Upgrade to Old Existing Concrete Beam

[Everynameistaken]

Hi All,

I am working on an old building retrofit. The building was built in the 1940s of R/C, and the beams I have a question about are 12" wide and 24" deep. They are continuous beams with three bottom bars that lap at the supports and three top bars the are only over the columns. The beams do not have any shear ties at all! We have some limited testing data for the old concrete and rebar but not very consistent, so we are also considering the conservative historical strength data from a range of sources including CSA S16 - Chapter 14, and historical ACI codes.

The beam concrete is in fair condition and there is some obvious corrosion of the reinforcing that is starting to result in rebar section loss.

Our calculations show that the beam, using current LSD load and CSA A23.3 resistance factors, has a D/C > 1.0 in flexure and right at 1.0 for shear. We are considering upgrade options for these beams, but the access to the underside is very limited and tall scaffolding would be very expensive. So we are considering adding concrete and reinforcing to the top of the beam to increase the effective depth at midspan for positive bending and add more new flexural steel for negative bending. Access to the top of the beam is much simpler meaning all the work can be completed top down.

I have attached a basic sketch here of our concept and looking for some thoughts of others on the forum to see if you all agree that this addition can in fact increase the capacity.

We will need to roughen and dowel the new concrete to the old to transfer the shear flow, which we think will generate increased flexural capacity. I am not sure if adding the extra topping concrete can also be counted on for increased shear capacity?

Do people agree in this approach? Any thoughts or comments are much appreciated!

 

[HTURKAK]

Pls provide more info. ( span length , loading , at least a sketch showing the structural plan.) I would consider other options ( external prestressing, plating from sides , adding steel profile to the top , FRP retrofitting )





Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

 

[JAE]

Are these singular beams with no slab between them? Or is there a slab between them and your additional topping extends across the entire level? Your sketch seems to imply singular beams.

If singular beams, how is the load getting into the beam if there's no slab to impose load on them.

If a slab exists, that's a lot of topping load you are adding to an already overstressed condition.

I'd be very careful about assuming the topping adds to your shear capacity. I could see some initial slippage or other irregularity which might impose more shear on the lower section first, failing it and then the upper topping would have no way to support all the shear.

 

[ALK2415]

your approach is for (-ve) bending regions. you need to strengthen the bottom side of slab (+ve) bending region

 

[Everynameistaken]

Hi All,

Thanks for the initial comments

These are continuous concrete beams only, no R/C slab between them. There is a pair of them that supports a small crane that rolls on a rail above the beams.

The load is applied from the crane in a couple of specific locations to pick specific items up, so only Self weight and then point loads from the wheels of a crane lifting.

Access for things on the side and bottom is difficult which makes steel plates and FRP tricky and expensive. Since there is access to the top already then the topping is an achievable solution from a constructability perspective. I will also look at the size of steel beam that might be required to support the crane completely support to support, that might work, but I suspect it will be larger!

So the idea is to add the topping "glued" to the top of the existing beam with extra bars for -ve moment and then the topping adds extra depth to increase the leaver arm of the positive bending using the existing reinfrocing.

The beams span about 25ft or so and there is a couple of spans but only one crane so only one span is loaded with live load at a time.

The topping is only a small added dead load as its just on the beams. I can see if the shear flow is not accounted for correctly, then there could be some slip and now the load will go to the stiffest item which is the original 12"x24" beam, but if there is sufficient interface shear then there should be no "slip" between the original and the topping. then we can get the flexural increase. But I will check to see if the Vc is sufficient to carry the crane and the extra concrete weight in the original, 12"x24" beam with no ties.

 

[JAE]

I'd be concerned about the shear. You have zero shear reinforcement and a plain concrete beam in shear, while it can work in theory, is no match (in my opinion) to the impact/dynamic loads of a moving crane. I'd go with a new "take-all-the-load" steel beam above the concrete beam and let the concrete beam have a rest for say 20 years.

 

[ALK2415]

Im afraid you are talking about such crawler crane
you should think about adding steel beam to support above structure with proper anchorage

 

[TLHS]

If you've got the clearance and the span isn't too long, it's likely cheaper to just slam a new steel beam over or under it. Second choice for me would likely be to strap steel channels to the side.

If that doesn't work, then I'd start looking at strengthening the concrete. If there's corrosion, it's understrength in bending, and it's non-ductile in shear it's going to be a lot of fiddly labour and you're still not going to get it up to where you'd want it to be.

 

[bridgebuster]

@ALK2415 - 79th Street Boat Basin?

 

[ALK2415]

bridgebuster
the example given is applied also (i think)
these pictures taken from LinkedIn website. using steel beams are good solution and recoverable also

 

[bridgebuster]

@ALK2415 - It is the 79th Street Boat Basin in Manhattan. I thought perhaps you were working on it. I inspected the structure back in 1985.

 

[TLHS]

If you're really set on concrete based upgrades you can basically upsize the beam so that you can upgrade bending capacity and also get a more ductile and reliable shear load path. I've never done it, and it seems like a pain, but there's a reasonable amount of literature on it and I've seen it locally as a seismic rehabilitation strategy. Chip the outside of the beam as prep, put in stirrups an inch or so clear of the existing face and new bottom bars as appropriate. Then cast a new few inches of concrete around the outside of the existing beam to integrate those items. I'd likely do some supplemental doweling or similar to make sure I keep everything held tightly together.