Existing Beam Reinforcement

[EYFS]

Hi all,
I Hva and existing deep beam with a section of 20x60 cm . the existing bottom steel is 2 T16 which is below the minimum required reinforcement by 20%. (must be 2T20).

I am adding a load on the beam, but the issue is the following.
The beam without any additional steel can support the existing load, should I retrofit the beam by adding 1 bottom layer or can I leave it as it is?
(the beam is 70 years old and no cracks are shown-f'c=22mpa fy=300 Mpa)

Thank you,

 

[BAretired]

I am adding a long on the beam, but the issue is the following.

What does that mean?

2 - 16M bars bottom appears to meet minimum reinforcement requirements. If the beam as built is adequate to support the loads, additional reinforcement is not required. In any case, adding an additional bottom layer to an existing beam is not a trivial task. Leave it as is.

BA

 

[rapt]

You have not said

- which concrete strength has been defined, Cube or Cylinder.

- whether it is a T beam and the flange dimensions.

Also, is it really a Deep beam i.e. not flexural?

 

[EYFS]

Thank you for your replies.

Sorry its a typing mistake not "long" but "load".

its not a T beam...its an ordinary flexural beam With b=20 cm and h=60 cm.

The concrete strength is cylinder

 

[dik]

For small amounts of reinforcing, concrete strengths have little effect on the flexural strength. Does your code have requirements for reduced amounts of reinforcing? Does the beam meet minimum temperature steel? That is 0.2% or whatever? Does your code have provisions for less than flexural steel? The Canadian code permits less if the beam is designed for 33% greater moment. Is there sufficient steel for cracking moment? also sufficient steel for interior or exterior exposure? What are the actual areas of steel top and bottom and what is the moment and shear (factored loads)?

Dik

 

[Agent666]

In the NZ code you can also reduce the minimum reinforcement provided the strength is 1/3 greater than the capacity required like Dik has noted. However the reduction is specifically excluded if the beam is part of the lateral load resisting system (unsure if your local code has something similar but would pay to check before going down this route). The reason for this is with lower levels of reinforcement you are unlikely to get the distributed cracking required for energy dissipation in seismic cases, and its more likely that you'll get a single crack with the concrete cracking moment being higher than the reinforced capacity.

Depending on what the effective depth is (you have not noted this), you are so close to minimum steel that I'd tend to say its ok provided you have some margin in the strength department. One point of warning though, after 70 years the real f'c is likely to be much higher than 22MPa. Which obviously factors into a higher minimum reinforcement ratio.

 

[dik]

concrete strength likely irrelevant. Check the flexural difference between 25MPa and 50MPa for small reinforcing ratios.

Dik

 

[dik]

In CSA A23, "...flexural members with a clear span to depth less than 2 shall be designed as deep flexural members..."

Supports at 4' or 5' o/c?

Dik

 

[dik]

Check the output... note exposure.

Dik

 

[dhengr]

EYFS:
I would make a composite beam by putting a couple 12” deep channels, back to back, one on each side of the existing conc. beam. You say the existing beam will carry (is carrying) the existing loads, and the new channels should be sized to carry something more than the new added loadings. Jack the existing beam up, maybe to induce a bit of camber, so as to unload it, and then apply the two new channels. Use the predrilled holes in the channel webs as a through drilling template for bolts through the conc. beam. Epoxy the through bolts into the conc. beam and tighten the bolts down. This method should allow both parts of the beam to pick up their share of the total load, without over stressing the existing conc. beam or its rebars, before the channels start picking up some load.

 

[EYFS]

I'm using ACI. I made a coring test for the beam, so the actual f'c= 22 mpa.

I didn't found any closure is ACI mentioning the 33% load...both ways I have more than 33%.

So to sum up I have to add and additional bottom layer to reach the 1% from bottom, and one rebar to rech it from the top.

 

[Agent666]

Regarding third greater capacity thing, check 10.5.2 in ACI 318-08. Probably in a similar place in later versions.

 

[CANPRO]

Are you sure you have deformed rebar that is similar what you would expect to see today? Not sure how relevant these minimum percentages from current codes are if you're comparing them to 70 year old rebar that doesn't meet todays standards.

 

[BAretired]

I Hva and existing deep beam with a section of 20x60 cm . the existing bottom steel is 2 T16 which is below the minimum required reinforcement by 20%. (must be 2T20).

As(min) = 240 mm[sup]2[/sup]
As (provided) = 400 mm[sup]2[/sup]

How did you conclude the steel provided is below minimum?

The beam without any additional steel can support the existing load, should I retrofit the beam by adding 1 bottom layer or can I leave it as it is?
(the beam is 70 years old and no cracks are shown-f'c=22mpa fy=300 Mpa)

Can the beam support the existing load plus the load you are adding? If so, what is the problem? If not, how do you intend to add a bottom layer?

How do you expect helpful answers with the kind of information you are providing?



BA

 

[rapt]

Concrete strength (tensile) does affect the minimum reinforcement. Though many do not specifically say it specifically, minimum reinforcement is based on the cracking moment of the section.

