Add cast-in-place columns under existing 'over-stressed' concrete beams

[Torm]

Our job requires the installation of cast-in-place concrete columns (both 18" and 24" square) to be placed under existing floor beams in a building on several levels at several defined locations. the columns, after being poured, will require to be jacked to a specified 'pre-load' force. we have determined we will jack from the bottom of the columns and I would like some input on the type of jack to use and possible ideas on the bottom connection. Our first look at it entails installing a flat jack beneath the column baseplate, possibly with a load cell between the flat jack and the bottom of the baseplate to measure the load. The baseplate would be installed on anchor bolts, supported by the leveling nuts and shims, with a sufficient gap underneath to place the flat jack and load cell, and the columns would be installed up to the beam connection at the top. After the concrete cures sufficiently, we then plan to jack the columns upward to the 'pre-load' force. Any ideas on whether the use of flat jacks is the best option? what about ideas regarding the best way to measure the preload, and once the jacking operation is complete, being confident that the 'pre-load' force is maintained as designed? The engineer also indicates that due to 'creep' in the new concrete, the force may need to be re-checked 2-3 years down the line, then re-jacked if needed prior to finally grouting in the baseplate. Another idea thrown out had to due with the use of belleville washers under the baseplate to verify the pre-load, or possible load cells placed directly inside the anchor bolts. any ideas would be welcomed.

 

[hokie66]

I have never heard of doing this with cast in place concrete columns, but it is relatively common with steel columns. I would propose using steel columns, with the flat jacks at the top. With steel, you wouldn't have to be concerned about creep, and the grouted flat jacks themselves deform very little. The preload is measured simply by hydraulic pressure x area, and the jacks should come with tables to make this determination foolproof.

 

[dik]

Hokie's on the right path. Steel would be a lot easier to install, smaller and the adjustment could be built in... like a 'big telepost'...

Dik

 

[Archie264]

Be careful about supporting an existing concrete beam with a new column mid-span or thereabouts as you can induce stress reversals in the beam. I'd imagine this has been taken into account but I'd still rather say it than leave it unstated.

 

[structSU10]

How overstressed are these beams? Any way Carbon fiber or equal reinforcement is possible? It is expensive, but can be a nice solution that keeps the floor space unaltered. Or even a steel beam on either side, with tie plates under the beam to transfer loads or something? I'm sure you looked into other possible designs, but as archie said, better tha left unstated.

 

[JAE]

Archie264 has a very valid concern. Typical layouts of flexural reinforcement on concrete beams doesn't usually allow for negative bending within the span lengths.