concentric or eccentric?

[vato]

My 2006 siesmic design manual is in the mail, but I can't wait for the answer (which I have assumed will be in there).

I am designing a Special Concentric Braced Frame to be installed inside of some stacked shipping containers. Hitting the workpoint dead on is difficult to achieve for many reasons, mainly fabrication issues. So in elevation, how much "incidental" eccentricity can I have, and still call it concentric.

In the other axis, can I create a torsion situation, say the brace is attached to the side of a tube column. Am I still concentric? What if I resolve the torsion force (say shear studs on the column in the slab)? It feels like I am moving away from a SCBF if I do things like that.

Direct me to the correct literature if I ordered the wrong book.

 

[DaveAtkins]

I will defer to others who do a lot more seismic design, but I would say the kind of eccentricity you are talking about does not make your frame an EBF. For a true EBF, there must be a large space between the diagonals (we're talking feet, not inches) to allow a plastic hinge to form between the diagonals.

DaveAtkins

 

[Stillerz]

I have to ask....
You are building a frame that goes inside a shipping container?

 

[sandman21]

You can have a modified work point the seismic design manuals have an example of this. How much difference are we talking? AISC 341 might have requirements, I have not done a EBF

Can you? Yes. Should you? No. The loads for gussets plates get very large, ensuring proper behavior of the gusset plate under those conditions will be very difficult.

 

[IJR]

Stillers question is valid, what has this got to do with containers? You mean shipping the frame inside containers?

The idea of EBF is: you use less seismic force, get an economic structure, with the promise that you will allow the large displacements to occur without collapse. You are allowed to use less force because the EBF has an energy absorbing mechanism: the link. Unfortunately as the link yields, displacements increase. For an EBF to work, no part of the frame should ever buckle or yield. The only part allowed to fail is the link, and that has to only yield, and yield only in flanges, never in web(shear yielding not allowed). So if you use the EBF, you get an economic and a safe design, but it will cost you proper detailing, book keeping of displacements and stabilizing. I have seen many unstabilized EBFs, probably designed by fellow engineers who fast read books with the clean diagram showing the stiffened link only.

In zones 3 and 4, even for tall structures, a CBF will probably be OK. In zones 1 and 2, and for some small heights CBF will work. In zones 1 and 2 and some substantial heights, EBF may be a solution.

respects
ijr

 

[vato]

The frame is to be installed inside the shipping container, then the containers are stacked, and the frames are connected, box to box, at that time.

Getting the work point dead on is certainly difficult in this situation. We are looking at 65+ building height - D seismic zone, so I need a special concentric braced frame. Again, my apologies for not having the book yet, should get here today.

I will research aisc341 to see how much I can deviate from the work point. Hopefully this will address offsets in plan as well.

As usual, I have shown the architect how much room I need to do this without introducing any eccentricities, and I'm being pushed to push the frame into a much tighter space as well as trying to use parts of the container for the frame as well (just to make it a little more complex).

I do understand the concept of the eccentrically braced frame and I know that I am not trying to do that.

Thanks for everyones thoughts on this so far.

 

[ishvaaag]

Do not wait just for the mail, it is freely downloadable (or is there anything newer?)

http://www.aisc.org/WorkArea/showcontent.aspx?id=17638

Anyway, I think you will need to deal with the built eccentricities to ensure by design the structure remains a concentric braced frame by whatever the tolerances extant.

Then, I am an architect, I do NOT like the "container" architecture. This is even better advice than any technical I may give. These things were conceived to send away safely in normalized format merchandises, not to help people to live well. It is plain old Sullivan, form follows function (as we check strength by established methods). And nor the function nor the form are good for human dwelling. I am not between those that think that the future of humankind is to live like idiot beggars, and even less between those that see -or seek- an opportunity in disgrace.

 

[Stillerz]

This seems preposterous.
You are building a occupiable structure out of shipping containers in seismic D?
I am seriously asking.

 

[ishvaaag]

Believe it or not, Stillerz, there is a school of thought between some designers on that this is a kind of proposition that must be explored in practice.

 

[Stillerz]

I guess I could see that....but i would think you'd plan ahead. It sounds like in this case they are retrofitting shipping containers with frames. Like framing a house...after the fact.

 

[vato]

Many times the container is the reason for the building.
There is a lot of "love" out there for this sort of thing.
Although, there is very little understanding for the most part that we are modifying a very well engineered structure. It was not originally designed to perform as an element of a structural system. So the problem is actually very interesting, especially in this circumstance. The kind of problems that make you revisit principles, like exactly what defines one work point in a frame.

How close to the "work point" do the centerlines need to be without causing enough bending force to worry about?

Same question in plan view, when a brace introduces torsion into a column at the connection.

A little bit seems ok?, but I would prefer some code lanquage to quantify it.

If the force that may be created is resolved, then can you rely on the same response modification factors as a truly concentric frame, in plan and elevation?

 

[vato]

I just got the book.

"Concentrically braced frames are arranged such that the centerlines of the framing members (braces, columns, and beams) coincide or nearly coincide, thus eliminating flexural behavior."

 

[Stillerz]

I guess I would always consider using shipping containers in a building to be eccentric. If I also braced them, I owuld consider it an "eccentrically braced frame".
I kid.

 

[Stillerz]

vato- can you point to where that is state?

 

[vato]

page 3-2
I almost expected that sentence.

 

[Stillerz]

oh....i was looking at 341-05

 

[superbridge]

yeah, you can find in detail in AISC 341-05..
But I think torsion should be avoided by proper connection.



IN TODAY'S ENGINEERING PRACTICE, THERE IS NO SUCH THING AS A“NORMAL”OR "STANDARD" STRUCTURAL ANALYSIS.