Shear Breakout in Hollowcore Plank

[Kstructuralguy]

I need to check a hollowcore plank's capacity to resist shear breakout force applied by embedded anchors. The anchors are post installed, and embedded though the entire plank (top flange through to the bottom flange and through a hollow core, not into a web). The shear force is parallel to the span of the hollowcore plank.


To check the plank's capacity, I used ACE318-11 Appendix D ( Section 17.5 in ACI318-14) as advised in the PCI Handbook. Initially, I only considered the breakout area of the flanges for simplicity. The cores will be grouted at the anchors but since the plank has already been cast, the only thing holding the grouted core in place if there is a failure is friction so I cannot account for that to add to the breakout capacity. This calculation does not produce the required strength, so I included the webs of the plank within the breakout area.


This is where things become unclear. The basic concrete breakout strength in shear of a single anchor, V_b, (section D6.2.2 in ACI318-11) needs the required load bearing length of the anchor, l_e. So question #1: If each flange is 1" thick, the load bearing length, l_e, is 2" regardless of if the cell is grouted correct? Since the grouted section is not load bearing.

The bigger point of uncertainty is the "modification factor for anchors located in a concrete member where h_a < 1.5*c_a1" (section D.6.2.8 in ACI318-11). If h_a is taken as 2" (2* 1" flange thickness) then the factor comes out to over 3.0 (my c_a1 = 13.5"). This seemed straightforward in the initial calculation where only flange areas were considered. But now I wonder, could I consider each flange area separate, thus having 2 distinct breakout areas each with h_a = 1"? This would increase the factor further.

With the area of the webs also considered, I increased h_a to 8 since that is the total plank thickness and as a result the modification factor is reduced significantly. So in each calculation, the capacity comes out to be roughly the same. Intuitively, that is not realistic because I've only added to the breakout area while holding all else the same.

I searched ACI-318, the PCI Handbook, Manual for the design of Hollowcore Slabs, and various only resources for more information or testing on shear breakout in hollowcore planks but cannot find any information on how to approach this.

 

[TehMightyEngineer]

I'd probably have to do some digging to figure out my thoughts on the proper breakout strength.

But the first two practical thoughts I have are:

1) How confident can you really be in those numbers? You're definitely outside of the empirical data used to create the appendix D equations. I'd take any numbers you get as mostly a rough guess. If your capacity is not significantly more than the strength I'd probably find an alternative such as:

2) I'd look at ways to avoid this whole mess. Either using hairpin rebar reinforcement around your anchors to transfer the shear deeper into the span of the plan, or by using some sort of headed steel plate or embedded element instead of the anchor (one that develops the load into a more favorable breakout condition).

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)
American Concrete Industries

https://americanconcrete.com/

 

[Retrograde]

There are mechanical anchor fixings specifically designed for use in hollowcore slabs.

http://www.powers.com/product_09340.php

 

[Kstructuralguy]

I feel confident in the first calculation. Considering only the area of the flanges and neglecting the webs altogether is conservative and the flat slab analysis is applicable to the appendix D equations. Unfortunately, this analysis does not yield the required strength.

I considered a hairpin, but the issue of the plank already being in place presents problems. They would have to break out a large portion of the top flange to grout the bar in place. This also does not solve the issue of the core filled with grout not bonding to the plank itself. The hairpin would have to hook at the ends and cross into plank webs to prevent the hairpin and grouted core from pulling out. I am always hesitant to have plank webs cut unless necessary.

I don't believe an alternate anchor would help. I neglected to mention this, but the concrete breakout values I have include the entire end of the plank's area - meaning the breakout cone encompasses the full end of the plank and cannot be further increased.

I believe the best option is to split the load over 2+ planks. This can be done by backing the anchors further away from the edge and either using interface shear between planks or rebar anchors between planks to transfer shear. Doubling the concrete breakout area will provide a good amount over the required strength. Now I need to coordinate with the other engineer to modify the Tie-in detail. Another thing to consider is the plank's attachment to the building framing but that is beyond the scope of my design

 

[structSU10]

Does it make any sense to have the anchor attach to an angle on the bottom of the plank, which spans between plank webs? you can then use the shear strength of the web to carry the anchor force.

Are you projecting the failure surface over a 4' width? I don't quite understand what you are saying your projected failure cone is as it sounds much too large for an 8-12" thick plank - I would think at best you could project at a 1.5:1 (used for solid slabs) which would be 18" in all directions for a 12" plank. That also assumes that can project through the other hollow cores, which seems suspect.