Strengthening concrete beam with steel channels 3

[KutEng]

I have an existing concrete beam that is already showing excessive deflections under just dead load conditions. The beam will be loaded up with additional dead loads and we are required to strengthen the beam to a) prevent further deflections (and reduce deflection if possible but I believe the deflection is due to creep and can't be undone?) b)increase the strength of beam to take extra loads

I am a bit out of my comfort zone here as I don't have much experience with strengthening existing structures. I have suggested using FRP strips however the headroom is apparently already too tight below the beam and they can't afford to go any lower. We are now looking at fixing a steel channel or 2 to either side of the concrete beam to strengthen it however I am unsure how to design this. In other posts, I have seen terms like strain compatibility and shear flow which I think I understand as a concept but I'm unsure how to actually check this.

Would external post-tensioning be ruled out if I can't go below the slab?

Thanks in advance

 

[KootK]

there are services running along the top 130mm which cant be touched (read: can if we ask enough)

As SRE mentioned then, this is almost certainly a some version of truly reinforcing, with shear flow, rather than anything resembling sistering. If it's 6" channels, those will be six in channels expected to behave compositely.

 

[hokie66]

Rapt and AR,

I haven't seen a square twisted bar either. But according to the SRIA, they were used in Australia up until about 1963, when the reinforcing suppliers replaced the square bars with twisted deformed bars.

The twisted square bars had a similar strength as the twisted round deformed bars, but I would question the bond properties, particularly when the concrete is not of the best quality.

 

[Ingenuity]

I have designed and built a number of external PT systems for concrete beam (and slab) strengthening.

Some things to consider:

1. external PT adds little additional mass to the system ==> significant advantage;
2. specialized design and build ==> significant disadvantage;
3. able to 'tune' the recovered deflection in some instances - i.e. design the number of tendons (or jacking force) with some 'fat' and adjust in the field based upon actual recovery measurements taken during stressing;
4. short tendons do not work so well for economic PT solutions - disproportionate amount of hardware (and labor) for such one-off and/or short spans;
5. for short spans with not much available 'drape' it means you have to provide more axial prestress to obtain the equivalent uplift forces ==> not economic in many cases;
6. if there if difficult access to the beam ends for stressing, then internal stressing anchorages have to be used ==> can be expensive;
7. best to have a PT contractor on board early, so that details and costs are understood from the get-go. However, such a small project will possibly not attract any PT contractors;
8. depending on the extent of pre-existing corrosion damage, and all other options considered, removal and replacement of the member is sometimes more cost effective.
9. since the beam width is 230 mm am I correct in assuming it is supported by a engaged brick pier? If so, depending on the wall/pier details, this may be difficult for external PT to accommodate;
10. need to meet building code fire resistance requirements ==> may be able to wiggle-out of this if the current capacity exceed the current demand, but sounds like you do not;
11. need to check/satisfy ductility requirements of external tendons for moment demand, assuming your current reinforced section does not meet strength requirements.
​

For every 1 external PT strengthening project, we probably do 10 FRP strengthening projects, so keep that in mind for economics and competitive pricing.

 

[KutEng]

So it seems that:

External post-tension is the most effective but isn't a cost-efficient option
FRP is the most cost-efficient option but the owner does not want it
And adding steel members can be done but won't get the capacity I need

[flush]

Will I see a lot of resistance if I suggest shoring up the slab and knocking out the beam?

 

[oldrunner]

Probably should have started there. This topic has shown there's a lot of talented engineers out there.

 

[Ingenuity]

Will I see a lot of resistance if I suggest shoring up the slab and knocking out the beam?

Depends on the $'s and the client. Remove and replace will be noisy - so that is always a consideration too.

Start by costing/pricing out remove and replace option ==> lots of contractors can help price this out, so I would expect very competitive pricing (unless you are in a small rural-type town).

A 6m x 450D x 230W beam is only 0.62 m[sup]3[/sup] of concrete (1500 kg). Assuming access to the beam/slab top and beam sides probably 2 laborers x 8 hours x 3 to 4 days, so 50 to 75 man hours for demo with electric chipping tools. Add in disposal, new supply/install rebar, formwork, supply/place/finish/cure new concrete etc etc, add in overhead + profit, you have total price estimate. Existing services may be a headache - but that is common with all techniques.

Go the the client with the advantages of a remove and replace option: "Sell" it as an opportunity for you to 'engineer' the beam to the required capacity and serviceability...I would widen the beam > 230 mm (and add some compression rebar) and maybe shallow up the depth to 400 mm (L/D=15). Owner may really go for a slightly shallower beam (more headroom for his Range Rover )...with the added bonus that it is brand-spanking-new.