rb1957 said:
1) they design in margin because they anticipate neglect inservice ?
2) they "overdesign" because it is cheaper for them to use larger sections ?
3) they "overdesign" (really "underdesign") using conservative analysis because a tonne of analysis isn't worth a pound of steel ?; and of course ...
4) they're "lazy" = efficient with their time to maximise profit
2) Cheaper Sections: Yes, we would do this at times.... use "preferred" sections based on who was supplying the steel and which sections are easier for them to produce. Not really something that I got involved in. But, there were sections that were specifically excluded for us.
3) During my time there, the overdesign was usually related to being on a "fast track" project where the initial design was done with "estimated" mechanical equipment based on similar specifications from previous jobs. But, it then (usually AFTER the foundations had been poured, and the steel bids had been awarded), we'd find out which vendors one the bids and what equipment we'd actually be supporting. The structural group (which had been almost completely de-staffed) ramped up again, sometimes with totally different engineers than did the initial design.
If the "re-design" was too significant, then we wouldn't be able to make the target dates for construction (which were the basis for our company's profit). Therefore the initial overdesign was REQUIRED. The client LOVED this because it meant that a project which would have taken 3 years to complete just 20 years before could be completed in less than HALF that time.... And, there plant could be making all kinds of profit for them during that extra time of operation.
Honestly, I got very good at documenting (in my analysis models and my calculations) exactly which pieces of equipment were associated with which loads. That way when another engineer came in 6 months later, they just had to update the well organized calculation and see if the demand requirement changed enough to modify the design. Obviously, there were a lot of cases where the support beams were shifted here and there for different equipment dimensions.
4) Back in the 70's (and maybe into the 80's) a lot of these large Engineering and Construction companies used "cost plus" contracts almost exclusively. That means that the more expensive the engineering was, the higher their profits would be..... Think "government work". If we had to do a site visit, they'd figure out how to MAXIMIZE their costs so that their profit would be higher.