Pre-loading beams
Pre-loading beams
(OP)
We have a project where we are taking out a column in a steel office building and supporting it with two new transfer beams supported by two new columns. We have an engineer in the office who likes to pre-load the beam for dead load in these situations (sit the beam in place, jack up the supports until the beam deflects the calculated dead load deflection, then lock it into place) This, in theory, eliminates any settlement at the transfer and in turn any possible damage to finishes above. The other option would be to size the beams to limit deflection, likely getting beam sizes significantly larger than if they were preloaded.
Do people like either of the two options above better, and what are your brief reasons?
Do people like either of the two options above better, and what are your brief reasons?






RE: Pre-loading beams
another day in paradise, or is paradise one day closer ?
RE: Pre-loading beams
Even if beam depth must remain unchanged and a heavier beam of the same depth will limit deflection, I consider that preferable. Besides, labor and equipment to pre-load are not "free".
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Pre-loading beams
Sliderule a tend to agree, Jacking requires skill and experience from a contractor that I am not sure I will get.
RE: Pre-loading beams
www.structuremag.org/wp-content/uploads/2016/05/SF...
http://csengineermag.com/article/preloading-approa...
RE: Pre-loading beams
RE: Pre-loading beams
It seems to me that you have been asked to do the almost impossible, and then people are telling you, ‘but, don’t make it difficult’ or don’t expect to hire a contractor who knows his butt from a hole in the ground, he might have to think a little in the process. If the contractor can’t grasp/imagine this process, I don’t know if I’d want him on the job anyway. You might even specify that you want to observe and help with this aspect of the project.
I think your coworker/engineer is right on the money with his thinking. After all, you have to unload the existing column before you can remove it, and this could be done with shoring all over the place, but that’s not always so easy either, and involves jacking/lifting too. You certainly don’t want to start loading the new beams from where the structure is now. Although, you haven’t given much info. on what the real structural conditions are. You say two new beams and two new columns, and I assume you mean one beam on each side of the existing column, and then a new column out at the ends of these paired beams. You must have a fairly concentrated load at the existing col., based on the framing system, or you wouldn’t need the col. That means you camber the paired beam set over only a 3-4' length near center span (the existing col. location?), with a fairly small radius of curvature. The rest of the beam lengths remain fairly straight, if not cambered, up, slightly (exact concave shape down, to be determined ). This way the new paired beam system will pick up the load above almost exactly like the existing column does now. You bring in the two paired beams, either side of the existing col., bolt them together with a few cross frame stiff. pls. and lift them until contact at the center col. Then, you start jacking 18" inside to new end col. locations, leaving some working room for new cols. These two locations must already be fairly strong (or shored from below) to take the two new cols., and you already know about how much end delta you’ll get to unload the existing col. At this point, you are already carrying the D.L. and some L.L. from above by the two new paired beams. When the new cols. fit, with a .25" shim pl. atop the col. cap pl. (some thickness to be determined), bolt the paired beams to the new col. cap pls. and you are essentially done. Anything else allows the existing structure above to deflect too much before the new beams really start picking up their loads. What could be easier to accomplish the impossible?
RE: Pre-loading beams
www.PeirceEngineering.com
RE: Pre-loading beams
I have done this a few times for concrete column removal with new concrete beams, but not too common for steel.
RE: Pre-loading beams
RE: Pre-loading beams
rapt brings up a very useful 'tool' for such applications. Flat jacks (one of the many original patents by Eugene Freyssinet) are a very compact/high-capacity method of pre-loading.
I have used them on several projects - from small 8" dia to giant 36" diameter ones.
We have our own equipment, but the hydraulics are readily available for rental in the US from WB Equipment Link. I think WB also provide site-services too, across the US.
For smaller-diameter permanent flat jacks it is possible to pre-load the structure via flat jack/s with hydraulic fluid, then (without de-loading) pressure pump in 2-part epoxy resin into the flat jack using a simple hydraulic pump and 'rocket' whilst flushing/bleed out the hydraulic fluid. Avoids have to place shim/packing plates etc.
Here are some photos of a project we were involved with for a transfer beam with new micropiles that required pre-loading:
It does require some specialized experience, but not too complex.
RE: Pre-loading beams