Reinforcing Steel Beams

[steve111]

Yesterday we reviewed an existing structure that had a series of steel beams running along a theater addition. The original beams were reinforced for the increased snow load due to drift accumlation re the higher roof of the theater addition.
The original beams were W18x55's. The reinforcing consisted of welding 2" thick x 3" wide plate/bar steel sections to either side of the webs of the underside of the top flanges and on top of the bottom flanges again either side of the web. The connection of the steel plate/bar to the original beam was by intermitten fillet welds at the junction of the web/plate and flange/plate for each.
Anybody familiar with this or have comments otherwise?

 

[RG88]

I haven't seen it exactly like this, but I would imagine if the original stiffening repair could not be made to the top of the top flange or the bottom of the bottom flange (due to aesthetics, physical constraint, et al), the only way to add more section is just inside the flanges. I can see the need to add 'section' in a place as far from the centroid as possible to increase bending strength, but the size of the plate is quite large in comparison the thickness of the base metal.

Are you questioning the shear capacity of the welds or the fact that the plate is substantially thicker than the base metal?

It is quite possible that the plate was all that was on hand at the time. I have been asked to develop somewhat similiar repairs with surplus channels, angles, plate stock etc.

 

[steve111]

RG88
yes suspect this was the most simple way to add 'Section' due to existing physical constraints.
yes questioning first off the 'massive' reinforcing, seeing it is visually stunning, but from an analysis point of view yes questioning the ability of the welds to transfer shear and the overall capacity of the reinforced 'built up' section.
yes is possible that the only plate available was this size, at the same time to be adding such a large amount of section, if this was required, and then to connect it all with what appears to be at best single pass 1/4" intermitten fillet welds has me a bit stunned.
in any case we'll start crunching away here and see where we get to. Thanks for reply, your quieries (Base metal and shear) are my concerns as well.

 

[chichuck]

steve,

how did they get those big 2" thick bars, on the inside of each flange to butt up against the web, given the fillets at the junction between web and flange? On a W18X55, that is a fillet with R= 0.40". Did they bevel one corner of the 2" X 3" bar to make it fit? Because if they don't do something like that, I can't see how they could make the welds between web and reinforcing plate.

chichuck

 

[Watermelon]

Chichuck:

It may not be a 3" x 2" thick bar, but rather an angle 3" x 2".

Steve111 are you sure it's a solid bar?

 

[steve111]

Yes a solid bar and they bevelled the inside corner.

 

[ChipB]

You've got my curiousity...

What's the maximum shear force?

 

[m8589]

What about vibration analysis ? Long span....

 

[Taro]

Vibration checks are usually not applicable to roof framing.

 

[steve111]

sorry been abit busy, the beam in question is part of a gerber grirder cantilever/drop in span system, somewhat, so to speak, in any case it has a couple of support conditions depending upon location in building, either a simple beam, or a continuous beam pinned connected at one end and cantilevering over a column recieving a drop in span beam. 2.286 m cantilever, 12.192m between column supports, simple enough. in its worst case the factored shear over the column is 315 kn (70.9 kips) and a factored moment over the column of -629 kn-m (-464.2 kip-ft) and factored moment of +616 kn-m (454.6 kip-ft) +/- cl. span, Note all FACTORED. The beam is actually a W460x74 or W18x50.

 

[MrStohler]

I have seen reinforcing similar to this many times before (relatively large or thick reinforcing plates welded to a beam).

Generally the engineer who designed the reinforcing either knew or assumed that the existing beam was stressed near its maximum allowable stress AND the stress could not easily be removed prior to reinforcing (significant dead loads could lead to this). In order to prevent the beam from being overstressed with the new loading, a significant ammount of material would need to be added to ensured that the added stress is small in value and that the total stress does not exceed the allowable stress for the original beam. Note that this assumes the design of the reinforcing material is based on an ASD method.

Having said this this is a very conservative method of analysis. It does not seem to fit the loading mentioned in the original post (one would think that the reinforcing could be added when there is little load on the roof or the beam could have been jacked to releive stress prior to reinforcing. Also it does not account for the tremendous increase in strength of the section that an LRFD analysis would.