Pre-loading Steel Reinforcing under Ex. Conc. Slab

[StrucPEng]

Hi All,

I am looking for input, best practice/procedures, alternates, (any input really) on the pre-loading of steel beams used to reinforce an existing 2-way concrete slab. We have called for this to be done many times before but I am working on improving the process and making it more clear, contractor friendly and easier to design or specify on a typical project. A few items I would appreciate comments on are:

- Opinions on how much is enough pre-load, how much is too much?
- Best methods (We have used flat jacks air bags in the past)
- Procedures or sequencing of work
- Design considerations (Deflections, end reactions)
- Details of steel beams, adding stiffeners, etc.

Thanks in advance for the input!

 

[Agent666]

Usually the degree to which you preload is unique to each situation, depending on how much you're strengthening and why (deflection or strength issue), and capacities of existing structure and new steel, new connections required. Minimum preload should be used to achieve your desired outcome.

I've used flat jacks predominantly for this, strengthening existing concrete elements with steel. Then grout between steel and concrete member to permanently lock in the preload.