Tapped holes for baseplate

In my experience there is no huge issue tapping through two laminated plates. I'd weld the plate first. Typically this would be profile(laser) cut with the hole already in place. (though I believe there is some heat hardnening here that makes tapping harder).

The plate would then be welded to the SHS. Then after that you'd drill through the SHS and then tap. If the item is being galvanized then you need to retap the holes afterwards. Like you said with a 25mm plate and the right bolt length you probably don't need the SHS tapped.

One quick aside question. Where does tapped holes fit into structural codes? AS4100 is what I deal with, which I don't believe if specifically addresses tapped holes. (Though I've been sensible and used 'engineering judgment'.)

I avoid tapping through two plies - I would provide a clearance hole through the HSS and field weld the shop-tapped plate.

There are other options - Hollo-Bolt

human909, thanks - you're right that AS 4100 does not address tapped holes. I've had to scour a few different sources to calculate the thread length for these.

dvd, thanks for the reply, I'm leaning towards the clearance hole option since I think this is more likely how they will fabricate it. I've not really dealt with Hollo-bolts before but I will check it out.

If tapping the holes, the 25mm plate should be welded to the HSS before drilling and tapping. The use of the Hollo bolt is good. In these environs the 16mm plate (or more likely a 10mm plate) would simply be field welded to the HSS, or just the post itself.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

Thanks dik

Question on hollo-bolts - are these removable? I expect that the fabricator will want to fit everything up in the shop before disassembling and shipping it to site.

Hollo-Bolts are removable, but not reusable.

Your goals are not clearly defined for this - as dik indicates the handrail, or plate, could be welded to the HSS.

It is also possible to use a tube/pipe sleeve and bolt the handrail into the sleeve.

In general, most fabricators would be processing the plate and the HSS on different machines. Most of them would not want to drill and tap the holes after welding together. Because of this, it would be a good idea to have larger clearance holes in the HSS and only tap the plate - this will allow for fabrication and fit-up tolerances.