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Re: Structural System Determination

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Ah,  since I was called upon ... I didn't completely read the previous answers but I would agree with the overall thoughts.

Usually, selection of the structural system is a question of economics but there are a lot of factors contributing to the economics.  

In DC proper (as opposed to the the "DC area"), virtually all construction is concrete,  with quite a bit being post-tensioned, in order to allow longer spans.  This is due to the fact that both contractors and engineers are well versed in post-tensioned concrete design and construction, which in turns stems from the fact that there has been a post-tensioning supplier here since the 70s.  There is in fact a height restriction which amounts to a limit of 10 or 12 floors, depending on how tight you can pack them.  Concrete typically allows closer packing than steel.

Outside of DC proper,  it is still mostly concrete (because of the experience factor)  but steel use is increasing,  in part because some new steel plants opened up (one in Petersberg (?), Virginia).  

One problem with steel is that you have to be able to transport the pieces - if you need a 90 ft piece and there is a turn somewhere along the way that wouldn't allow a 90 ft piece to get through, you may be out of luck.  The same holds true for precast but precast plants seem to be more common than steel plants - steel plants require a fairly sizeable capital investment.

Boston is similar to the Detroit area.  Heavily unionized and subject to weather that puts cast-in-place concrete at a disadvantage.  Almost all construction in Boston is steel,  but there are some very good concrete contractors.

Nominally the architect is the one making the decision but usually they will ask the structural engineer for guidance.  As in everything else,  everybody has an agenda.  Given a choice between a precast and a post-tensioned parking garage,  most structural engineers would be able to come up with lots of reasons it would be better to have a post-tensioned garage.   One thing they probably wouldn't point out is that if it is a precast garage, their work will be limited to designing the foundation,  the precaster typically designs everything else.

Normally the owner doesn't get involved unless it is a question of cost - I designed an office building out by Dulles a couple of years ago- we did the schematic design as post-tensioned concrete but one of the contractors bidding the job said they could do it cheaper in steel.  So the owner said he wanted a steel building.  So I designed a steel building.  Last time I checked it was still standing.


Gail Kelley