Compressive axial force across section resulting in increased flexural rigidity

[Engineering05]

Hello

If you evenly post tension or pre tension (ignoring all losses to simplify) say a concrete column resulting in no net bending across the section do you increase the flexural rigidity of the member in some way? Will it lead to less deflection? At the most fundamental level solving for the uncracked (gross) and cracked stiffness does not rely on any forces across the section - only the mathematical first moment of area. The force in pre/post tensioning elements has nothing to do with that calculation. If anything it will help the section remain in an uncracked state under higher service load due to the additional compressive stress across the section and thus yield a greater flexural rigidity where we would otherwise be forced to switch to either an uncracked or effective (via tension stiffening) flexural rigidity.

Is there anything else to this line of thinking other than a yielding a larger cracking moment under pre/post tension? I'd be interested to hear all your thoughts on this.

 

[bookowski]

Google post tensioned shear walls - it seems like that is what you're getting at (same idea). They are used to increase stiffness by remaining uncracked. I've only seen it for wind drifts in tall/slender buildings (with limited success due to cost/complexity). There is also some seismic stuff about the walls being able to 're-center' themselves but that doesn't sound like what you're looking for.

 

[KootK]

In the absence of time depended effects, I see it as you've described: the dominant effect being the delayed onset of cracking. Obviously, any bonded, unyielded prestressing reinforcement would also stiffen the section similar to how conventional reinforcement would.

With time dependent effects included, things would change a bit. Creep will result in a disproportionate amount of your prestress being carried in your conventional reinforcing at the expense of the precompression in the concrete. That would mess with the cracking moment yet again.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Klitor]

I was mulling about this, doing 16m tall precast warehouse in a pretty high seismic zone (0.24g)

Deflections were pretty big (large selfweight and all) using cracked sections
I reasoned if I use smaller collumns and prestress them, they could remain uncracked under SLS seismic load.

In the end I couldnt find examples of it being done (aside from few curiosity pics in some textbooks) and local contractors never done it before so I scratched it and just thickened the columns (thereby also increasing selfweight -> more seismic force...)

Anyone here done something similar?

 

[Teguci]

do you increase the flexural rigidity of the member in some way?

Yes. Concrete rupture will not occur until the tension stress from outside forces exceeds the pre-compression stress [Pe / Ag * (1 + e c / r^2)] + concrete rupture (root 3 f'c +/-). Pe = tensioning force, e = eccentricity (0 for your example)

Will it lead to less deflection?

Ignoring long term effects, yes in most cases.

Is there anything else to this line of thinking other than a yielding a larger cracking moment under pre/post tension?

Long term creep will be greater. If you need to increase deflection performance for a given concrete section, the PCA has an article on using higher f'c concrete (increases E -

http://www.cement.org/cement-concrete-basics/products/high-strength-concrete

). You can get about 12 ksi max here on the east coast and 19 ksi on the west coast (North America).

Until rupture is exceeded, the element can be treated as totally elastic.

 

[IDS]

The previous posts imply that although a uniform prestress force will reduce short term flexural deflections, it will increase long term deflections due to the effect of creep. (That may not have been intended, since it was not explicitly stated, but that is the implied message that comes across to me).

This is not correct (in my opinion). The effect of creep and shrinkage can be modelled as a negative (i.e. compressive) prestress applied to the reinforcement. Since prestressed reinforcement starts of with a tensile strain the total flexural deflection will always be less than for a section with no initial stress, and a virtual negative prestress due to the effect of creep and shrinkage.

Axial strains (both short term and long term) will obviously be greater for the prestressed section.

The only proviso to the statement above is that if a flexural member is restrained against axial strain then the combined effects of flexure, creep and shrinkage may well result in greater cracking and deflection than in a reinforced section.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[KootK]

The previous posts imply that although a uniform prestress force will reduce short term flexural deflections, it will increase long term deflections due to the effect of creep.

I certainly did not intend to imply this. My stance is that a prestressed column would be flexurally stiffer but, as a result of long term effects, not as stiff as would be predicted when long term effects are ignored.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Klitor]

Nobody bit on my rambling, just to add one more thing - I did the calculations of long term losses...it worked up to 35% prestress losses on my (axially lightly loaded) column.
Beams usually come up to 10-15% max (Ive seen some engineers just take 20% and dont bother calculating)...

 

[PicoStruc]

I might be stupid...

But in my opinion, Tension increase geometric rigidity and compression lower geometric rigidity !

For example, considering P-Delta / p-delta effect in analysis will increase the period of a building because all column and walls are in compression !

 

[KootK]

and compression lower geometric rigidity !

I don't believe that this will be the case here. Prestressing generally introduces no additional, net compression on the concrete cross section. As such, it should not impact P-delta stuff.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.