The minimum reinforcement reduction should never be used in a determinate member, and should be used with caution in indeterminate members. In reality, if less then minimum is supplied at one hinge location/moment zone, the capacity should be assumed to be zero there and treated as a pin in analysis to redistribute the moments top other parts of the member. I know ACI318 does not say this but it should!

As others have said, the reinforcement supplied would appear to be sufficient for a rectangular section.

If you are using the normal beam minimum reinforcement rules, they assume a T beam which requires more reinforcement due to the increased section modulus.

 

[Agent666]

BAretired, has the minimum reduced in latest versions of ACI318?

In the 2008 metric version clause 10.5.1 has 0.25*bw*d*sqrt(f'c)/fy =426mm^2 with d assumed to be 550mm, second code provision of no less than 1.4*bw*d/fy = 513mm^2? (our NZ code has the exact same provisions). This is 20% over as originally noted. Curious where the 240mm^2 came from in the code?

Original poster has also noted after 70 years that 22 MPa concrete strength is present, minimum reinforcements are based on the specified strengths, so if that is available could use that. Might be something like 17.5 or 20MPa, which gains a little bit.

I too am confused if it had sufficient strength why anything further is being proposed.

I wonder what ASCE 41 has to say on the subject, as it evaluating an existing structure. I'm not too familiar with it unfortunately, perhaps someone else can comment?

Our local Guideline for evaluation and strengthening of existing buildings do let us use what is termed the 'probable strength', which is more of an average than the lower 5% characteristic strengths typically used for new design.

 

[dik]

Concrete strength (tensile) does affect the minimum reinforcement. Though many do not specifically say it specifically, minimum reinforcement is based on the cracking moment of the section.

Not sure what you are aluding to? For flexural strength with reinforced concrete, for small amounts of reinforcing in beams, the concrete strength has little effect. The jud value is nearly the same, only slightly reduced for the weaker strength concrete. Flexural plain concrete strength is enhanced, and greatly diminished on first crack.

Dik

 

[Agent666]

Dik, the issue is if the uncracked concrete strength based beam capacity is higher than the ultimate capacity of the beam based on the reinforcement after cracking. Consider a beam being loaded progressively with an increasing load, you load it to the point where the concrete is about to crack, it cracks and suddenly you have the scenario where the reinforcement cannot support the load, in a determinate member this means collapse. In an indeterminate member it means some redistribution and still possiblity of collapse depending on the rest of the structure having sufficient strength for the redistributed actions.

There are also serious implications for seismic, a single crack is bad, forcing multiple cracks with distributed nature is better for energy disapation/ductility. A single crack accumulates all the plastic strain in the reinforcement within a very short length and leads to low cycle fatigue and ultimately bars fracturing. It's like repeatedly bending a bar over a short length, it breaks after a few bends back and forth, however bend the same bar over a longer length and it can be cycled for many more cycles without risk of fracturing. This type of thing was identified as a real issue within recent New Zealand earthquakes, in particular in walls where there were cases of the vertical reinforcement progressively fracturing due to much higher in situ concrete strengths than specified, and only minimum reinforcement ratios being provided. Our codes have recently been amended with changes to minimum reinforcement in many members (most notably at wall end regions, doubling previous minimum reinforcement requirements to combat specified concrete strengths vs final in situ strengths being up to 2 times that specified). Closer control over maximum long term concrete strength in mix design is also now required.

 

[BAretired]

BAretired, has the minimum reduced in latest versions of ACI318?

Not so far as I know. I was relying on my faulty memory which gave: As(min) = 0.002*b[sub]w[/sub]*h or 240mm[sup]2[/sup]; however that was intended to apply to slabs, not to beams.

The 1994 version of A23.3 (Canadian Code) requires As (min) = 0.2 sqrt(f'c)*b[sub]t[/sub]*h/f[sub]y[/sub]. Using f'c of 22Mpa and fy of 300 mPa, As(min) should be 375 mm[sup]2[/sup], very near the value of As provided.

ACI 318-63 required a minimum value of p = 200/fy, so As(min) = 200/(300*145) = 0.0046 bd or about 500mm[sup]2[/sup].

I don't know which code is in use by the OP, but if the existing reinforcement is adequate to support the total load, I would be inclined to waive the minimum reinforcement requirement.

BA

 

[rapt]

Thanks for answering for me Agent66. Saves me a lot of time.

Unfortunately some design codes continue to use a constant value to define minimum reinforcement even though their initial logic was based on concrete cracking moment. The 2 worst are ACI318 and BS8110. For example the old BS8110 still says .0013 for slabs and walls. This was correct about 60 years ago when concrete strengths were 20MPa or less. Unfortunately the code writers did not envisage the increase in concrete strengths or thought future code writers would update the codes. Better to put in a formula that covers all concrete strengths.

More modern codes have recognised the problem and related minimum reinforcement to concrete strength. AS3600 changed to cracking moment in 1988 and Eurocode is nor dependent on concrete strength. Unfortunately some are still way behind