material for the weight. Using the new information, I would suggest replacing the top angles (where the transformers sit) with at least S3x7.5 A36 steel. Those beams are slightly heavier and should give you more strength than the angle, especially in twisting. It wouldn't hurt to also provide a horizontal brace between them directly under the load.

[Bob Garner]

'nuther suggestion if I might (and I don't mean to bugger up the original design):

How about inverted chevron bracing in the long direction. With inverted chevron bracing, the braces in the long direction go from the bottom of the columns to the center of the long beam.  This provides lateral bracing and also provides support in the center of the beam supporting the pots.  In building design, by code, you're not allowed to consider that this bracing supports the beam and you have to design the beam as if the braces weren't there.  But I believe you have a "non-building structure" and you can consider that this bracing also supports the center of the beam.  I'll have to leave it to you regarding what codes you have to hop to.

Some good news:  Looks like SE Georgia is indeed in a "soft" seismic area.  Electrical equipment is generally considered "occupancy classification IV, essential facilities".  This is a code classification so don't expect it to make any sense.  But considered with your location, you're in seismic design category A, the least restrictive of all.  Although you may not have to figger seismic loads, you still need to allow for lateral wind loads or some minimum lateral load so the structure doesn't fall down when that 200 pound electrical supervisor leans against it.  Some people say if nothing else, design for a lateral load of 3% of the vertical load.  This is not "official" though, just a guideline.

Anyway, best of luck with this, and I'm happy to help out in any way I can.  Of course this is "off the record" help for liabilty reasons or whatever.  But I can't let a swamper go without whatever help I can provide.

Bob G.

[Spike Wilbury]

But I can't let a swamper go without whatever help I can provide.

I like that ^^^^