Post Fixed Punching Shear Bolts 2

[pappyirl]

Hi People,

Wondering what your thoughts are on post fixing punching shear reinforcement with an anchorage only on one surface.

Typically we would post install a bolt with washers top and bottom mimicking the reinforced bar equivalent to enhance punching shear on an existing slab.

Recently a number of clients have requested increasing punching shear capacity without access to one of the surfaces, top or bottom. The inclined bolts do cross the shear failure plane and have an anchorage of some length past the failure plane. They do not however provide the equivalent tie that a bar/bolt anchored top and bottom would provide. So depending on which surface is not available, the punching shear tie anchorage occurs at some point outside the analogous truss strut and tie. See attached for a basic description of the design philosophy.

My understanding was that the truss analogy for shear relied on a strut and tie with the punching shear providing a tie between both. I'm struggling to comprehend how this system actually works.

Thanks.

 

[r13]

This way might work (provide fastening plate) with localized weakness as circled in red - inadequate length to develop tensile strength, leaves regions without reinforcing.

 

[WARose]

I'm not sure I understand what are you trying to do.....is this an attempt to provide a increase in (two-way) punching shear capacity by having anchor bolts crossing the failure plane?

If so, that would not be acceptable (IMHO).

 

[pappyirl]

Thanks, yes enhance two-way punching shear in an existing slab.

In my opinion enhancing punching shear on an existing slab requires anchorage on both faces. Good to see agreement.

 

[JLNJ]

We are not in agreement.

Bolts with washers top and bottom doesn’t really work either.

Whose methodology is this?

 

[pappyirl]

Torqued bolts anchored top and bottom set in an epoxy resin. Recessesed if necessary for durability or fire considerations.

The inclined methodology is based on a Hilti design which i am struggling to justify.

 

[r13]

In theory it can work, as bars across shear plane that increases the shear resistance through tension. The reality is, it is highly unreliable, except for an urgent, temporary fix, before a permanent solution is found.

 

[hokie66]

Shear reinforcement has to be anchored at both ends, so this has no chance of working.

Punching shear is not to be trifled with. You need to be investigating a solution with a bracket or column enlargement below the slab.

 

[pappyirl]

Thanks, my thoughts exactly.

Attached link for those of you who are interested.

https://www.hilti.co.uk/medias/sys_...chnical_information_ASSET_DOC_LOC_2521845.pdf

 

[r13]

I think I understand hilti's claim that epoxy essentially works as bolting the bar, and pre-tensioned thru proper torque, similar to post-installed anchors. However, as pointed out, there are potential weakness as the shear stress/crack is highly unpredictable.

 

[JAE]

The inclined bolts do cross the shear failure plane and have an anchorage of some length past the failure plane.

You don't know WHERE those failure planes are. That is the concern here.
The diagonal shear failure planes are not some singular line of cracks but are waves of overlapping cracks in a zone anywhere from the column face outward - perhaps several feet.

I can't imagine drilling hundreds of diagonal holes in a donut zone around a column - from below no less. Sounds like someone's pipe dream.

And if you don't know exactly WHERE the cracks are, there's no way you can determine how much length of development you have past the crack.
So some percentage of your epoxied bolts aren't effective and you don't know how many.