Punched holes in Structural Steel 1

[haggis]

Hi all.
I am engaged in the design of overhead conveyors in a major car assembly plant. I also design the supporting structural steel for theses conveyors. Structural Engineers don't be alarmed that a Mechanical guy is doing this...all the support design goes to a local Structural Engineer for review and certification. As welding to the building steel is prohibited,we use a WT6 and L 4 x 3 x 3/8" clips to attach to the OWSJ. The WT6 being on the u/s of the joist (back to back angle bottom chord) and the angle clips over the angles. The WT's and L clips (vertial leg cut to suit)are bolted together clamping on to the bottom chord of the joist. My question is: Is there any difference in drilling or punching the holes in the WT's and angles. I've been told by one Engineer that punching the holes can cause problems later with cracks propogating from the holes. Is this possible?
Thanks in advance for any opinions.

 

[IFRs]

Driling is preferred for critical members. I think that Mack trucks does not allow punching on their frame rails.

 

[ishvaaag]

If I would have such concern I would punch the angles, dimensioned if necessary bigger than necessary. This way both the serviceability and safety of the structure as a whole and the reusability of the costlier main member would be greater.

There must be something in your question for in some codes and places it is mandated that the fabrication is to be made punching to a smaller diameter than required, then drilling-reaming to the required diameter. Hence the finishing by reaming is likely understood to lay final superior properties for the fabricated holes, in more than allowing (maybe) bigger geometrical perfection.

 

[Koz]

Hi
I'm a little new to this structural game but I have been involved with fabrication for years.

The sections you are using are mild steel, pretty ductile stuff. Catepillar and other truck fabricators use high strength steels (I don't know the type) for their truck frames.

Punching tears the metal and leaves little micro-cracks in the surrounding material of the bolt hole. That's why sub-punching and reaming is done. AISC considers these micro-cracks in their calculations for tear-through and block shear rupture. Check AISC LRFD for better and exact information. We use as-punched holes in most connections. We ream sub-punched for critical holes such as cantilever connections for sports stadiums holding thousands of people over the heads of thousands of other people, or when the engineers impose the requirement on us. The same is done for fracture critical members, generally made from high strength steels, and are considered non-redundant. The micro-cracks from punching will have greater impact as the steel gets harder. The owner bears the cost for these additional steps.

Drilling is fantastically expensive if done with a conventional drill. Hole-Hawgs are commonly used to drill large holes if the sections are not too thick.

Koz

 

[Lutfi]

Drilling will yield a much better and cleaner hole with no micro cracks. However, it is very expensive and time consuming compared to punching.

I my 23 years of experience in the structural design and mainly steel, I would say that majority of the steel fabricators in the US punch the holes. They punch them 1/16 inch larger than the bolt to facilitate installation. If you have concern with fatigue and stress concentration due to particular joint geometry, drilling may be the way to go.

If the structure you are using will utilize standard steel rolled shapes and you want a steel fabricator to do the work, expect that you will have punched holes. If you specify and insist on drilled holes, expect to pay premium and lengthy fabrication period.

Good luck

 

[haggis]

Thanks to all who answered.

I take it punching the holes is not a critical issue and if one Engineer refuses to certify the connections,I should consult another.

 

[CWIC]

AISC (ASD) permits punching holes (M2.5) within thickness limitations. The Specification for Structural Joints Using A325/A490 Bolts 3.(C) also permits punched holes.