Pre-Stress hollow core slab panel design 1

[D.Jaya]

Hi Structural Engineers

A developer(also a Contractor)want a building to be construct with a steel structural frame and pre stressed,pre-cast(PSPC)hollow core slab panels to be laid on top of the steel frame with a concrete topping to finish the building for a office.(pl. refer attached sketch)

My assignment is to carry out the design of the steel frame & to calculate the jacking force(how much PS force to apply)for a PSPC hollow core slab.

Steel framework design is straight forward. But since i am not much experienced in PS design work, I am having some points to get clarified. I can apply the Standard Pre-stress formulas to get the values of Prestress force & the eccentricity but I need the following facts also get cleared. ( As the sketch shows the maximum span of supporting beams to be 4.4 m ,design load is calculated as 12 KN/m2 and the maximum tensile stress of the chosen PS strand(7mm dia) to be 1670 N/mm2 & also the assumed section profile of the hollow core slab is shown in Fig:iii)

Questions:

1) Can I select the Concrete strength of PSPC slab as Grade 30 (30N/mm2)

2) Is it correct that I will calculate the jacking force by using the standard Pre-stress formulaes and add 20% for the losses and divide it by 6 (in this case)to find the jacking force of a single strand?

3) PSPC slabs will be laid on UB's with a simple connection.So I will assume the slab panels to be simply supported and will calculate the BM as for a Simply Supported beam. is it OK? do i need to provide a single strand at top level at close to the 2 side corners of the slab panel.

4)How do I provide PS strands for the cantilevered portion ( 1 m projection)? Do I need to provide PS strands at top and bottom of the panels which to be laid at the corner sections?

5)Can I optimize this PSPC hollow core section? What are the other facts do I need to consider.
Thanks

 

[cooperDBM]

You should talk to some of the eligible hollowcore providers in your area for guidance on specifying their product. They will probably do the detailed design themselves based on your requirements (e.g. layout and loading).

For your own education you can obtain a copy of the PCI Hollowcore Manual available for free at Link. It may not suit your design code but it will give you a basic understanding of the product.

 

[jayrod12]

Agree with cooperDBM. Generally speaking as the EOR, the design of the hollow-core slabs is generally delegated to the supplier. They woudl submit sealed shop drawings covering teh design of the panels themselves to suit the loads and information on your drawings.

Many of them will provide tables for your use to perform the preliminary sizing of the panels.

 

[JLNJ]

What is your lateral force resisting system? How do the beams and slabs contribute to it? We just had a video posted in this forum of a building similar to yours which collapsed due to gross incompetence and a lack of a lateral force resisting system. We don't want to encourage incompetent people performing structural engineering.Link

Where is the project located? Hollowcore comes to the site prestressed from the manufacturing facility - you don't need to apply any jacking forces. Are you sure you are talking about hollowcore and not some other kind of post-stressing system?

Maybe you need to find a local professional who can help you. Soliciting random people on the internet for advice on matters you don't understand can have dangerous consequences.

 

[D.Jaya]

Thanks for all of your replies

JLNJ (Structural)

The lateral load resisting will be provided by the shear walls and the internal & external wall(brick walls)arrangement.Those wall arrangements not shown on the attached sketch.

I saw the video (related to the link you provided) which appeared in an earlier post. That is a different scenario.Thanks for your advise but there is no such competancy issue here.

The building is located close to Colombo City- Sri Lanka. Here The contractor himself want to manufacture the hollow core slabs by doing pre-stressing.He is having the necessary equipment.That is why it is required to calculate the jacking force.No post tensioning will be done.

 

[BAretired]

Normally, the design and manufacture of hollow core slabs is performed by the supplier, who has a considerable amount of expertise in the design and production of such units. Has your client produced hollow core slabs before? Or has he recently acquired the necessary equipment?

The jacking force and expected losses should be specified in the code. Are you using the ACI code?

30 MPa concrete may be adequate for design, but steam curing and higher f'c may be preferable to permit early release of tendons. That would be a question for your client to consider.

Simple span slabs may need some top steel to control camber.

Cantilevered slabs must be reinforced both top and bottom.

A design load of 12.0 kPa seems unusual for an office building. Is that superimposed load?

The size of 120 x 600 is not standard for hollow core slabs, but may be okay.

BA

 

[rapt]

There is a lot more required for the concrete design than simply selecting a prestress force for the strands.

You need to get an experienced composite slab designer involved based on your questions.