Tolerance for Beam With Cope End

Question

I need to have some beams fabricated with copes on the ends. They will be set on top of existing steel trusses, and used to replace undersized purlins to shore up existing rafters (hence the need for the copes).

My structural engineer specified 4" long copes. There is a cope at each end. The distance between the edge of the flanges on the steel trusses is 186".

The question is a matter of practicality. If I have the fabricator make the coped length of the beam exactly 186", I feel like I may have a difficult time getting the beams in place. So I'd like to have a little tolerance to give me room to slide the beam above one truss and then pivot it into place above the other.

How much tolerance should I allow for to make this easy on myself, the installer of the beam, without diminishing the integrity of the design? In the CAD drawing below, I have just arbitrarily chose to show that the coped end-to-coped end distance should give me 1/4" of play on each end of the beam.

I don't want to overthink this, but these suckers are heavy and installing them is going to be fun enough without having to fight tolerance problems.

Is there a tolerance rule of thumb for this situation? What tolerance would you use?

Answer 1

The short answer is yes, and it's expected that there is clearance for constructability. The Steel Erection manual allows for these types of tolerances. Did you try asking your steel fabricator what is typical? Or check your general notes, that is where you will find this type of information. Or ask your structural engineer. If you mess up, you don't want to have to tell the jury that you got your advice from a bunch of yahoos online :)

Answer 2

What I would suggest is add a p cut in the flange of diameter slightly larger than the thickness of the horizontal flange. With that you maintain dimensions and manage a tolerance fit without introducing a weakness that may lead to web failure. A square cut could be catastrophic. Once the beam is in place it is only a matter of diagonal positioning to align with the correct final destination without wiring about tolerances.

As a prevailing precaution against the web tearing, I would consider welding a diagonal strengthening rib above the angular T beam vertical cut or add a flange to the end of the beam where it is full size up to the middle of the web before the p cut position.

Answer 3

Your safest best is to run a simple FEA with mesh analysis, but you can calculate shear stress and strain by hand and determine the factor of safety using $S=\frac{M\cdot y}{I}$ (max moment * distance over inertia).