Steel Structures Foundation

[jjrg]

I am a Junior Engineer and I am interested in Steel structure to foundation connections details, here in the Dominican Republic 99% of buildings are made of concrete and those one made of steel are put together with the foundation via and end plate (these are light storage buildings). How would I attach a 10 or more story steel building to a foundation? (If I wanted a fixed base for the structure) We are in a very seismic active zone having two plate borders region in our island. Would it be better if we considered our buildings with a pinned attached to the foundation instead?

JJRG

 

[Imagineer]

Check out AISC Steel Design Guide #1 "Column Base Plates". In general there are 3 different methodologies depending on the columns loads: 1)No moment, therefor, no eccentricities; 2)small eccentricities, when the resultant is within the kern of the plate; 3)large eccentricities. Different equations apply with each case. You will have to do a couple of load cases because I've noticed that they control different aspects of the baseplate and anchor bolt design. Typically, I end up with calculations for case 1 and either cases 2 or 3. One will control the thickness of the plate and the other will control the anchor bolt and pier design.

To be conservative, I would design the column as if it was pinned and design the baseplate, etc. as if it were fixed.
Imagineer

 

[dougantholz]

JJRG,
A good place to start is with the AISC design guide, and of course the AISC LRFD design manual with further details base connections.
When you are talking about tall buildings, you have to of course consider the drift of the building Vs. what the architect wants. If you can add enough brace lines so that you can pin your bases I think this is a better engineering practice. You can model the building with pinned bases, but you naturally would have some fixidity due to the weld of the base plate. This would add to the strength of the buliding, which is always good.
One way that I have done it is to have the columns carry through the ground floor into a basement where they attatch to footings (assuming piers/piles or mat for a larger building). You then only need a very stiff first floor diaphragm to drag the force out to very stiff shear walls/ basement walls. If you can handle the shear from the force couple in the columns into the baseplates (easily accomplished with 4-6 1" diameter bolts, bang you have fixidity.
That's just the way that I have done the details. Admitedly you are in a higher seismic zone than I am, so your drag forces could be very high. If you can't drag the force out of the braces, then try making the bottom floor out of steel or concrete shear walls instead of braces.
Hope this helps
Doug

 

[whymrg]

Another approach for fixed base is to use channels welded to column or stiffeners welded to column and base plate. This will reduce thickness of base plate.

You may find this in Chapter 3.3 of "Design of Welded Structures" by Omer W. Blodgett. This is very good book which cost less than $20 and distributed by book division, the James F. Lincoln Arc Welding Foundation, P.O. Box 17035, Cleveland, Ohio, 44117-0035, USA, fax No. 216-361-5901 (you may verify price and postage).

Good luck.

 

[engcomp]

I like your philosophy, Imagineer - "design the column as if it was pinned and design the baseplate, etc. as if it were fixed" - hang the client's money.

That's what clients think we do, anyway. And if something goes wrong when we are less conservative, who gets blamed?

Good on you, Helmut

http://www.engs-comp.com/

 

[Imagineer]

EngComp,
The difference in material cost is minute. Of course, the client's bean counters won't actually LOOK at the numbers to see that. But, that's when you go into the "safety" discussion. Which is the real reason for designing worst case, anyway. I'd love to find a situation where I COULD design for best case but, alas, I doubt that will ever happen.
Imagineer

 

[engcomp]

I have a computer program called OPT_SHED, which designs all elements of a portal frame building on the basis of cost-effectiveness.

After the program determined the wind loads from your site information, it lets you choose a cladding from many alternatives - a dearer cladding requiring fewer supports may cost less overall.

The program finds ALL viable solutions for columns, rafters, purlins and girts. As you select a solution, which could be small columns with big rafters or big columns with small haunched rafters or any combination between these extremes, the program returns a cost index.

Column bases may be fixed or pinned - you'd be surprised what difference it makes.

Look for the program here, regards, Helmut engcomp@ozemail.com.au

http://www.engs-comp.com/