Safe Design

[m63]

I was checking an old concrete slab for a heavier loading and found that the slab was well over designed at the time and very suitable for new loads. This’s got me thinking of the new limit state designs that structural engineers perform nowadays. What are margins of safety that you guys allow yourselves when designing structural members beside what is just required by calculations and codes, what is your office practice or your personal comfort margin? Knowing that code loadings are minimum and if not told otherwise what would be appropriate to account for without unreasonably spending owner’s money while performing safe or over safe design.

 

[JAE]

I think that there are always the round-up factors where your unity (load /capacity) is in the 0.85-0.9 range.

Its sometimes based on your personal judgement as to the certainty of the loads. If you are suspicious that there may be increases in loading, new mechanical units, future floor toppings, or just owner directed changes (I'm sure you've all had THOSE kinds of owners before) - you might choose to be a little more safe than 0.85 - perhaps 0.75 or more.

 

[DaveAtkins]

I typically design as close to the limit as I can. The load factors and strength reduction factors inherent in structural engineering exist so that one can do this. However, on industrial projects, I design up to about 95% of the limit. Industrial buildings tend to experience more life cycle changes than other building types, and the extra 5% can come in handy.

DaveAtkins

 

[ctcing]

Our experience tell us to slightly over estimate de thickness of slabs and probably reduce rebar cost. We also determine our safety factors on floor vibrations or noise reduction in the case of apartment buildings

regards