Connection experts - steel angle and concrete

[ATSE]

Connection experts: I need some critical feedback on a special connection.
The attached pdf is a 90%.
1. Existing bridge sidewalk is 12" thick at tip.
2. New 16" ductile iron pipe, supported at 9' oc. 166 plf, plus seismic for SDS = 1.0. moment, shear, and axial is a little complicated, but the governing action is moment.
Mu = factored moment = 50 k-in.
3. The continuous #4 bar (probably Fy=40ksi since constructed circa 1965) and the perpendicular hooked top bars #4 @18" certainly provide strength and confinement, but may not be sufficient.
4. The limit states are easy for steel and bolts, except I'm not sure how to analyze concrete breakout at the top upper right corner. That is, the (3) bolts in shear that tend to break out the top corner - or at least cause linear cracking.

 

[ishvaaag]

In normal situations where the weight is prevalent essentially you have the shear to be transferred through the connectors to the sidewalk. Through bolts is a repeated question in this forum and maybe should be better addressed in codes but whilst one can think for a start as in shear friction respect the steel shaft, and as mechanically fixed anchors to concrete, since add to the free bolt inside the hole spalling forces and should be conservative.

When in such approach, and considering that even if the concrete must not be cracked (out of the sidewalk action) you may not yet meet the recommendations or geometrical checks given for evaluation of anchor bolts. For those cases a FEM 3D analysis can be enlightening respect if the tensile stresses added by the shear forces imparted through the angle are such as to endanger the adequacy of the detail.

Not much difficult to make: just 3 extrusions in autocad (sidewalk, hole through boolean, and rod) then export to say algor or visual nastran, put fixed the cantilever and apply forces to the rod (or better 3 rods) and you have at sight the principal stresses.

Everything tensile, (especially physically risky when over say 30 kgf/cm2 at which concrete may statistically start to crack) must be properly counteracted by reinforcement or the detail won't be valid. Code-like situations may be far more stringent than that, and in fact, seeing this is a local detail where reinforcement (even in the combined action of corner rebar and hoops or stirrups) is unlikely to properly restrain tensile stresses in every direction, you should seek if feasible to meet the specified characteristic tensile strength of your concrete (met 95% of the times); this is when you are sure your concrete won't crack from this action.

If not, as entirely usual in reinforced concrete, you must rely in the reinforcement scheme (that, by the way, you can readily model by its inclusion in the 3D model with non-slipping contact for the non-cracked state) to counteract as said the present tensile stresses forming the cracks.

Then you would be lead to ascertain if the crack widths are premissible and this is more difficult undertaking for if separation between cracks and width of the same are evaluable from code literature and alike for flexural cases, I don't think have to met that in what I have read in bolt contexts, and the closer thing that comes to my mind respect proper reinforcement in localized zones is the spiral reinforcing schemes at postensioning points, or parallel meshes under point loads.

Yet since you also name seismic loads, if at some time flapping upwards, the angle integrity or connection may be put at risk by repeated cyclic prying action, and this shouldn't be forfeited for lifelines, I think.

 

[ishvaaag]

Merely reading a question just below than this at the moment, I have remembered that I have used (with entire success to its now modest life of 2 years) epoxy adhesive under compressed plates to enhance the shear transfer to more desirable levels near some edges, spreading the shear transfer to wider zones than what in your case would be the contact surfaces at the sheared bolts. This trick that I used for new you might use for retrofit with more justification if only you find it feasible in some way inject the adhesive, and no doubt the top localized tensile stresses would benefit decreasing to lower values from the shear load being distributed on a wider surface.

 

[delagina]

just looking at the pics wont the REBAR prevent those concrete breakout

 

[dhengr]

ATSE:
You could improve your perceived moment on that connection, and thus the shear on the 3 bolts, by making your canti. beam deeper. Instead of using a W6x15, consider using a W16x36 as follows: approx. 28" long; split the web on the diag. with 10" height near the conc. sidewalk slab and 6" height out at the tip; of course, this cuts 2 beams; the bot. flg. is horiz. and about 2" above the bot. of the slab; the top flg. is an 8 or 10" wide bent plate to be horiz. on the top of the slab and it takes 2 A.Bolts, and slopes down and is welded to the cut web. Add an 8" wide x 11" deep x 3/8" end bearing plate on the 10" end of your canti. beam, at the conc. slab and under the bend in the top flange pl. If you are worried about cracking and pull out, etc. and then divide whatever your shear is by 3 for an average bolt shear, I think you are kidding yourself. At least one of the 3 bolts will be yielding something, at a much higher shear than the others, before all 3 come into play. I don’t think I would worry about the .25" web stiff. pl. at the canti. tip. I would be tempted to have someone map the top hooked bars and the cont. #4 at them, at about 9' o/c spacing, and give or take a bit, I’d center my hangers over those hooked bars, if that didn’t get to messy and look too funny in terms of hanger spacing. Could you hang this pipe and hardware from the sloped underside of the sidewalk, with a cut-web beam as above, or does your “(E) pipe not shown” prevent this. This would eliminate the tripper plate and nuts up on the sidewalk.

 

[TXStructural]

You are adding loading to the concrete-rebar bond within the hook development length. Extending the connection beyond the development region will improve performance. It is likely that heavy loads will result in cracking of the tip of the sidewalk away from the remaining concrete in proportion to the stress applied to reinforcing across the cracks. More closely spaced supports will reduce crack width, since you will engage more steel in tension.

It is also doubtful that the rebar at the other end of the cantilever has excess design capacity. If load factors were used, they provide an allowance for such an added load. You can use this added capacity up to the factored design load.

As dhengr says, increase the vertical leg: you can add a strut or gusset below; there will be less stress on the bolts, which will reduce stress at the top of the concrete near the bolts. This doesn't address overall capacity, but will reduce local stresses.

Consider drilling into the end of the cantilever at or below the top rebar and use adhesive (HY 150) for anchor rods inserted past the hook development length of the bar. (In lieu of the thru bolts.)

Finally, use the number of fasteners/anchors needed, but putting more in doesn't necessarily increase the capacity of the group, once the capacity of the concrete is exceeded.

 

[dik]

Can you use Hilti HIT TZ epoxy anchors and maybe reduce the number of anchors and the depth of drilling required? They also come in higher strength.

Can the roller accommodate a skewed installation with a hanger located on either side of the web? to minimise torsion on the beam section.

Do you require the stiffener plate at the end of the beam?

Dik

 

[ATSE]

Thanks to all for the useful comments.