Compund steel Beam Cover plates

[Ahmed A. Alamin]

I know that cover plates are added in steel beams to improve the bending resistance of the beams. Theoratical cutt-off points of these plates can be easily determined, but it said that actual cut-off points must exrend beyond theoratical cut-off point "so that weld on the extension can develop the load in the plate at the theoratical cut-off" i never understood this what do they mean by this, can somebody explain this please

 

[jdonville]

Say you have an I-shaped beam and you cut a circular hole through the beam from flange to flange - i.e., you have now removed a significant part of both flanges and the web at a given point. Say the hole is 6inches diameter and the beam is HP12x53 which is approximately 12in flange, 12 depth and tf = tw = ~0.5in.

You need to have the beam resist a major-axis flexural load near the capacity of the intact member at the exact point where the material was removed.

So the point at which you need your cover plate(s) to enhance the beam in flexure is the ~6 inch length of beam where the material was removed. And, in addition to the 6in length where the cover plates are working, the cover plates must extend beyond the point where the excess demand is required by the length required to transfer the portion of the load carried by the cover plates from the intact beam flanges via the welds.

So if you had 2in x 3/4in cover plates - 2 on each flange, and the design tension load in the cover plate is 50ksi x 2 x 3/4 = 75k/plate. Assume the plates are connected to the flange by 1/4" fillet welds using 70ksi electrodes (I'm not going to look up the AISC required fillet legs). The length of plate (and weld) required per each side of the 6in hole is = 75k x (2^0.5) / (2 x 0.6 x 0.75 x 70ksi) x 1 / (1/4") =~ 6.75in. Therefore the cover plates would need to be 6.75in x 2 + 6" = 19.5 inches long.

So the theoretical cutoff is the edge of the hole through the beam, but you need to extend the cover plates an additional 6.75" (minimum) on both sides of the hole to allow the weld to transfer the required load to from the flange to the cover plate and back again.

 

[KootK]

i never understood this what do they mean by this, can somebody explain this please

There was an excellent and highly detailed thread on this topic a few years ago that you can review here: Link. Below, I've reproduced the part that I consider to be my major contribution to that discussion. Certainly, if you can spare the time, I'd recommend reading the entire thread and digesting all of the opinions expressed there.

If you have any questions after reviewing the informatin below and in the linked thread, do ask. It's an interesting topic and I'd welcome the opportunity to discuss it further as I'm sure others would as well.

Try this on for size:

In these scenarios, we design our welds to satisfy two requirements:

1) We design our intermittent/continuous welds to deal with the increment of moment at each location. This is the VQ/I stuff for elastic situations and the analogous formulation for plastic situations.

2) We design our intermittent welds and termination welds such that, when taken together, they are capable of developing the entire force delivered to the reinforcing at the point of maximum moment.

Additional observations:

1) When reinforcement extends the full length of the original member, termination welds are technically not required.

2) When reinforcement extends only partial length, the VQ/I welds (set out precisely and varying) will not adequately resist the entire force delivered to the reinforcing at the point of maximum moment. Additional termination welds are required to make up the deficiency. These are the MQ/I welds or the analogous formulation for plastic situations (often conservatively taken as As x Fy).

I've attempted to summarize all this graphically below.

 

[BridgeSmith]

Just as you have a development length for reinforcing, where a certain bonded length is required to transfer the force from the rebar to the concrete, you need a certain cross sectional area of weld in order to transfer the forces in the cover plate to the flange. Since you're limited on the size of the weld that is effective (and the shear strength of the plates) for this transfer, it has to be spread over a certain length to get the required area.

Rod Smith, P.E., The artist formerly known as HotRod10