How to Reinforce an Existing Precast Double Tee? 1

[mdmiller]

I have an existing precast double tee floor that according to the shop drawings was only designed for 40 psf LL and 15 psf DL. The owner wants to hang a 1/2 ton hoist and rail to the underside of the precast tees. I expect the point load from the hoist to be around 1.6 kips.

How can I reinforce the existing double tee to support the new hoist load?

THANK YOU!!!

 

[SperlingPE]

I have never encountered having to reinforce double t's.
Some thoughts:
Carbon Fiber
steel shapes attached to the sides of the stem

 

[JAE]

Or add steel beams between the stems and only use the existing double tees for lateral stability of the beams.

 

[Ron]

I like JAE's idea if you have the connections/span capability. I have used channels and flat plate to reinforce TT's. Carbon fiber works well too, but is better suited to overall strengthening rather than point loading.

Don't forget two things.....
1. Attachment scheme of the reinforcement has to miss all existing cables/rebar.
2. Strain compatibility between reinforcement and concrete. Check both the elastic modulus and the secant modulus of the concrete. Supplier should be able to provide both values to you. If you have no supplier info, do the best you can with similar generalized data.

 

[dik]

I'm not sure that strain compatibility is easy to obtain (I think it looks good on paper , but is difficult to obtain in the field) and I usually don't get too involved with it. I try to preflex the DT and secure the beam/channel/plates taking care of the strand location. Also use a steel locater... Sometimes the biggest problem is the attachment of the beam o distribution of the load to the supporting corbel if this is the support for the DT.

Dik

 

[Qshake]

You can get some good ideas for conventional strengthening and FRP strengthening of existing concrete members from the following:

Peter Emmons book on Concrete Repair and Maintenance. It's a bit elementary but if you've not done this before it can be helpful.

NCHRP Publication 293 or 294 (can't remember right now) which is about strengthening bridge members. Of course there is a lot on both reinforced and prestressed members.

Lastly, for the newer FRP methods use ACI 440. You will note that ACI 440 will require a certain strength for the existing in order to use FRP and you will also note some healthy strength reduction factors for the FRP. For the time being that's what we have to work with until more engineers and appilcations yield more in-service data.

Please note that while some methods mentioned in the publications above, you can apply them to TTs with modifications.

Good Luck

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[soukup73]

The carbon fiber option is ideal for this application - for one, there is no need for drilling into the stems and chancing contact with the tendons. Never drill into pt or prestressed if you don't have to - you never know where a tendon would be, even if you think it makes no sense for tendons to be at a certain location. Second - your point load is actually quite low and the carbon fiber could strengthen the stem with a fairly low amount of material used. Contact Fyfe Co. at

http://www.fyfeco.com

if you are interested in this application - they are the leaders in the carbon fiber industry.