Conceptual question on reinforced concrete design
Conceptual question on reinforced concrete design
(OP)
Hi,
This is a conceptual question on reinforced concrete design.
Given an arbitrary cross section subjected to a normal force F and a bending moment M (see picture), how do I tackle its design? Is it a beam with a normal force or is it a column with a bending moment? How do I tell?

Thanks.
This is a conceptual question on reinforced concrete design.
Given an arbitrary cross section subjected to a normal force F and a bending moment M (see picture), how do I tackle its design? Is it a beam with a normal force or is it a column with a bending moment? How do I tell?

Thanks.






RE: Conceptual question on reinforced concrete design
Code gives a limit on when it needs to be treated as a column.
RE: Conceptual question on reinforced concrete design
Leaving codes aside, and applying pure common sense together with the necessary backing math, how do I tell if I have to apply the nomograms for columns or the graphs/tables for beams ?
Thanks.
RE: Conceptual question on reinforced concrete design
I'm not sure exactly what charts you're talking about, but they likely have the appropriate phi factors embedded in them so you ARE still applying the code.
ACI doesn't really recognize beams or columns, it is more a matter or tension-controlled, compression-controlled, or transition.
Finally, when talking about a cross-section and not a member there is only a strength consideration since buckling can't be considered for a given cross-section without analyzing it on a member level.
RE: Conceptual question on reinforced concrete design
Why would you not want to consider this? If you are trying to ingnore compression reinforcing then you really need to treat it as a beam and superimpose the compressive stresses over the bending stresses.
RE: Conceptual question on reinforced concrete design
To quickly check the "interaction" is done quite easily in excel or mathcad... or if you conservatively ignore compression steel it can be solved by hand if you assume tension steel yields... which it most always will... then you can back our your phi factors.
RE: Conceptual question on reinforced concrete design
I just wanted some clarifications, as more often than not codes tend to overshadow the fundamentals of reinforced concrete with design constraints that many engineers follow blindly without putting the things in the right context.
RE: Conceptual question on reinforced concrete design
RE: Conceptual question on reinforced concrete design
RE: Conceptual question on reinforced concrete design
RE: Conceptual question on reinforced concrete design
RE: Conceptual question on reinforced concrete design
RE: Conceptual question on reinforced concrete design
How would the order in which (I'm assuming that's what you meant) loads are applied matter?
Even if you apply an axial force first, then apply a load perpendicular to the cross section, the p-delta effect will be exactly the same.
RE: Conceptual question on reinforced concrete design
It's rare event, you are thinking the applied load in direction of the gravity, mine is in reverse.
RE: Conceptual question on reinforced concrete design
RE: Conceptual question on reinforced concrete design
Try the excerise at home:
Case I - plate subjects to uniform selfweight only with axial load (W+F+M). Then add equal upward pressure (-U).
Case II - plate subjects to uniform selfweight and equal upward pressure (W+(-U)). Then add axial load.
Draw stress diagrams to see the differences.
Hint: For case I, there is lock-in stress due to selfweight (permenent deflection), whereas, case II has none since W=U in magnitude.
RE: Conceptual question on reinforced concrete design
In a 2nd order analysis, you are right- it would change the stress distribution.
It must be said, however, that your scenario is terribly improbable and has no bearing on an actual analysis problem.
From a practical standpoint, your second scenario is most likely trivially advantageous (how much so depends on several factors including the magnitudes of the loads and the flexibility of the member), although I can't imagine how you can possibly design for such a specific scenario. You always design for a worst case.
But I'll cede the conceptual point.
RE: Conceptual question on reinforced concrete design
There are quite a few real word cases if you deal with fluids. That 2 cases were specifically constructed for ease of visualizing the effect of loading sequence on internal stresses. Try think deeper before reject.
RE: Conceptual question on reinforced concrete design
I'll ignore the patronizing tone for now.
This is a structural engineering forum. Yes- structural engineering covers a wide array of structures. However, unless specifically stated otherwise, one assumes buildings and "building-like" structures when forming opinions on the subjects covered in this forum.
Admittedly, I am ignorant as to the specifics of design of structures like vessels or containers or whatever else you have in mind when you are saying there are "quite a few" real world scenarios where uplift exactly equals gravity loads.
The principles certainly are the same, but the practical, experience-related aspects of design can differ greatly.
For example, it is obvious that in your line of work this may be an important consideration, but I have never (which doesn't mean much due to limited experience), nor do I know of anybody who has ever (which means a lot more, as I've worked with engineers with decades of experience) had to design any member differently due to the "sequence" in which the loads are applied.
RE: Conceptual question on reinforced concrete design
Mike McCann
MMC Engineering