Loading Philosophy

[KootK]

This is kind of a philosophical post. The important stuff is at the bottom so just skip to there if you are pressed for time! Thanks.

ACI 8.8 states that a designer should check a column for:

1. max axial load plus corresponding moment.
2. conditions producing worst moment/axial ratio.

My question is this: is it sufficient to consider only these two cases when designing concrete columns? Can all other permutations of load reasonably be expected to fall within the interaction diagram as long as these two points do?

If others are considering more than just these two situations (are you??), then how are you identifying the more critical scenarios? The problem seems to be compounded if you consider that the amount of P-delta magnification expected in a member is, in part, a function of the total gravity load associated with a given LC.

As my limited experience grows, I am beginning to view loads more as tools for reasonable proportioning rather than "real" anticipated events that need to be considered in every possible form. Am I out to lunch here?

Examples:

1. For multi-span continuous beams, considering multiples of complete span loadings (between supports) does not necessarily yield the absolute worst possible forces. Really. Pretty close though.

2. Wood truss guys put chase openings in floor trusses at the center of the span because it is the mystical location of "ZEREO SHEAR". Hence we didn't need webs there anyhow, right? If you consider loading half the span, it is actually the location of wL/8 shear. I'm sure it's fine -- but it's never checked.

Perhaps then, checking points (1) and (2) on the interaction diagram is a reasonalbe way to proportion a column. Even if it is conceiveable that a situation may arise which would fall beyond the limits of the interaction diagram.

Even if loads are not just "tools for proportioning", they are undoubtedly rooted in probability. With this in mind, what then would be the probability of one of these freakish load cases (checker load patterns alternating from floor to floor and the like) actually orccuring? Small enough to allow me to sleep well I think.

What do others think? C'mon...tell me I'm a danger to myself and the public at large.

 

[rapt]

Adam,

I would also check the case of highest moment with corresponding axial as this could definitely be worse than the case with the worst moment/axial ratio.

This still does not guarantee that all cases are within the interaction diagram, especially if slenderness is being considered.

For lightly reinforced columns where the shape of the interaction diagram is very curved it is probably ok, but for heavily reinforced columns where the shape is much closer to a straight line, it may require checking of more points.

P.S. The dangerous designers are the ones who accept code clauses like this, and are willing to take the design short-cuts offered by them, without questioning the logic.

 

[Trussdoc]

Regarding the duct chase issue in wood floor trusses....the older (circa 1982) software was not sophisticated (pin-connected modeling etc.)enough to analyse this configuration, and TPI permitted an "empirical" design that assumed zero shear at the dead center of the truss with an adjustment for the shear at the ends of a centered duct; an adjustment was also made for an offset up to 2'.

This was reasonably safe in residential use as loads are generally pretty much uniform. However, with cantilevers, point loads, and multiple span trusses coming into play, and the HVAC guys wanting ducts in very specific locations, current design software takes load cases into account, along with an exact analysis of the duct opening as part of the overall design.