Thread Strength of HHS Stock

[terminus]

I need to field tap 4"x4" 1/2 HHS square stock to mount an equipment cabinet. As I am an electrical type I am unsure if the thread depth (and material) may be suitable for my end requirement - to withstand an earthquake. Here is what I know:

The steel - as above
there are four bolts (corners) per cabinet
I am using 1/2 bolts (thread count/material TBD)
the Service load is 4748lbs
the bolt will project 1.5" above the frame
Pull-out strength per bolt is 2374lbs
Shear load is 1941lbs (2 bolts per side)

Will this work? Can the tapped hole provide the required strength? Any suggestions on the size/thread of the bolts? My field installers are not machinists - can they effectively drill and tap in the field?

Any help would be greatly appreciated.

 

[diamondjim]

I would put in the extra 4 bolts between the
corner bolts, ie, an extra bolts on each side
between the corner bolts. Especially if this
is going to be a field service installation.
You might be able to drop down to 7/16 diameter
with more bolts. The extra measure of safety
is warranted if you are overcoming earthquake
loads. I would also use course threads for
a field service application. Fine threads are
supposedly stronger but need better attention
to drilling and tapping.