what exactly is shear lag

huangyhg

what exactly is shear lag?
i am currently working on modelling of an existing steel concrete composite bridge (rc deck atop steel ubs) and understand shear lag reduces the effective width of the sections but could do with understand what exactly it is, why exactly it occurs and when it needs to be considered if anyone knowledgable fancies helping out?
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this concept should be described in any steel text.  additionally, the design criteria are in the tension members chapter of the aisc steel spec.
essentially, it's a phenomenon affecting tension members at the connection.  if one part of a member is connected, and another is not, the portion of the gross area at and around the connected area becomes overloaded and the part that is not connected is not fully stressed.  to accomodate this, we reduce the effective area at the connection.
check out my spiffy braced frame isometric.  if you were designing the hss braces, you could take the full gross area for the tension member away from the connection.  however, since it's only connected by four welds at the gusset plate (2 on each side), the portion of the gross area near the welds is fully effective, and the portion away from the welds (facing hss walls in the isometric) are not as stressed.
shear lag is inversely proportional to the length of the connection -- i.e. if the welds in the picture are longer, there is less shear lag.
you should rarely design tension members to their full capacity, because it will require doubler plates or additional reinforcement at the connection, which is generally much higher fabrication cost than upsizing the   
i'm assuming you want to know about the effective concrete width in your composite beam.  
a flange element that is farther away from the beam takes less compressive stress than a flange element directly over the beam.  shear deformations in the slab relieve some of the compressive stress in elements further from the beam, or ribs.  we call that shear lag.  
it's accounted for by taking an "effective width" of the slab for the composite beam or t-beam.  the effective width should be calculated such that a uniform compressive stress over the effective width will yield the same compression force that is actually developed in the compression zone of the slab.