Stair Stringer Bolt Design Question

[zrck99]

I am putting together a set of calculations to justify the details/design that the a steel fabricator put together for a very typical office building stair set. They plan to connect stringers to walls with 1/2" diameter wedge bolts at 2'-0" o.c.

I've shown the detail in the attached pdf. My question is, won't gravity loads cause the bolt to rotate at the face of concrete because of the 3" flange width standoff? I assume people do this all the time so I'm just curious if anyone has seen a good justification for it. Some of the other guys in my office suggested adding 3" square metal shims in between the wall and inside face of the channel web. I agree this would add some stiffness but it still looks a little indirect to me...

Any thoughts?

Thanks

 

[phamENG]

Agree it's a problem. That's a lot of eccentricity for an expansion anchor that size. Why are you attaching it to the wall? Typically we span from landing to landing and bolts aren't needed from the stringer to the wall.

 

[zrck99]

I agree with you. I would typically design stringers/landing headers more as beams and just do stronger end connections. This setup more looks at the stringers/landing members as ledgers but yeah, I agree that it isn't great. A 1/2" diameter kwik bolt 3 has a substantial amount of shear and tension capacity but the hinge deal kind of throws those numbers out the window in my opinion. I assume that when calcs aren't needed, their system works just fine so long as they create enough clamping force by torquing down the anchors but that is tough to justify in calcs.

 

[phamENG]

Yeah, in reality there's a balancing act where friction between the stringer and the wall is resisting the shear while the bolt is just providing the normal force....until it slips and anyone on the stairs has a really bad day. I'm not quite as worried about the anchor/concrete interface as I am about the steel in combined shear and bending. Can you even get one with a threaded stud long enough for the application?

What's the span on the stair? Just a basic, stable, 2-bolt connection into the landing will usually carry the loads for the stringer unless it's a monumental or oversized stair. Or is there something unique here that's driving them away from the landing connection?

 

[r13]

The 3" eccentricity is real, and the bolts will see both bending and shear, when the channel moving down naturally. The bolt will fell tension only right before pull out by gravity.

 

[phamENG]

when the channel moving down naturally

Agreed. But to move down, it has to overcome friction from the steel to concrete/masonry wall interface that is generated by a normal force (unknown pretension in the bolt which will occur during installation as the stringer/ledger is pulled tight to the wall). I for one would not count on this in design and it doesn't seem like zrck99 has any desire to do so, either. Just discussing how this system probably performs when installed without engineering review.

 

[JAE]

I don't think any bolts are intended to bend like that. The force is coming down through the channel web. The resistance is at the face of the wall. Most bolts have some ductility but are not meant for this.

We have bolted single plates to the wall directly and then welded stringer flange tips to the plates as an alternative.

Or you could use a tube, and provide holes for bolt installation access on the wall side, then close up and grind smooth the hole.

 

[KootK]

1) I share the concerns and have an extra of my own. I'm not sure that we actually have a reliable design method available for checking concrete breakout when bolts are used with standoffs this significant.

2) I like a seat angle bolted to the wall under the bottom flange is that would be visually tolerable.

3) I like the shim idea so long as separate shims are welded to each other and the whole stack is welded to the channel such that shear transfer can be plausibly said to occur close to the wall. With this detail the eccentricity would presumably be rectified via moments in the the adjacent stair framing (treads etc).

 

[zrck99]

JAE -- Your comment reminded me of this detail that we have used in the past. I feel a lot better about it with the plate added like this, but would feel much better about it with a bolt above and below the channel flanges rather than right in the middle.

phamEng -- they are calling out 1/2" Dia. x 8" wedge bolts in their details. I believe that I could force them to spec long Hilti Kwik Bolts that would work for what they are "trying" to do but just don't really like the connection in general. The stair is in my opinion, very typical. Just landings on either side connected by stringers with a pretty typical rectangular cavity for it to fit within. We are kind of wedged within the rectangular cavity so if a failure actually occurred, I don't think that it could physically collapse to the ground but again, it's just a sketchy detail that I would feel more comfortable avoiding.

retired13 -- Agreed

 

[BridgeSmith]

I wouldn't count on friction to stop it from bending. I'd be requiring a plate washer (shear tab?) that attaches to the edge of a flange of the channel for the bolt to pass through at the face of the wall to eliminate the eccentricity. I'd consider adding a sleeve for the bolt between the channel and the plate washer for ease of assembly.

Alternately, adding the bolt tab below the channel, attached to the bottom flange, would also work, and would seem to be a better position, anyway. If I'm visualizing this correctly, what's shown would result in the end of the anchor stud sticking out into the stairway, wouldn't it?

Rod Smith, P.E., The artist formerly known as HotRod10

 

[zrck99]

Anyone see any issues with just having them add these plates then sizing the anchor for shear and tension from the couple force?

 

[BridgeSmith]

Won't the stair treads resist the torsion on the channel that would produce tension on bolts? Or am I thinking of something other than what you're working with?

Rod Smith, P.E., The artist formerly known as HotRod10

 

[canwesteng]

I like that best out of all options - pretty clean load paths. Seems like the fabricator is spending a dollar to save a dime though with all this anchoring to concrete, would I be right in guessing they are supply only?

 

[JAE]

zrck99,
More like this:

 

[zrck99]

JAE -- Ha, a nice catch. I agree, without the welds we are going to have issues for sure.

 

[phamENG]

I agree JAE's is the best solution if you have to treat it as a ledger. But then I can't get away from the fact that you're saying this is a typical stair between two landings. What is happening at the landing? If there's already a connection there and the stringer is able to span without intermediate supports, even if you have to bump the bolt size up an 1/8" to eliminate the ledger connection you'd be saving them money...right?

 

[r13]

Just visit Hilti and get technical design information for 1/2" Kiwi Bolt 3 (ASD):

fc' = 4 ksi
L_embd = 4-3/4"
T = 2575 lbs
V = 2415 lbs
Steel - Concrete friction coefficient = 0.45

The trouble is I couldn't find the bolt with length required for the installation as shown by the OP. Must be another type by other manufacture. You need the spec.

Count me out, as I spent too much time in the shop and late for the party. Agree with BridgeSmith that shear friction in this case is a dangerous proposition, and JAE has the better idea. One question, is there any reason why the channel can't be flipped?

 

[KootK]

This is pretty clean and mechanically respectable. One would need to give some thought to gap management. I'm not so sure about constructability though. I imagine that the bolt would have to come before the stringer.

 

[JAE]

That's a good detail, KootK.

 

[KootK]

That's a good detail, KootK.

Thanks. Maybe better suited to landings than stringers though for construct ability reasons.

1) With an adhesive bolt, I imagine you'd want to install that first and add the stringer after. Tough handling and bolt locating. You might get around this with an extended adhesive nozzle or something but that would probably impact QC.

2) An expansion anchor might be able to be installed with the stringer in place but then:

a) You've gotta hold this giant stringer in place rather than just the plate in your detail and;

b) I might check with Hilti to ensure that even a mechanical anchor could be installed successfully through the gap.

One question, is there any reason why the channel can't be flipped?

The way that stair stringers and landings are normally framed internally leads to the flanges naturally wanting to point outwards from the walking surfaces. If they point inwards, it's a messier look, connections get tougher with coping etc, and, in some cases, there's stuff to catch your feet on if your a clod.