Most engineers have heard the saying “garbage in-garbage out”. It was a phrase developed for software engineers, meaning, if you enter garbage data into a software program you will get garbage out.
I Ike the modified version of “garbage in-gospel out” because it reminds me the output I get from my structural analysis program is only as good as the assumptions I enter into the software. Too often we believe the computer print out as being “gospel” or the absolute truth.
There used to be a school of thought that mandated an engineer only use software that he or she created. That way the engineer would know all the limitations of the software and account for unknowns with a little more conservative design.
Why do I bring this up, well, I had a meeting today about camber in prestressed pretensioned concrete beams. Specifically about how to get closer to the “actual” camber deflection dimension.
The problem in determining camber for long PPC beams is knowing the exact concrete mix properties and the initial and 28 day concrete compressive strengths. Without this information they best you can do is approximate the beam camber and use methods in construction to account for variances.
But you don’t necessarily have this information when you design the beam and prepare the bid documents, months before a letting.
Precasters typically use hot mixes (strengths about 20% above the design strength) so they can pour beams and strip them in a one day cycle. Quick turn arounds mean more money but cause me some problems in my design.
Say I ask for 7000 psi initial strength but I actually get 10,000 psi from the factory. Fine by me, but here is the point. Don’t ask me to calculate a beam camber within 10% of the actually camber based on my design strength of 7000 psi. Either live by the limitations of such a system with some realistic error bars or readjust the camber numbers after the beam strengths are known. (Not blaming precasters, I used to be one)
You cannot get accurate camber numbers out from a computer program unless you put accurate strength numbers in, so realize that and live with the fix in the field.
Otherwise, GIGO. See I got there.