Direct Anslysis?
Direct Anslysis?
(OP)
This is probably a simple question for some so if it is I apologize.
When designing a steel structure using the DAM. What forces do you design the base plates and foundation for? Do you design them for a regular pdelta analysis neglecting the DAM results? Or do you use the results from the DAM?
When designing a steel structure using the DAM. What forces do you design the base plates and foundation for? Do you design them for a regular pdelta analysis neglecting the DAM results? Or do you use the results from the DAM?






RE: Direct Anslysis?
RE: Direct Anslysis?
I don't think the question of second order effects on framed structures can be considered simple or obvious. I am not familiar with the DAM, but in reading about it briefly, I understand that it uses a notional lateral load of 0.002ΣP at each storey to represent erection and manufacturing tolerances. This is presumably based on an erection tolerance of 1/500 times storey height. For a 10' storey height, this would be a tolerance of only 0.24" per storey.
The Canadian standard uses a notional lateral load of 0.005ΣP which was recommended by Clarke and Bridge based on research performed at the University of Sydney, Australia. This appears to be considerably more conservative than the DAM and I am not sure why there is such a large discrepancy between our two codes.
Whatever assumption you make, I agree with StructuralEIT that the baseplate and foundation must be designed for the notional load combined with the second order effects of gravity loads operating over the resulting eccentricity, i.e. the PΔ effect.
BA
RE: Direct Anslysis?
I am not familiar with the Canadian Standard. From my uses with the DAM I do know that part of the analysis requires you to modify the axial and flexural stiffness of the frames. Does the Canadian Code require you to do the same thing?
RE: Direct Anslysis?
that does seem like a big difference in notional loads. Does the Canadian Standard require a stiffness reduction factor, similar to the DAM?
RE: Direct Anslysis?
BA
RE: Direct Anslysis?
RE: Direct Anslysis?
Now, the DA method specifically allows this stiffness reduction to be ignored for things like drift or deflection design. This is because these are deflection based values, not strength based values.
Considering both of these things, the demand moment and axial force of your column bases should be based on a DA method including the stiffness reductions.
However, if you are looking at rotation or deflection limits for your column bases, then you could ignore the stiffness reductions.
RE: Direct Anslysis?
If I remember correctly, the stiffness reduction is to account for softening of the steel at strength level loading with regard to second order effects. The reason I say that is that even though you use the stiffness reduction for the analysis, you only use that to get the forces - you don't use a reduced EI or EA on the design side when calc'ing the capacity.
RE: Direct Anslysis?
"Softening of the steel".... Not sure what you mean by that.
You make a good point in that these stiffness reductions are used in your analysis and do not apply to capacity calculations at all. Though that is somewhat off-topic....
My basic point remains the same. You use the full DA method to come up with the force and moment demand at the base of the column. But, you can neglect the DA stiffeness adjustments if you are more concerned about deflections or rotations at the column base.
RE: Direct Anslysis?
I agree that serviceability considerations (drift, etc.) should not include the reduced stiffness factors.
RE: Direct Anslysis?
We design for a notional horizontal force of 0.5% at each storey. Analysis of the bare frame is carried out, and 'alpha cr' value calculated for the whole frame. This is a measure of susceptbility of the frame to second order effects. Second order effects are either neglected, or first order effects amplified, or a full second order analysis carried out depending on the value of 'alpha cr'.
The code (BS 5950 or EC3) specifically states that notional horizontal force effects are exculded from foundation loads as they are not 'real' loads.
VB
RE: Direct Anslysis?
RE: Direct Anslysis?
RE: Direct Anslysis?
RE: Direct Anslysis?
I recognize we're dealing with tiny loads (relatively speaking), but whether we're talking about an out-of-plumb structure or a notional load that is equal to the out-of-plumb dimension or actually modelling the out-of-plumbness the base shear is really there.
RE: Direct Anslysis?
User Note: The notional loads can lead to additional
(generally small) fictitious base shears in
the structure. The correct horizontal reactions at
the foundation may be obtained by applying an
additional horizontal force at the base of the structure,
equal and opposite in direction to the sum
of all notional loads, distributed among vertical
load-carrying elements in the same proportion as
the gravity load supported by those elements.
RE: Direct Anslysis?
If you model an H/500 out-of-plumbness to your structure and you run the analysis for gravity loads, there will NOT be a net shear at the base of your structure. There will be a net moment, but not a net shear. Make sense?
The Notional Loads are fictional loads that are intended to approximate the destabilizing moment that results from the out-of-plumbness of the structure. But, they are not real loads.
RE: Direct Anslysis?
I understand that the total base shear (when you look at gravity AND lateral elements) is 0, but who ever looks at base shear for gravity columns. They all have some based on the displaced shape, but I've never considered that in design unless the column was intentionally sloped......... and even then it's base shear doesn't figure into the actual lateral analysis.
RE: Direct Anslysis?
What's your new handle?