Pre Loading Steel Beams

[sk_90]

Hi all,

I know there have been some discussion on pre-loading steel beams, but does anyone have a good specification or step by step for a contractor to follow on site?

Typical example would be a relatively long beam typically 7-9m for a single storey masonry wall above for an opening i.e. house extensions.

Any feedback would be much appreciated

Thanks

 

[jdcollins]

I've done this once before, but no longer have access to the drawings/specs. That being said, the basic process was we determined how much we wanted to "pre-load" the beams. Agreed on 80% of the existing structure dead load similar to a post-tensioned slab. We used beam loading/deflection equations to determine the required induced deflection along the beam. We noted that after the induced deflection was achieved, that sloped shims be placed from each side of the beam to ensure bearing.

Our situation was fairly easy, because we had plenty of access to the underside of the existing structure and it didn't have any HVAC/Electrical at the underside.

 

[Tomfh]

It depends on the site constraints and how much deflection you need.

Basic method is to measure the unloaded beam position with a laser level (or similar) and deflect the beam until required deflection is achieved, then you grout.

To deflect the beam you can use wedges, flat jacks, bottle jacks, chain blocks (if you have something below to pull on), etc…

 

[Agent666]

I've only ever used a flat jack (I think that's the name), you place them between the structure being supported by the beam and the beam and pump grout into it until you meet a pressure that equates to a predetermined load which reacts against the structure and beam. Much easier than trying to measure a deflection. Once grout sets fill the remainder of the gap with grout. Determine the load form an appropriate analysis.

https://engineervsheep.com

 

[sk_90]

Thank you all. OK so in theory we advise on the deflection to be achieved and the beam is deflected on site by means of steel shims or timber wedges. The contractors will probably hate me for specifying jacks but if we need it then we need it.

Interestingly enough, I haven't seen much industry guidance on this matter on smaller simple buildings. Does anyone know of any good reference just for my information.

thanks

 

[Tomfh]

I've only ever used a flat jack (I think that's the name), you place them between the structure being supported by the beam and the beam and pump grout into it until you meet a pressure that equates to a predetermined load which reacts against the structure and beam. Much easier than trying to measure a deflection.

I’ve had a bad experience when relying solely on that method. As with pulling on tendons, the pressure/load isn’t always going where you think. A faulty gauge, a blockage, and your result can be meaningless. A deflection check (ie checking the actual strain in the steel) can be more decisive. It’s easy too. A $100 laser level and a tape measure will give you mm accuracy of beam deflection.