huangyhg

welding vs bolting on existing structure during upgrade
i would like to seek opinion of the forum members on
welded vs bolted structural steel connections on existing steel structures.
the questions which come to the mind are:-
1)whether it is good practice to weld on to a hot dip galvanised existing steel structure,considering that during hot dip galvanisation,changes to microstructure of steel
penetrates well below the surface.
2)which is easier and takes lesser time, drilling bolt holes on a existing steel structure or welding ?
3)pipes spools with wall thickness upto 75mm are routinely
welded on site, why then are we reluctant to weld
structural steel sections on site ?
rgds

1. it seems to me that welding should be done before galvanizing wherein possible.
2. welding generally would take less time, assuming a nominal thickness of steel plates to drill through and the number of holes to drill.
3. in general, it is widely acknowledged that field welding isn't something that should be attempted without necessary precautions to prepare the field area to a level of that with the shop. preheat is important as is the skills of the welder. most shop welders weld day in and day out. mosts field welders weld on an infrequent basis. add to this the lack of inspection that may be required for special welds and bad practice along with fatigue related problems and you've got potential problems.
regards,
qshake
eng-tips forums:real solutions for real problems really quick.
galvanized steel can be successfully welded, however it is very "unpleasant" to do so. the (molten) zinc coating tends to "pop" and "spit" during the process. most welders don't like to do this.
i have to respectfully disagree with qshake on the limited usefulness of field welding. in certain applications, such as splicing very long (driven) steel h-pile, field welding is the norm. this is performed with the (partially driven) h-pile in place; field conditions can be "atrocious". however, when a skilled welder performs the work, there are rarely any problems.
as a gc who has done a significant amount of steel seismic retrofit work i always ask the engineer to consider bolted connections in lieu of welded if the existing steel is galvanized. it takes a considerable amount of time to properly prep the joint for a weld if it has been galvanized and it never really gets clean. it is faster to drill and bolt than it is to properly prep galvanized steel for a weld.
another aspect to consider is the qc. it is much easier (in my opinion) and economical to perform qc on bolted connections. once everyone is on board with the ro-caps, skidmore tension proofing, torque, and "turn-of-the-nut" the process can move along quite nicely. it is also easier to get craft workers who can bolt than it is to get someone who can properly weld galvanized steel.
came across some detailed information on your question

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