Shear Stirrups 1

[vincentpa]

For shear stirrups in concrete beams, I have always used #4 bars minimun and 60 ksi steel. However, in PCA Notes, the design examples use 40 ksi steel and I have seen some drawings from the 1990's where the the specified steel strength was 40 ksi. Why would they use 40 ksi? I thought 60 ksi is more common. Is there a good reason for 40 ksi?

 

[UcfSE]

40 ksi used to be more common. It is "softer" and much much easier to bend and cut.

60 ksi bars are more common now and mean less bars and therefore less labor to fabricate. Grade 60 bars are much harder to cut and bend. Sometimes they just break instead of bending.

Some fabricators may still use grade 40 for stirrups and grade 60 for longitudinal bars. I have never mixed the two.

 

[dik]

Some earlier grade 60 stuff was pretty brittle. I saw a #3 stirrup drop and hit a concrete curb and fracture into two pieces... Some consultants in these areas still use all 10M bars as 300 Grade (40ksi).

dik

 

[vincentpa]

I am in the northeast and I haven't heard anything about brittleness of 60 ksi rebar. When I was in Washginton D.C., I did a lot of concrete design, and I always used 60 ksi. Now I am in SW PA and I got to thinking that maybe it is not good to use 60 ksi. Now I am even more dubious after reading about the steel becoming brittle. Have they resolved the problem of brittleness with 60 ksi rebar? What do you use up north?

 

[JAE]

My take on this is similar. For many years all rebar was 40 ksi. When 60 ksi came out, engineers at first just converted over - but soon found that the specific recipe for 60 ksi steel was brittle - per the above comments.

But the brittleness was specifically found in the #3 bar stirrups (per dik) and so many engineers were directed to use the 60 ksi for the longitudinal steel, and use the 40 ksi for the stirrups.

After that, the CRSI recommendations were that #4 stirups were OK at 60 ksi, but the #3's were the worry. Soon after that (I'd say in the mid - 1980's - the recipe was changed in the 60 ksi product and the brittleness was alieviated.

 

[vincentpa]

Thank you all for your input.

 

[jike]

When they first start making grade 60, there was two problems:

The carbon content in the steel did not have sufficient controls on the amount of carbon and it made the steel brittle. This was later corrected by a change in specifications.

The bend radius on stirrups was too sharp for the higher strength (brittle) steel. Some engineers had the scary experience of going to jobsites and noticing stirrups with cracks in them. The immediate solution was to go back to using grade 40 stirrups. ACI shortly after revised the bend radiuses to prevent cracking.

This has been corrected long ago, but it still amazes me how often this question comes up.

 

[vincentpa]

I think that it comes up for two reasons.

1. Engineers look at old drawings and specifications that are not updated and see 60 ksi steel for main reinforcement and 40 ksi for shear stirrups. Then they think maybe they are doing something wrong if they are using 60 ksi shear stirrups.

2. PCA Notes on ACI 318 gives two examples using 40 ksi shear stirrups as if they are the typical choice. That is unfortunate. They give no direction as to which grade of steel is used.

 

[Lutfi]

keep in mind to use closed striiups when you have torsion. Regualr ties will not work.

Regards,
Lutfi