Simple Shear Unstiffened Steel Seat Connection

[mappryan]

Let me first preface this by saying I am a jr. bridge engineer and have zero experience with this type of connection. We are attempting to hang a "fascia panel" in one of the spans of our bridge to make up for the drastic difference in superstructure depth. The adjacent spans have 6' deep beams while this span has only 3' deep beams. Span is ~70' long. Therefore, we will be using a FRP panel that is 3.5' tall in order to line up at the bottom of the 6' beams and will cover a portion of bottom flange of the 3' beams. I have attached drawings so see those. I am trying to come up with the structural support for this FRP Panel system. I have a 16x16x1/2 HSS in order to limit deflections. I am having trouble with connecting the HSS to the pier. I wanted to use the Simple Shear Unstiffened Seat Connection that I am finding in my steel manual for simplicity. AISC LRFD 2nd Edition 9-128. A few things:

1. I purposely made the angle only 8" long to keep the edge distance for the anchor rod and angle small. I have a lot of reinforcing in that portion of the pier so I need to have the 6-5/8" min. dimension I am showing. I cant find any examples where the seat is not at least the width of the beam. Is this feasible? Is there any maximum edge distance requirement that I would violate if I kept the anchor rods in same place but made the angle extend the full 16"?

2. My plan for the HSS to angle connection was going to be slotted holes to allow for the expansion. However after further reading here it seems that slotted holes are not a good idea for expansion (sometimes okay if loads are very small). Is this true even if my loading is normal to the slot? My loads seem fairly small (Service Reaction around 3.5 kips). The HSS is only supporting itself and the panel. Is there maybe a better way to go about this.

Any help would be much appreciated as I said I am really struggling with this one.

 

[ash060]

What about wind load? 70' is a long way to go. I would guess that you would have service issues with that kind of span.
Also, you could put an embed plate into your pier and attach to that.

 

[JedClampett]

With my massive 60 seconds of exposure to this, I would make the following suggestions;
* Use a wide flange shape instead of a HSS. It's more efficient for deflection.
* If you do that you can use a standard double angle connection. If you'd like, you can slot the holes, although that does mess up your concrete to angle connection. If you do that, than the next suggestion helps.
* I would embed a plate with welded studs in the concrete instead of trying to miss reinforcing.
* It would seem you would have a substantial wind load. Don't forget to design for that and it's deflection.
* My spidey sense says that a 16 inch sections spanning 70 ft. is a bit much. Consider deepening your section. someone might want to hang a scaffold off that member some day. Even if you use a tube, consider one that is deeper and narrower.

 

[JedClampett]

ash060, you sniped me.

 

[hokie66]

As a counterpoint to Jed's suggestion, the wind loading is eccentric, so the tube is good torsionally, while a wide flange is not.

As a practical thing, why would you want to do this? A short span should have smaller members than a long span. Why try to conceal that fact?

 

[mappryan]

The middle span is short because it accommodates a future rail corridor and therefore it also has to have a much higher vertical clearance then the 2 adjacent spans. The idea of the fascia was to conceal this vast difference in the beams and then could be removed if and when the future rail corridor is put in place. The client thinks this vast difference in beam heights would not be aesthetically pleasing. I tend to think this fascia panel may be even worse, or at least to someone who knows what they are looking at. I settled on the tube for 2 reasons. 1) my wind load is ~30psf and therefore my lateral deflection due to wind is ironically almost identical to my vertical deflection due to self weight plus dead load of panel. 2) Torsion due to the eccentricity of the wind loading. I know the HSS16x16x1/2 section is pretty stout but I do want a pretty strict limit on my deflections in both directions. I guess I am still wondering if a narrower than beam width seat angle is feasible? And is a slotted connection okay for this application to handle the thermal expansion (very small expansion ~0.375" total or 3/16" each end)? If i were to switch to some sort of embedded plate, isn't the typical connection to an embedded plate welding? How then would I be able to handle the thermal expansion? Thanks for all responses, I really appreciate the input.

 

[dhengr]

Mappryan:
If that void space below the beam is that obnoxious and annoying why not make the facing material out of a printable and replaceable material and sell it as advertising space. It would pay for itself, as opposed to the community using its lack of funds for such a project. And, next month they will want a tax increase for what they claim are real serious needs.

Watch out for the differential expansion and contraction of the different materials. And, with the details you show right now, with the wind loading causing a varying, and vibrating, lateral and torsional loading, isn’t the top of this fascia liable to actually slap against to bottom flange of the existing girder at the top edge of the fascia?

You have as much need for horiz. bending strength as you do for vert. bending strength, and the torsional loading and torsional vibration are going to be difficult to deal with, given your current details. Why don’t you make these structures fairly light box-section space trusses, maybe 30" square by 70 long? That is, 4 corner longitudinal chords, angles turned in, then whatever diags. are required for vert loading and lateral loading. Then, on the (btwn. the) outer long. chords attach vert. zee’s or channels with toes in, every 4 or 5', and small bolt holes pre-punched, for fixing the panels in the field.