How to handle uncomfortable technical disagreement/concern over design 3

[Lion06]

I have a situation in my office where I'm involved in a design and disagree with a principle that, in my mind, could have significant safety considerations. I've already aired my opinion, but others disagree, because it's just been done in other applications without problems. I just don't buy into that philosophy, especially when there is no technical literature on the subject.

I genuinely have concerns, but I don't know what else to do given that I've already put my concerns on the table and they've been dismissed.

 

[PEinc]

SEIT,

I've designed micropiles for both tension and compression using threaded bars without any lower nuts and plates.

http://www.PeirceEngineering.com

 

[sandman21]

You do not sign the drawings; you should voice your complaint but ultimately you are not responsible for the drawings. Take it as a lesson learned for when you sign drawings that is all you really can do. I would not go out of my way to have my complaint in e-mail form. You are only inviting trouble with no real benefit to you. I have had to design things I did not like, quite a few times, I voice my complaints at the time and moved on. If the person signing the draws wants to use old standards, fuzzy logic, or poor engineering judgment, there is nothing you can do other than letting building official or state board know.

 

[rowingengineer]

once again I agree with Hokie. My gut feel is development length should be about 50-60dia.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[TXStructural]

A machine thread has no development in concrete, but a headed anchor rod (or the typical nut and plate washer should develop the full strength of the rod. As long as the anchor rod extends far enough into concrete, and laps with the rebar extending into the pier, the anchor will be developed. You avoid App D considerations by providing confinement in the form of ties or spirals, and by crossing the failure plane(s) with reinforcing.

The most likely tension-failure plane in the pier concrete is a horizontal cross-section near the end of the anchors, which will be restrained by the rebar crossing the plane, and developed fully above that plane.

The pier reinforcing needs to be developed above the failure plane, extending to a depth of pier sufficient to hold down the structure. Headed or hooked reinforcing can be used to reduce development length.

Use of A615-conforming threadbar (Dywidag, SAS, etc.) as an anchor rod requires more consideration than using A1554 anchor rods. Bars and nuts are different sizes, and special details will be required. Mechanical splices may be preferred to laps with the high loads described, and threadbar can be connected to standard A615/A706 rebar with typical mechanical splices for rebar.

John Turner CSP PE
CRSI Greater Southwestern Regional Manager

 

[Lovepuzzles]

I was intrigued by the situation and thought of a detail for consideration.

I can't pretend to fully understand what's been said, but to resist a force one needs a static component, right?

Wouldn't welded components in concrete resist better with a perpendicular head/washer/disk than none?

I think there's also the consideration of ethics.

Best of success, StructuralEIT

 

[TXStructural]

Lovepuzzles: It sounds like you are talking about headed bars.

In accordance with ACI 318-08 12.6, headed bars can develop the reinforcing bars (or anchor rods) in tension over a short distance (l_dt), which is roughly 15 bar diameters.

A few practical considerations:
-Pier cages are sometimes placed and then cut to length, or otherwise cut to length in the field.
-Congestion may be an issue, resulting in conflict between anchor rods and pier reinforcing, particularly where anchor rods are connected as a unit before placement.
-12.6 requires minimum spacing of headed bars.

Headed anchors are not permitted for development of bars in compression, per ACI 318-08 12.6.3. (Similar to the restriction on hooked bars.)

 

[rowingengineer]

Maybe in the USA thread bar dosn't develop in concrete, but in australia it is possible. we will even use smoth bar for fitment development.


An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[TXStructural]

Rowing,
Threadbar which has deformations equal to A615/A706 rebar will develop. Machine thread "all thread" does not develop to any usable extent.

A straight, smooth bar will not develop, even at hooks. Adhesion between portland cement concrete and smooth steel is negligible.

 

[hokie66]

Says who, TXS? Saying that all thread does not develop usefully is a very broad statement. And if hooks of smooth bars do not develop, there are a lot of beams all around the world with shear stirrups that are in trouble.

 

[GregLocock]

http://www.sciencedirect.com/scienc...serid=10&md5=97c2b442187dc25290f109dd1a033b65

may help

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[apsix]

"A straight, smooth bar will not develop, even at hooks. Adhesion between portland cement concrete and smooth steel is negligible."

Perhaps our concrete comes with extra stickiness.

BS8110 Table 3.27 lists bond/lap lengths for plain bars (ie those with no deformations) which are generally about 80% greater than for deformed bar.

It stands to reason that all-thread bar bond lengths will lie somewhere between that for plain and deformed bars.

 

[rowingengineer]

Thanks greg, here is a simlar article but free.

http://wpage.unina.it/gamanfre/pape...nchored smooth bars in existing rc frames.pdf

TXS,
I can see that in high seismic regions that smooth bar should be considered like a deformation anchor. That is to say: the development length should be such that little to no deflection along the bar anchorage should occur before plastic behaviour of the structure. While this requirement may limit the use, it shouldn’t negate the use.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[PEinc]

TXStructural said, "A straight, smooth bar will not develop, even at hooks." However, per FHWA Publication No. FHWA-SA-97-070, Micropile Design and Construction Guidelines, Implementation Manual, June 2000, Sec. 5.E.5 Grout to Steel Bond Capacity, Page 5-30; the typical ultimate bond values range from 1.0 to 1.75 MPa (145 to 254 psi) for smooth bars and pipe and 2.0 to 3.5 MPa (290 to 508 psi) for deformed bars (ACI 318). This bond for deformed bars is a litle different than the value I quoted above from a different FHWA Manual.

http://www.PeirceEngineering.com

 

[hokie66]

If smooth reinforcement has no reliable bond development, I wonder how pretensioning works. I know, the wires are twisted, but they are still smooth.

Interesting to me that people depend on modern glue for developing threaded rods, but don't like to depend on portland cement glue.

 

[rowingengineer]

That is a good question hokie, maybe it is because it has rusted a little so the awnser to the riddle maybe that we need to rust the smooth/threaded bars a little and the bond will be good.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[dik]

Hokie... smooth wires, unless they are greased have some bond... albeit a smaller value. With P/T stuff, there is a slight Poisson effect that tends to make the ends a little fatter...

Dik

 

[TXStructural]

I was speaking from a code perspective. ACI 318 does not recognize development of smooth, mild reinforcement. As for strand, it does have to be very clean to bond. It also has a greater surface area per unit length, and the twist of the strand increases the bond. The FHWA pub mentioned does not recognize smooth bar in the discussion of types of reinforcing.

I am certainly not saying that there is zero surface bond between steel reinforcing and portland cement paste. Deformed bar develops primarily because of mechanical interlocking of the deformations, paste and aggregate, and surface bond with cement plays a significantly lesser role. Surface bond is susceptible to factors which do not adversely affect development of deformed bars or wire to the same extent.

As for ASTM F1554 all-thread, there will be some development, but again, it is not comparable to A615/A706 deformed bar.

 

[hokie66]

TXS,
Thanks for clarifying your earlier posts. I think we all agree that deformed bars are the way to go in most cases.

 

[Lion06]

I really didn't just let it go, so I kept looking and I found that the threads on Williams bars meet ASTM A615 for bar deformations, so this is the bar we'll be using. This is helping me sleep much better.

Thanks to all for the input and discussion.

 

[PEinc]

All of the GR60, GR75, and GR150 threadbars from DSI, Williams, Skyline, and SAS Stressteel meet A615. These are the bars commomly used for tieback anchors, tiedown anchors, and soil nails.

http://www.PeirceEngineering.com