Another Seismic Question

[Lion06]

I'm using an OMF in a high seismic zone (It's a one story home), and I'm looking at the connection design.

I'm using HSS round columns with WF beams and I'm looking at the connection design. I see that the hinge can be in the column or the beam for a OMF, and that the connection needs to accomodate 1.1 times the capacity of the member that the hinge will form in (including the ratio of Fy,avg/Fy,min).

It also looks like prequalified connections are not required for OMF, just that the connection capacity must be adequate per above.

So, here's my situation. My column and beam capacity are reasonably close, but the hinge forms in the column. I go through and determine what the connection capacity needs to be. All is good so far. I'm using HSS through-plate moment connections for the pipe column to WF beams. Now I design the plate thickness and bolts to transfer the momet - I picked larger bolts so I have a reasonable number. Next I check block shear of the flanges of the beam, and, of course, it fails. It's more than adequate for the demand, but the capacity is now less than the column.

Since block shear is a brittle failure, I'm assuming that this means that I need to either provide more bolts so that block shear doesn't reduce the beam capacity so much, but I can't find anything to confirm that.

Also, I'm unclear from AISC 341-05 if the welds from the pipe to the through-plates need to be full-pen welds for a OMF.

 

[ishvaaag]

In spite of that I of course think that we better implement corrective measures whilst we don't know how better do, the point is that I think that the procedure to investigate the solicitation for one target level of striking earthquakes remains defective, elastic or not, if we have to still apply an overstrength procedure.

If one knows what is the proper level of response, the concomitant happenstances anywhere in the structure should be reasonably readily determined, and you wouldn't need to overdesign anywhere in the structure above what that way determined.

I think it inconsistent to say that we only expect demand to attain some level and then so hugely (sometimes) amplify some details "because we are unsure" or "because we want ductile failure". Well, I don't want failure, ductile or not. Nor I am so adamant on ductile behaviour, if it stands what targeted.

I am not saying that to in any way diminish the advantages that ductile failure can bring to one structural system or set of structural systems or type of structure, but to emphasize that what we need to add is knowledge, not blindly material.

 

[ishvaaag]

Just on that line (there is a history and a philosophy to everything) I remind some article at one of ACI's Special Publications where the US' sent investigators to some high intensity earthquake in Chile effects were clearly impressed by the sound structural behaviour of chilean buildings with shearwalls, that, they observed, had a higher in-plan area ratio to what supported than was common in USA. The success there came not from flexibility, but from sheer strength. Not an unexpected surprise since a lot of US practice was along the lines of moment frames being immunes to earthquakes (that also had been discovered by some were not, and Northridge proved so), and so flexibility was having the upper hand.

So against earthquake a set of alternative possibilities remain open.

 

[JoshPlumSE]

Ishvaag -

You can design structures exactly in the way you describe. It's called "performaced based design". It usually involves some form of more complex analysis....push over analysis or non-linear time-history analysis.

I'd guess that the cost of engineering the structure increases by a factor of 10. Generally speaking this is only done for existing structures that cannot be shown to be sufficient with traditional methodology.... Because that's the only time clients are willing to pay that much for their engineering!

Also, engineers will take on more potential liability when they choose to design in this manner. That's very important here in California. We engineers are outnumbered by lawyers by a large margin.