ACI 318 App D - side blowout

[archeng59]

Working on a project where we have steel embed plates with headed studs welded to the plates. The embed plates are installed in concrete columns. The capacity of the embed plates are controlled by side blowout because the headed studs are too close to the sides of the column. ACI 318 is vague regarding the use of supplementary reinforcing. My question is this: without supplementary reinforcing, I need to increase the column width so side blowout is not an issue or add supplmentary reinforcing through the shear cone of each stud near the edge. The architect says no-go on the column size increase. So, if I add reinforcing through the shear cone, it appears to me that side blowout can be ignored. Is that how anyone else interprets the use of supplementary reinforcing? I have vertical bars at each corner of the column and each face. Those bars pass through the side blowout zone. I will add more column ties in the area of the embed plate. Just not certain that is enough to satisfy the side blowout issue and get a larger capacity for the embed plates.

 

[JedClampett]

Since the code gives no guidance, I would go to the references cited and see what they say. I would be wary about ignoring any mode of failure.

 

[archeng59]

Jed, I am wary of ignoring a failure mode which is why I asked the question. What "referenced cited"?

 

[JoshPlumSE]

I'm not really an Anchor Rod / Pullout cone expert. But, my company spent some time on this recently.

Some useful references that we found:

1) ACI 349.2R-07: Guide to Concrete Capacity Design (CCD) Method - Embedment Design Examples

2)Eligehausen, Mallee, Silva, "Anchorage in Concrete Construction" Published by Ernst & Sohn in 2006

3) Comite Euro-International Du beton; "Design of Fastenings in Concrete - Design guide". Published by Thomas Telford Services 1997.... Probably not as useful for US engineers as the others.

I'm not sure why, but it seem like the Europeans have really lead the way with the anchorage research.... Thankfully, all the books / guides I've listed above are published in English.

I hope that helps.

 

[archeng59]

Thanks, Josh.

 

[TXStructural]

If the loading is parallel/axial to the anchorage, consider using deformed bar anchors (DBA) instead of headed stubs. Make the DBAs long enough to develop as laps with main reinforcing (using rebar development length) and confine using normal ties. This negates this side blowout as the failure mechanism.

318-08 App D commentary says that "An explicit design of supplementary reinforcement is not required. However, the arrangement of supplementary reinforcement should generally conform to that of anchor reinforcement shown in Fig. RD.5.2.9 and RD.6.2.9(b)." These figures show typical reinforcing methods to increase resistance to side-face failure.


John Turner CSP PE
CRSI Greater Southwestern Regional Manager

 

[archeng59]

The loading is not parallel to the anchorage.

 

[TXStructural]

Failure of a chunk of concrete from shear loading of the anchorage is more "break out" than "blow out", sorry for my confusion.

A single leg tie between the face bars in the column should work (depending on needed strength.) Using hairpins around the studs, [per RD.6.2.9(a) or (b)] tying them back to the remaining concrete mass is another way to go.

 

[archeng59]

I am at home and do not have the exact numbers, but the embed plate capacity is 180 kips when side breakout/blowout is not an issue. When the edge distance is such that breakout/blowout is possible, the embed plate capacity is approximately 60 kips. If hairpins or other supplemental reinforcing is used, would the capacity be 180 kips or would I only get a 40% increase above 60 kips? That is my question. I need to support a 125 kip load.

 

[Lion06]

If I have a harad time getting headed studs to work (which is often the case for high loads), I weld rebar to the embed plate and get away from App. D completely.

Be sure to spec A706 rebar and low hydrogen E90XX electrodes. The E90XX electrodes allow for smaller fillet welds from bar to plate to develop the bar.

PCI has tables giving the fillet weld size needed to develop various size bars for different electrodes.

 

[archeng59]

I used weldable rebar instead of headed studs. Could not get the capacity needed. Either I am not calculating the capacity correctly, Appendix D is wrong or all of us have been very lucky in the past. Makes you wonder how so many buildings were constructed over the last umpteen years using headed studs on embed plates and no catastrophic failures.

 

[TXStructural]

I don't wonder about why older work doesn't fail more often. In part, this arises with stronger concretes, leaner structures, more specific loading, and more exact computational methods.

"Back in the day", beefier connections and members allowed more room for error by being stronger than required. Less precise control of material properties frequently meant errors to the high side in an attempt to avoid weakness.

Designing with precise tools means less room for error, so errors will start showing up as failures sooner or later.