Countersinking Structural Bolts 2

[Deker]

I've got an architecturally exposed steel connection that I'm trying to make pretty and I'm considering using 3/4" ASTM F835 flat head screws so that the fastener heads are flush with the steel. The member is a fascia channel that will be continuous past the tips of wide flange cantilevers providing support on the backside of the channel. Bolts will be loaded in shear with a strength level demand of 2 kips / bolt.

The concept is to provide an end plate at the tip of the wide flange that will bolt to the web of the channel, with the bolt heads countersunk into the web. Since the height of the bolt head is the same thickness as the web, I'd be adding a welded shim plate to the backside of the channel so that bolts aren't bearing solely on the sloped surface of the countersunk hole. Holes in the end plate, shim, and web would be standard holes with a nut on the backside of the assembly (no tapped holes).

Does anyone foresee any issues with the proposed connection? I haven't used flat head screws in structural applications before, so not sure if I'm overlooking something. I'd appreciate if anyone would share their thoughts, experience, admonitions, etc.

 

[GC_Hopi]

What about clip angles shop welded to the channel and field bolted to the steel members?

Here is something

 

[Deker]

Thanks. I'm considering a welded connection, but I'm not thrilled about a bolted connection to the supporting wide flange beams; they'll be tapered down to 6" depth at the tips supporting the channel. Channel is an MC13x35. Nice find on the manual reference for bearing on countersunk holes.

 

[azcats]

Can a regular fab shop accommodate making the tapered hole?

I'd lean more towards the above suggestion of reconfiguring the connection to provide bolts that don't need to penetrate the channel web.

ETA: Can you provide a sketch of the relative orientation of the members?

 

[Deker]

Not sure if your average fab shop can automate the countersink, but worst case I imagine they can punch a standard hole and follow it up with a piloted countersink in a mag drill. Can't post a sketch right now, but the tip of the net 6" deep wide flange cantilever will be supporting the 13" channel from the back side of the web. Top of channel will be 3" above the wide flange. I can use the full depth behind the channel for the connection if needed.

 

[Deker]

Framing arrangement is shown below. As I mentioned, proposed connection is an end plate welded to the TSB and bolted to MC web. Bolt heads would be countersunk on the MC side. Would probably add a welded shim on backside of MC for additional bearing and bolt through the whole thing with standard holes.

 

[SnTMan]

How about button heads, reasonably attractive looking, no c'sink required.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[dvd]

Are you looking to use hot-dip galvanized flathead (or button head) screws? Know that HDG ASTM F-835 flathead screws are not readily available and can be subject to stress-corrosion cracking and/or hydrogen embrittlement. There are specific bake-out requirements prior to plating for these fasteners based on tensile strength, so if you start custom plating fasteners, make sure both you and the plater understands what they are doing.

 

[Brad805]

How many bolts and what size of project? If a small quantity it is easy to countersink. I can do it in my own little shop for projects. If thousands needed, then it may limit the number of fab shops. May not be the most cost effective detail regardless.

 

[GC_Hopi]

This is an exposed steel so maybe you want to showcase the bolt head. Or you can go with tension control bolts since they typically have a round head and not a hex. Or get a cap for the bolt heads.

 

[Deker]

Brad - Total number of bolts ~150. Aesthetics take priority over costs here (within reason).

Thanks to everyone who responded. Seems like most here have an aversion to countersinking the bolts. I'll also pitch the idea of button heads or TC bolts to the architect and let him choose.

 

[SnTMan]

Didn't see it mentioned but hole pattern tolerancing can be a problem as well...

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[GC_Hopi]

I would say its more a foreshadowing of the aversion that you are likely to get from the contractor. We have all heard it... grumble grumble we never do that, it will delay the schedule, why are we doing that, we are not doing it... but like you said money talks as long as the owner is willing to open their wallet.