Bonded Post Tensioning / Parking Garage

[bvbuf]

I have a parking garage with 60' x 20' bays. The owner would like to design for blast loading. Unbonded PT is no good for blast and mild steel is questionable for the 60' span.

I believe bonded post tensionsing is the answer however I can't find any design information. PTI does not publish a method for design. I've never done this type of design before so I am looking for a step by step method to design beams and slabs with bonded post tensioning. Can someone point me in the right direction?

My beam depth is limited to 40", my slab will be 6". Does anyone have an opinion about using a conventional reinforced concrete beam for a 60' span?

Thank you
BVBUF

 

[dik]

I've been involved with several parking structures with 60'+/- spans (Generally 58'+) using 36" dp concrete beams. They will be about 14" to 16" wide to handle the several layers of conventional rebar and can have significant shear reinforcing. The concrete beams are spaced about 15' o/c and frame into 14"-16" dp columns by 3' wide (some of these are heavily reinforced with the uppermost column generally governing (less axial load). The continuity of the columns is considered for the beam design to reduce the mid-span moment. If the column is loaded from the opposite side by another beam (possibly at a different level) the opposite beam is supported on a corbel and bearing pad, else the shear through the column becomes too great. Be prepared for hairline cracking distributed the length of the beam in high moment areas at about 2' to 3' o/c.

In practice, the maximum live loading will be less than 15 psf. If the heaviest vans/trucks are used to determine the loading, this will bump up to 20 to 25 psf. I've used this to 'help determine' the degree of fixity considered between the beams and columns.

 

[dik]

Should have added that you may be able to 'pare' the slab down to 5-1/2" or so, with long term deflection calculations and modelling the beam width into the system. The weight saving helps with long term beam deflection... (Done with CSA code and not ACI; that may make a difference.

Also a significant savings in rebar by using the actual bar cut-off locations for both top and bottom bars. Top bars get staggered over the support and bottom bars are approximately 1.6 times the span and half the required area. They are continuous through the support with the adjacent span making up for the other half of the area required.

 

You can design bonded post-tensioned parking garages in a very similar way to unbonded. The ACI 318 rules for minimum rebar are different, and there are more considerations for fitting the anchorages, but it is not difficult. Bonded strand is more efficient for ultimate flexural strength, although usually the effective forces are a few percent lower.

VSL have a very good 2 strand system for slabs

 

[bvbuf]

Thanks for the help. I spoke with a representative of PTI who discouraged my design with bonded post tensioning. His opinion was that there are only a few contractors that do it well. (VSL is one) They do all their own engineering and do not sell their design methods.

Thanks dik for the ideas using conventional steel. Looks like that is the way I'll go.

 

[dik]

You can use the rough sizes for a place to start... Check details for hooks with large bars for geometry and interference... some areas get pretty congested! Try to minimize the slab thickness and maybe consider using a proprietary surfacing. Also use proper expansion joints if exposed to 'hot and cold' climate. Expansion joints that work are expensive; cheap ones don't normally work so well... also look at means of providing a gutter at joints as well as 'grit' traps. Even though the slabs is designed continuously, I usually put a groove at each face of the beam in the top of the slab and caulk... Goldblatt groover or something similar. It's wide enough to get a proper caulk. There is economy in calculating the actual cut-off lengths for the slab as noted. Because of the large amounts of steel in the beams you may be looking at 3 or 4 cut off locations for the main steel. With parking garages, there can be a lot of 'similar' beams as well as slabs and the economy of doing a refined analysis is normally there.