cantilever hollowcore design 4

[hx200]

I am designing a Hollowcore slab cantilever 7 foot out. I need to check both service and strength. I think I should design the bending strength of the section at cantilever for 1.2D+1.6L+1.0(P*e) with mild steel on top. the hollow core provider think for strength design we don't need to add the P*e which is considered internal force.

I didn't find anywhere in the code back up my thought. Does anyone have thoughts/experience on this.

Thanks a lot

 

[ash060]

Check the hollowcore suppliers literature, they usaually have allowable cantilever loads listed.

 

[hx200]

Thanks for the reply, are you talking about PCI hand book or something, I couldn't find it?

 

[BAretired]

I agree with you. The P*e term is a negative moment which adds to the applied negative moment. I think you should treat the prestress force as a dead load and use 1.2( D + P*e) + 1.6*L.

BA

 

[rapt]

For ultimate strength Pe is an internal action, not an applied load.

It should be included in the calculations of the section capacity. It will possibly reduce capacity depending on the depth to the PT strand relative to the neutral axis and centroid of the compression force.

PS I still cannot believe that anyone will sign off on a design based on load tables from a suppliers literature!

 

[BAretired]

In a hollow core slab, the prestressed reinforcement is at or near one face of the slab, ordinarily the bottom face. It is an applied load, albeit internal, applied by the prestressing strands near the bottom of the section. In a cantilevered slab, it causes moment every bit as real as externally applied load. It causes tension on the top fibers and must be resisted by top reinforcement.

BA

 

[concretemasonry]

The hollow core business (especially 8" and 10" thick is a volume business because the sales are based on price. The units are typically made in 200-400 long beds with continuous reinforcement and cut to length, unless it is a lower production wet cast product.

Anytime you vary from the standard products, you raise the cost and delay the production and delivery. If you are convinced you need to vary from the locally available products they can be be obtained and cost premium and delivery time could be reduced if it is a large project.

I would suggest you contact the largest local supplier (who probably has more products and engineering)to determine available options and costs. Then you can design and specify what you choose and let the suppliers fight for the job. Specifying and CPI certified plant is a help, but most (80%+) good suppliers are certified and maintain great records for the future.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[frv]

rapt-

I agree with you that we shouldn't blindly follow manufacturers load tables; but do you go through hand calculations when specifying Hilti anchors? Do you test assemblies when specifying Simpson hangers?

It is in the manufacturers best interest to provide accurate, reliable information to the specifying professional. It is up to us to interpret their published capacities correctly.

 

[dik]

From PCI Hollowcore Manual

 

[BAretired]

According to dik's reference, the bottom strands (unless they are debonded) are considered in determining stresses but not in determining ultimate capacity.

I am not quite clear on why the negative moment at the support resulting from the bottom strands should not be added to the applied moment when designing top steel.

BA

 

[hx200]

BA,

Thanks for the in put. I just couldn't get my mind strait on this issue. If the concrete cracked and only 2" for stress block in compression, the PT force still will increase the concrete compressive stress in the stress block.

This could:
1. reduce the capacity of the section
2. Might cause structural failure model with concrete crushing which is not a ductile failure we desired for concrete design.

I think we still need to consider the PT force for ultimate strength design.

 

[BAretired]

hx200,
I agree that the PT force will increase the stress in the compression block. The second page of dik's reference says that when the bottom prestress is very heavy, a strain compatibility analysis should be performed.

For the example in that article and probably for most practical situations involving hollow-core slabs, the depth of the compression block would not change enough to significantly alter the moment resistance of the section. But, you are correct...it should be considered.

BA

 

[shin25]

In past I have debonded strands within the length of cantilever and then carried on with regular concrete design.

 

[BAretired]

shin25,
The danger I would foresee in debonding strands in a long prestressing bed is that you debond the wrong portion of the strand, but there is no question that debonding would work. What do you use to debond the strand, a plastic sheath?

BA

 

[rapt]

BAretired.

It is still an internal action. Its effecvt on ythe moments in the section will come out i n the capacity side of the calculation if the calculations are done properly by strain compatibility. As suggested above, it will cause a deeper neutral axis due to the compression. Also, As the tendon is below the neutral axis, its effect on the section capacity will be to reduce the capacity.

RE the comment about Hilti anchors. yes I will believe test data for something whos capacity can only be determined by test. That is also why I accept the shear capacity logic in concrete members.

But for things I can calculate based on accepted principles, I will calculate it and not accept some salesman's capacity charts!

 

[BAretired]

rapt,
I agree that the compression caused by the lower tendons will reduce the moment capacity of the section because the depth of compression block will increase, resulting in a slight reduction in moment arm. However, in normal situations, the reduction would be so small as to be almost negligible.

But, by all means, if you are concerned about it, calculate the effect and place a little more top steel in the slab to compensate for the slight reduction in moment arm.

To summarize, you need to provide enough top steel to resist a factored load of 1.2*D + 1.6*L (in Canada it would be 1.25*D + 1.5*L), taking into account the additional compressive stress in the lower fibers of the slab resulting from the bottom prestressing.

BA

 

[Teguci]

Just wanted to note that hollow core plank that is exposed to weather may exhibit some deterioration after a few years. This is especially the case where the top of the slab is in tension. If this is a problem, consider using a solid slab.

 

[dik]

Many precasters don't recommend that their HC planks be exposed to the elements...

Dik

 

[Seversen]

Our standard practice for a cantilever of that size is to provide prestressing strand at the top of the hollowcore section to counteract the prestressing strand in the bottom of the section. We do not debond strand in our sections due to the difficulty in getting the plastic sheath for debonding the strand in the right place and keeping it there while pouring a 400 foot bed.

The prestressing force in the cantilever is an internal force and is not included in the factored loads.

 

[kris1976]

About hollow core cantilevered slabs (UK).
Slabs are designed by manufacturers acting as simply supported members with bottom tendons. So they have cetificates for such action. However manufacturers allow for cantilevers but their length are limited (from my experirnce up to 1500 mm circa). It is just calculated as single reinforced beam section.
The reason of limited cantilevered length are:
1. Hollow core slabs have precamber so your unloaded cantilever will have some deflection at start. Hollow core manufacturers are not very willing to give value of this deflection as it can varry and they do not know this exact value.
2. To achieve cantilevered action of plank some end cores need to be opened gently and filled with structural concrete.
Usually only every second core or every third core is opened and filled with concrete. For short cantilevers is easy to assume some bar lengths to get correct anchorage.
For longer cantilevers you need to provide this length and manufacturers need to take it into account. And they are not very happy when more than half of slab need to have filled cores. Also they can not give you warranty that bars will be cast in core with min top cover so usually cover is assumed much more greater.

In your case: about 2.1 m cantilever I would talk to precast planks manufacturer at begining of projrct design
Regards,
kris