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Re: need help

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Daniel:

The second part of your initial question asked about how do you know
whether you have achieved an economical or efficient design. From a
practical standpoint, the efficiency would be evaluated based upon cost
of the entire building, not just the structural portion. A purely
efficient structural design may not always yield an efficient total
building design. For example, deeper floor beams may be structurally
efficient but will add to the total floor to floor height and therefore
will increase the cost of the exterior walls, mechanical runs , stairs
and elevators. Your main objective should always be the lowest total
cost for the whole building.

That said, they may be specific areas that you can investigate. For
example,  a rigid frame  controlled by stress can be optimized by
designing members with stress ratios as close to 1.0 as practical.  A
rigid frame  controlled by lateral deflection will require the engineer
to evaluate which is more efficient: to stiffen the columns or the
beams. This can be evaluated by looking at the deflection component
controlled by the stiffness of the column and the deflection component
controlled by the beam.

The approximate lateral deflection component can be derived from the
portal method. Column deflection due to column stiffness= fixed-fixed
columns with reverse curvature due to story shear. Column deflection due
to beam stiffness = beam end rotation times the column height.

Beyond this, keeping the details simple and repetive will make for cost
efficencies which will help minimize the total building cost.

Jim Kestner, P.E.
Green Bay, Wi.