Post-tensioned barrier cables loads on concrete columns in high-rise construction 1

[StructuralJoe]

I've got a high-rise concrete structure with post-tensioned barrier cables. The cables produce almost 8kips "pre-jacking force" and almost 6kips "final effective force". These forces vary and in some cases produce torsion in additional to lateral loads on my columns. I reviewed ACI-318 11.5.1c and in most cases the barrier cables are less than the Threshold Torsion and therefore it can be disregarded. But there appears to be a few location that may be a problem. Does anyone have a design example for reference, I have checked all of the texts I can find and I haven't found one yet? I can't find any examples of concrete columns with axial, shear and torsion at the same time. I have checked shear and it appears all of the columns will have less shear than Vc, if this is the case are additional stirrups required?

Thanks in advance

 

[dhengr]

StructuralJoe:
What are these “post-tensioned barrier cables?” I assume they span (are stretched through) a number of bays btwn. the dead end and the stressing end, and that the cables run through a pipe/tube void at each of the interior columns over the full stressed length. Are these just barriers during construction or are they at parking levels in the bldg. where they may see a several kip lateral load to prevent a vehicle from going over the edge. In the case of the parking structure the cables will see very high tension loading from any lateral vehicle loading. Remember, a lateral load on a stretched cable leads to very high tensile reactions at the cable ends, and your 6-8k tensioning force is additive at this instance. Either at the stressing ends, dead ends or the interior tube/void locations, you have a chance to design some fairly clean typical, pre-assembled, cages of reinforcing bars, closed ties and hardware which would just fit in with (fit within) the regular column rebars and ties. At the min. you need your cages to hold the extra hardware in place. But, they will also distribute the concentrated loads out into the column, where the regular reinforcing can take the added loadings/stresses. What do you expect to find in some special textbook or exactly identical worked out problem? Don’t we design columns/compression members every day, which have added moments and even torsion, and more or less combine stresses and reinforcing to cover the combined conditions?

 

[Handy_Andy]

The commentary in ACI 318-19 R.22.7.6 Torsional strength, states that the nominal shear strength Vc provided by the concrete is assumed to be unchanged by the presence of torsion. Seeing as your shear load than Vc, I would think you would be ok to design the main reinforcement with interaction diagrams and the stirrups for torsion, then check that the longitudinal reinforcement meets the torsional requirements, if not increase it.

 

[XR250]

6-8k tensioning force is additive at this instance.

Is that true, DH? Seems like the tension only increases above the preload if the applied force by the vehicle causes a tension above and beyond 6-8k.
Similar to bolt pre-loading where an additional applied tension does not increase the bolt stress until it exceeds the preload.

 

[EZBuilding]

This is a good resource for this type of work:

https://www.post-tensioning.org/Portals/13/Files/Pdfs/Education/Technote14.pdf

 

[JLNJ]

Star for you EZB. That's a great paper.

Vehicle forces get big quick when you increase the speed and reduce the deflection. 5 mph sure seems like a small number, too small in my book.

 

[EZBuilding]

I agree that generally - design the column for the combined forces it will observe. In practice, however, I would also provide a typical detail identify stirrups at minimum required spacing for shear at a minimum.

 

[dhengr]

StructuralJoe:
In the case of the bolt holding two pls. tight together at their faying surface; The bolt is tightened and elongates (causing bolt tension) and the two pls. provide an opposite reaction as they are compressed together. When you load that joint in certain ways, pulling the two pls. apart parallel to the axis of the bolt for example, you do not add additional tension to the bolt until the to pls. start coming apart, parallel to the bolt axis. At that point any added forces will tend to add tension to the bolt. Up to that point, the added force is just counteracting (reducing) the compression btwn. the pls. In your case, with your cable system, the added reaction from the vehicle lateral loading tends to further load the cable in tension, and that further loads the columns, or to further tighten the compression btwn. the two pls.

 

[XR250]

Ahh, makes sense.
Thanks

 

[StructuralJoe]

Thanks everyone, I unfortunately didn't see any responses when they occurred I am catching up now (I may need to change a setting). I got side tracked on a few other issues. The PT barrier cable design is by specialty engineer. I am just concerned about the loads being much higher than original designed for. We initially had 3kips each at a maximum 4" eccentricity. From the shop drawings I see a maximum of 7 to 9kips at a 10" eccentricity and the columns are already cast.

I still haven't found any good textbooks with examples of concrete columns with Axial, shear and torsion. Does anyone know of any? I am going to post again as a new thread.

Also this is for a project permitted under ACI-318-11 although I would consider a example of this loading to a more recent code.

Thanks!

 

[r13]

IMO, for cable through column with eccentricity, I don't think axial load matters, but the torsional stress will be in addition to any horizontal shear the column subjected to. Thus, you shall check and design the shear reinforcement, then check and design the torsional reinforcement, and add the results together. If nothing is required, provide minimum shear reinforcement per code provision should be adequate.

 

[StructuralJoe]

retired13,

ACI code does provide calculations for Threshold Torsion, if the torsion is less than the threshold torsion then it can be ignored and in this calculation axial load is a component, as axial load increases threshold torsion capacity also increases.

Logically I believe that increased axial load should increase torsion and shear capacity of the column.

I can work through the code I was just hoping to find a good reference example for comparison. None of my old text books check torsion in columns and it seems that every software program ignores torsion on columns.

PTI bulletin states it typically doesn't govern but I still feel it must be checked.

Thanks!

 

[r13]

So you already had ACI equation for torsion design, other than numerical example, what you think you are short/lack of? I don't recall axial load as a component to torsional effect (mainly a shear phenomenon), but I wouldn't argue with the code, if they require such load. You should have that information already. Don't you?

 

[r13]

I think WARose and EDub24 answered well on another thread of yours. Yes, the torsion design, if reinforcing is required, you will have additional longitudinal reinforcement, as well as stirrups, which is in addition to the stirrups for shear.

 

[StructuralJoe]

Correct, I was just looking for design examples to double check my work. Still haven't found any textbooks anywhere that address torsion on columns and thought that was strange. I am beginning to believe they don't exist...

 

[r13]

You may get some help from the linked paper, that take you to torsion design step by step. Looking for the section "Torsion" near mid-point of the paper.

Link