pre-cast vs. post-tensioned parking garages 1

[jedge]

I have to generate a letter to a state university system comparing the advantages and disadvantages of post-tensioned cast-in-place parking garages vs. pre-cast parking garages. The purpose of the letter is to facilitate the university in choosing one of the two systems for a new 630 space parking garage. I need to generate the letter this weekend. I have the following:

Precast - lesser construction cost, long term maintenance costs higher, less freedom in layout (use local precasters typical bay sizes), need for shear walls or K-walls for lateral force resisting system
Post-tensioned - more open feel, less surfaces for migratory birds to nest on, costs for crack repairs due to restraints against elastic shortening, concrete frames resist lateral forces so no walls to hide crime.

Does anyone know of a web site that makes a comparison between precast and post-tensioned parking garages?

 

[pinehurst]

In direct answer to your question, I'm not aware of any publication directly comparing the two systems. From our own experience however, you will generally find that the precast system costs significantly less compared to cast-in-place, post-tensioned; largely due to the requirements for a traffic bearing membrane required for cast-in-place concrete parking garages and the duplicity of the precast.

In my own opinion, the long-term durability of pre-cast concrete makes it a difficult system to beat. However, they are plagued with leakage problems because of the number of joints. Annual maintenance is strongly recommended to flood test the parkade to flush the chlorides and identify the joints in need of repair.

Regarding cast-in-place, post-tensioned parking garages, based on the number of post-tensioned structures we have repaired, I'm very reluctant to recommend any such system for parkades, even for the newer extruded sheathed tendons.

As you are aware, there are many arguments for each case, it depends on your priorities. If cost is number one, you will likely be using a precast parkade; if long-term protection against leakage is a priority and the Owner is vigilant, a cast-in-place, post-tensioned parkade, protected with a membrane may be the most appropriate.

If you want to discuss it further, feel free to contact my at john.w@ckpeng.com.

Regards,

John

 

[dik]

You might consider providing a preliminary design of both systems and get a cost from a contractor... either systems work well and a post-tensioned slab can utilize precast elements. For appearance, you cannot beat a good precast facade. You may want to look at the use of bonded/unbonded systems and effects of corrosion on the strand. The system chosen will likely have an affect the economics of the system.

 

[dbuzz]

Would your Client consider a composite steel and concrete design? These are gaining acceptance in Australia. See the attached links:
Economical Carparks - A Design Guide

http://www.onesteel.com/systems.asp?systemID=63

Economical Carparks - A Guide to Fire Safety

http://www.onesteel.com/productspecs.asp?specID=54

 

[jedge]

Thank you to Pinehurst, dik, and dbuzz.

The leakage problems identified by Pinehurst was something I had not considered.

This project is a design-build project and the contractor has already determined that the precast option costs $500,000.00 less than the post-tensioned option. Thanks Dik.

The information on composite steel design is handy. I'll have to review the information from a vibration, fire resistance, and life span point of view before I present this information to the University. Thanks dbuzz.

 

[dik]

and get a costing for the composite steel part... I've used composite steel in past and it works well... deflections, vibration, depth of structure and corrosion resistance... pay particular attention to control joints...

If design build, you have the advantage of having a contractor in your pocket to help with cost estimating... remember, it's usually the system chosen that determines the economy... not a tight design...

 

[jheidt2543]

Since you already have some preliminary budget costs for the two systems, why not take it one step further for your client and produce a life-cycle cost analysis for the two systems? This would take maintaince into account and show the real total cost to the client. The $500,000 difference is big money and even though first cost seems to be the decision maker these days, a life-cycle cost analysis approach would provide a valuble service to your client.

Let us know what you find out.

 

[uscapm]

PT caparpark structures are more laborious. However, even for many asian countries with cheap labor, precast system is still cheaper and preferred. Although, economies of scale is probably a big factor.