Fitted bolt connections 2

[Ipetu]

The fabricator wants to use bolted connections on a truss. There are a few connections where slip critical connections will not work i.e. we will need a lot of bolts. So I am trying to see if I can use a fitted bolt connection for these connections. We are using 3/4" diameter A325X bolts. What size of hole is required to ensure a fitted bolt connection? Is this addressed in the Steel Manual or any other reference text?

 

[paddingtongreen]

Why not just use bearing bolts, I never yet saw a truss that needed slip critical connections.
BTW, slip critical is a joint designation, a requirement for very few connections, I think you mean High Strength Friction Grip bolts.

Timing has a lot to do with the outcome of a rain dance.

 

[paddingtongreen]

Ipetu, sorry, I didn't really read your post very well last night, it was late.

Bolts are "X" when the threads are excluded from the shear planes and are only specified for bearing connections. Unless you are using thin material, almost all A325 and A490 bolts are X bolts because of the limited thread length.

You do not need fitted holes, 1/16" over the bolt diameter is the standard.

Bearing bolts in standard holes are normal in bolted trusses.

Once or twice, I have used the HFGBs for a joint where I needed the joint to be slip critical for one (service) load case and the higher bearing strength for another (extreme) load case.

Timing has a lot to do with the outcome of a rain dance.

 

[Ipetu]

paddingtongreen,

Thanks for the response. My concern was the 1/16" over bolt diameter space when using bearing bolts in standard holes. Will this 1/16" over dimension not cause the connection to "slip" and then the cumulative sum of "slip" in all the truss connections lead to unacceptable truss deflections? Or am I being over cautious?

Thanks again.

 

[paddingtongreen]

Ipetu, you are being too cautious. In structural steel, the holes are punched with 1/32" accuracy, so the 1/16" allows for two bolt holes to be off in opposite directions and still allow the bolt installation. You have to accept that structural steel is what it is, it is not clockwork. Probably all of the trusses will sag about the same amount, but again, It is structural steel so you only know within limits where the bottom of the truss will be. In over fifty years, I never saw a truss sag noticeably for that reason.

Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

In general, I agree with paddingtongreen. Bolted trusses do deflect due to tolerance holes, but this is not an issue in most structures.

If for some reason the bolts are required to be tight in the holes, I would call them pins instead of fitted bolts. In that case, you have to specify the tighter dimensional control necessary to make tight pins work. I had a job once where a stayed and strutted awning structure at an airport required tight pins, the fabricator used tolerance holes, the columns bent to accommodate the pins, and the fabricator had to do a lot more work to straighten it all out. So it does happen.

 

[paddingtongreen]

Just out of interest, when I first got on the drawing board, before there were HS bolts, they used "turned and fitted" bolts for what we now call "slip critical" connections. For these to work, the structural parts had to be assembled on the ground, clamped together, and a hole drilled through all of the pieces in one go. The turned bolt was inserted and the next one drilled. These bolts were turned on a lathe, to an exact diameter and in a shape similar to the shoulder bolt.

Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

Thus, the trade called 'fitter and turner'. Fitted bolts were more common in machine work (as they still are), while rivets were used in most building and bridge structures.

Paddington, you must be older than I am. A325 bolts have been in relatively common use since about 1960.

 

[paddingtongreen]

I started in 1951 as a drafting room clerk, in London. We used some large riveted plate girders back then, nobody trusted the welding. I remember that they pushed the friction capability when those High Tension bolts came out, I don't remember using them in bearing for a long while. We were designing and building oil refineries, and we had some structures that had to be non slip and demountable so rivets wouldn't do and black bolts or wedge bolts couldn't do the job.

Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

A belated welcome to the Eng-Tips forums, Paddingtongreen. We need more old hands to keep some of the young folks in line.

Yes, they did emphasize the friction type connections in the early days of high strength bolts. Now, we rarely worry about slip in the connections.