Site welding square hollow sections 2

[asixth]

What is the general acceptance of specifying site welds to square hollow section (cold-formed)?

An existing beam requires some strengthening so I have designed a square HSS strut on a 45degree between a square HSS column and a wide-flanged beam (bearing on column). I have detailed a T-plate cleat to be site welded to the column so the strut can have a bolted connection.

The steel fabricator/detailer has come back and asked whether they can site weld the strut to the existing column and beam because they have surplus lengths of steel that they would like to get rid of.

My thoughts were because it is a site weld it is not going to be the same quality as one performed in the workshop. Secondly was that the wall thickness of the HSS is only 4mm (3/16") so I have been told there is a possibility of "blowing a hole through the HSS" and finally, any temperature effects will create a greater stress in a welded connection than a bolted connection which has a little tolerance.

What are others opinions with:

1. The quality of site welds (are they specified often)?
2. Can a hole be blown through a HSS with a thin wall thickness?
3. Is thermal expansion critical in the design of welded connections?

Any reference material would also be appreciated.

 

[Ron]

Site welds are common. Is the quality the same? It can be. You will need to specify the standards and decide the type of weld you want, whether complete penetration, partial penetration, or combination.

Having said that, keep in mind that several of your welds will likely be "out of position", meaning that they will have to be welded in the vertical and overhead positions, whereas in the shop, you will generally be welding in a flat or horizontal position...much easier. Next you have to make sure the welders are qualified in all the positions they will encounter and using the welding procedure that you specify. So as not to get exotic with anything, I would specify the shielded metal arc welding (SMAW) process or flux-core arc welding (FCAW), as both of these can be done without shielding gas, which is difficult to control in the field.

The thickness you have specified is weldable without blowing through; however, you must remember that not only will you be welding in the field, but some "fit-up" may be necessary in the field as well. Field fit-up is more difficult in that you are likely to have more or inconsistent gap in the weld profile, thus making deposition of a quality weld more difficult. A good welder will have no problem with 3/16" material.

Another issue you have with the thickness is that of nondestructive testing. Your geometry is not readily amenable to radiographic testing. That leaves ultrasonic testing, magnetic particle testing (MT), and liquid penetrant testing(PT). Generally, ultrasonic testing for weld flaw detection is not done on thicknesses less than 5/16" under American Welding Society (AWS) criteria. That leaves MT and PT methods, both of which are surface methods and generally can give you little or no idea of proper penetration, fusion, or weld root details.

Make sure you review any submittals by the steel subcontractor with care. Review for weld details, pre-qualified welding procedures, weld profile, welder qualification, visual inspection procedures and nondestructive testing. Check all against AWS requirements, or its equivalent if you are not in the US.

Thermal expansion during welding is not a big issue, but may induce residual stresses. Most often these are ignored in small weldments.

My suggestion for references:

AWS D1.1 - Structural Welding Code-Steel
Metals and How to Weld Them - Lincoln Arc Welding Foundation
Other literature from Lincoln Arc Welding Foundation...most are excellent.

 

[BAretired]

The strut will be in compression. Depending on the widths of the column and strut, the 3/16" wall of the column will bend inward under load. That will affect the amount of load carried by the strut and the amount of support given to the beam.

The T-plate detail is better as it reinforces the column wall and limits local bending deformation. Welding to the column is also much easier.

I wonder if using up surplus material is a valid reason to change the detail you have drawn up.

BA

 

[rowingengineer]

Agree with Ron, but also Is the column external to the building thus that you need to hot dip galvanised the SHS post? If so, consideration needs to be given to how you are going to reinstate the protection method. “Cold galv” isn’t anywhere as good as hot dipped galv in my opinion.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[Zambo]

I don't like the term cold galvanising if used to refer to a paint. Basically this is just a suspension of zinc in an organic solvent or a zinc rich paint. It isn't galvanising at all, but may be used to touch up danage to galvanising as the galvanic protection remains for small damaged areas.

If anything cold galvanising refers to electroplating.

 

[asixth]

thanks all (Ron gets the star)

The beam was originally design to cantilever 8' however due to site constraints it had to be increased to 14' so I need to provide the strut to reduce deflections. I will look into the recommended references.