huangyhg

increasing shear capacity of a w beam
folks,
how does one increase the shear capacity of a wf member for increased loads? say i have a w24x68 and i need additional shear capacity due to increased loads, how do i strengthen the web for the additional shear?
it seems like adding a web plate would help, but i can't seems to visualize the load path through this web plate. can anyone shed some light into this?
are there alternate ways to increase the shear capacity of a wide flange beam?
many thanks
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not for a wide flange. if you had a slender web section, you could add web stiffeners to change the aspect ratio of the web panel for buckling.
but all wide flanges have compact webs; this means you can't get any additional capacity from this method.
others will correct me if i'm wrong, but as far as i know, the only way to get additional capacity in your case is to add a web doubler plate.
@frv,
would a web doubler plate would be the same as a plate attached to the web or will it not get attached to the web but be parallel to the web on each side running flange to flange?
doubler plates run from flange to flange. getting the shear forces out of the beam and into the supporting element may be difficult unless the beam is in some sort of bearing condition at each end.
@steelpe,
assuming that i have a uniformly loaded beam and due to increased loads, i need more shear capacity at the the ends (for arguments sake, say 1/8th span length at each end)
how does one reinforce the section for shear closer to the ends. i assume that if these doubler plates are used then these doubler plates will need to be connected to the connecting
a lot of welding, but the doubler plate is the most direct solution, either add on to one side, or both sides of the web. the minimum thickness of the plate shall be 1/4" to avoid excessive distorsion during welding. also, this is my personal opinion, the plate shall bear against the flanges, maybe through a full-pen weld, to share the bearing to allow uninterrupt shear flow path. the inner edge of the plate can be bevelled, or the plate can be placed a distance from the fillet (2 sides strengthening preferred for this case to form a box) to avoid interference.
i call the above solution a no brainer, quick and simple (from design point of view). if the additional shear is relatively small, you may want to explore other solutions, that are potentially cheaper, by fool around with equation vq/ib - when increase in q is less than increases of i, b, the combined shape will gain shear strength.
i have never run into the situation you are trying to deal with. i have only designed doubler plates to reinforce the webs on columns in moment connections. in these types of connections the plates are "full pen." welded to the flange. this requires the plate to be set a slight distance off of the web of the column (your case beam) to avoid the radius at the flange/web interface. this would make connection at the supporting element very difficult.
reinforcing only part of the beam may be possible after all it's no different than only reinforcing at moment connections only. depending on your loading condition you should make sure you don't overstress the existing web. unless you plan on relieving the load from the beam during construction by shoring the 100% of the dead load will be in the existing web and the live load would be shared between the web and the doubler plate.
how many cases do you have?

i am actually helping a colleague of mine in figuring out the best way to increase shear capacity of the beam.
the story goes like this: the job was designed 2 years back, but is going into construction now. a tenant (walgreens) requires a higher clear story height than what is provided. we are having to cut down w33's to w24's. moment capacity is achieved using cover plates, but shear is becoming an issue.
hence, there are quite a few cases where the beams will need additional shear capacity.
after looking into the design of doubler plates for moment connections in aisc 341-05 you can have the doubler plates "fitted" between the radiuses. however some of their details show welding within the "k" distance, which i was always told to avoid.
you can also attach the doubler plates with fillet welds.
from working with fabricators, cost of these things becomes a problem. not knowing the stage of construction, it may be cheaper to replace the beams with a section that will work for both shear and bending.

one easier solution, if there is no obstruction, you can weld plates to both side of beam from flange to flange to form a box using fillet weld. for static load, stitch weld should works.
hmmmm.....i'm thinking mechanical may have gotten involved a little late.
there are so many ways to skin this cat. ask two engineers the same question, get three different answers.
most cost effective is steepe's replacing of the beams. if you're already taking about adding cover plates to the beam, you may be able to get by with a w27, and talk the client into it.
if the steel has already been purchased based on w24x68 beams, in my opinion, a variation of kslee100 would be best:
i'd run the deepest channel i could on both sides of the w24 web. these could be bolted (or welded) to the web based on your shear transfer. your beam is going to be 50ksi steel, your channels a36. you could analyze it as a combined section, but, given the different grades of steel, i'd do them individually. share the load between the beam and channels based on your moment of inertias. the beam would take i(beam)/(i(beam)+2i(channel)) percent of the load, with the remaining load divided equally between the channels.
this would still allow any kickers coming from the bar joists, to still attach to the beam. this may eliminate your need for cover-plating. don't forget about your end reactions. probably would need to full pen weld the channel to the clip angle and increase the thickness of angle.
hope this helps

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