BASE PLATE WELD

[sedesigner06]

I have a moment frame building and the column is taking a pretty healthy moment around 600 k-ft. To resist this large of a moment with a 20" pipe i have to use 1 3/4" weld to the base plate. This weld seems way to large to be excecuted in the field but i dont know of another solution.

Any suggestions?? Your help is much appreciated

 

[slickdeals]

Although I haven't done it personally, could you use stiffeners welded to the pipe to "spread" the load out? Seems like that would help with keeping your base plate thickness down as well.

 

[AELLC]

Why is the weld a field weld, normally it is shop weld, and secondly, why doesn't a full penetration weld work? Use a inner pipe stub as the back-up.

 

[sedesigner06]

AELLC,

You are right i spoke wrong this would be shop welded but steel seems very large. Me being green in this area would a full penetration weld theoretically require half of the thickness?? Sorry for my confusion. Thanks

 

[civeng]

I agree with AELLC. Full pen welding will give you the same moment capacity in your weld as in your tube column. If your tube column is strong enough, then the full pen weld should be also.

 

[paddingtongreen]

sedesigner06, the weld the others have mentioned is a full weld where all of the preparation is done on the outside of the pipe and a backing strip is placed all of the way around the inside to contain the weld. It is used where you cannot get to the backside to chip finish the weld..

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[JLNJ]

Wow. Something seems not right. Wouldn't the pipe fail right above the weld?

What's the pipe wall thickness?

 

[sedesigner06]

JLNJ,

That was my orginal thought but i have went through how the moment was developed three times and get the same answer. Seems large...

The pipe is very large itself 20" diameter 1 1/4" thickness

 

[JLNJ]

I would be wary of a fillet weld this large.

The tricky part will not be the weld, but the design of the baseplate and its anchorage to the foundation.

As far as the modeling goes, you might look at the footing rotation to see if your moment will actually be developed. This will hurt your structure's deflection but might help your structure's stresses. And it should be a more reasistic model.

 

[sedesigner06]

The column actually sits on a girder beam that the baseplate is going to sit on, and is supported on a concrete wall that is inset 7" (A reason for such the large moments) . Very odd layout but its what the exsisting conditions are and we have to make it work.

And yes i am wary of a fillet that large ITS HUGE!!!

Thanks for your input

 

[racookpe1978]

I've had to weld them that large - both above the plate and below the plate - but it takes a while and you need good preheat.

1-1/4 thick "pipe" wall?

Try to change the connection to spread the laod out, and try to limit the fillet to 1/2 inch.

(Euro design (thickness across the throat) or US standard (length of the fillet leg)?)

 

[Galambos]

Using the effective weld size, described in AISC Design Guide 24 pg.8, we can match the weld size to the strength of the base metal.

Using LRFD:

Shear Yield Deff = 30.2 * Fy/Fexx * t
Shear Rupture Deff = 22.6 * Fu/Fexx * t

For A500 Gr B material Fy=46, Fu= 58 and assuming Fexx=70 ksi, a 1 7/16" fillet weld would develop the plate. To me at least, your calc doesnt seem too far off if you are stretching the capacity of the section.

 

[sedesigner06]

racookpe,

The weld is a weld that is all around the pipe so the only variable i can design to is the weld thickness, and im using US standard design.

Galambos,

Thank you for the input and im pretty confident in my number using this process just was trying to think so a solution to avoid the big of a weld.

 

[azcats]

Wouldn't CJP weld would greatly reduce the amount of weld material that needs to be laid?

A 1.75" fillet has a cross sectional area of 1.53 sq-in.

If you used a CJP with a 3/8" root and prepped the pipe with a 30° bevel it would only require 1.01 sq-in of weld. Although you'd need to back that up with some size a fillet.

Or you could meet in the middle with a PJP weld reinforced with a fillet.

 

[sedesigner06]

Im thinking a CJP with some sort of backing is the best direction to proceed as AELLC sugguested.