Rebar Bender & Min Bend Diameter

[jocose]

I'm confused why many Rebar benders on the market, and many I have seen used on construction sites, don't appear to meet the ACI recommendation for min bend diameter.

I was recently on a job that used a Rebar bender similar to this one:

https://www.youtube.com/watch?v=IOQBxGYF4zM

The resulting bend on a #4 bar was slightly less than a 1 inch diameter, a far cry from the recommended 3". Is there any explanation for why these products don't meet the specification, is there some detail I'm missing or are they just ignoring the standard?

The majority of jobs in my area use Rebar benders similar to this one. The building inspector signs off on these jobs, so that is where my confusion comes from.

 

[TXStructural]

Bends tighter than the specified diameter are not permissible.
The reasons for this are primarily:
1) to avoid damage to the bar during bending which impairs the tensile strength of the bar. Tight bends frequently result in outright fracture - bars are tested to a bend tighter than that permitted in design in order to assure product quality. Tight bends may also result in microscopic damage which is not evident.
2) bends tighter than the ACI/CRSI minimums do not reliably perform in concrete. When subjected to full tension loads, tight bends result in excessive crushing of concrete inside the bend and failure of the bar.

As far as equipment goes, anyone can make a device and sell it. The one in your video is directed at the non-structural market. Low grade and ungraded No. 3 rebar may or may not survive a bend like that, but I could see the appeal for residential use. I hope you don't have people using a device like this on building construction projects.

CRSI's position is to have fabrication done by a fabricator who knows the rules and materials. In the US, the cost of buying fabricated bar is always less than paying someone on a jobsite to cut and bend bar. This is especially true by the time you factor in waste and injuries.

 

[BUGGAR]

"The majority of jobs in my area use Rebar benders similar to this one. The building inspector signs off on these jobs,......."

Where are you?

 

[jocose]

Where are you?

Rural Eastern WA. We have a lot of 1000-5000 square foot homes being built right now as people from Seattle move over here for the cheep land and to live in the country.

These are residential construction projects not commercial or civil. Specifically it's used for foundations, and retaining walls. I've never seen one of these bars crack or break, but really think that the standards should be being met regardless. Most of it comes from local building supply stores like Home Depot.

 

[BUGGAR]

Remember all those lawyers that essentially forced engineers to stop designing multi family buildings in Seattle due to all the HOA lawsuits? They're on their way to Eastern Washington right now!

 

[MacGruber22]

The majority of jobs in my area use Rebar benders similar to this one.

That is why you should be getting shop drawings no matter it residential, commercial, etc. Contractors don't know the ACI 318, nor are they required to by law.

"It is imperative Cunth doesn't get his hands on those codes."

 

[TXStructural]

Contractors should know ACI 301, not 318. 318 is design, 301 is construction. If I even see anyone except a designer using 318, it is a red flag that the CDs were incomplete.

That little soapbox rant done, ACI 301 does cover minimum bend diameters. These are based on both steel properties and concrete behavior in situ. The main reason the required bends are as they are is concrete behavior - chiefly crushing concrete inside the bend and concrete splitting at the bend. Also any weakness due to a tight bend would be at the point of maximum stress in the bar arising from a tight bend acting as an anchor.

When looking at the places where field bending is often used - the residential and light commercial markets - the stress in rebar is typically very low. Sections are frequently stronger than required due to low demand and minimum reinforcement requirements. Very few of these bars are ever stressed to capacity, particularly at bends. Most structures never see full design loads. This is why failures are few and far between, even with imperfect construction.

One other thing I forgot to mention is that fabricators often achieve the correct bend shape and radius using pins (mandrels) that are smaller than the minimum bend. Frequently this is done as a segmented circle, with a series of small bends to create an acceptable radius. This is an acceptable practice and maintains the strength of the bent bar and of the steel-concrete composite section.