Repair Strap Inter-rivet Buckling Calculation 6

[CRACKKILZ]

I am familiar with several methods for calculating the inter-rivet buckling stress (Fir) of a repair strap in compression. I would like to know what other engineers use as an "effective" strap area to calculate the actual inter-rivet buckling load of the strap? In the past I have been told to use 2(fast. diam) x thickness. Others have suggested an effective width of 30t with a corresponding area of 30t x thickness. Any recommendations would be appreciated.

 

[SuperStress]

Since most of the inter-rivet buckling methods I'm aware of are independent of sheet width, I'm assuming you need an effective width in order to calculate your internal stress in built-up structure.

While I've seen plenty of analysis that uses 30t, Flabel presents a less arbitrary method in his book "Practical Stress Analysis for Design Engineers" starting on p.366. His method is based primarily on edge conditions and compressive modulus (for elastic buckling).

Post again if you don't have access to this book and I'll try to summarize his method for you.

SuperStress

 

[CRACKKILZ]

SuperStress thank you for your response. I do in fact have the Flabel book you mentioned, it is one of my best resources.

As you suggested I looked at page 366, and that is not exactly what I am looking for.

To better articulate my original question perhaps an example is in order. In Flabel (page 421) there is a graphic showing a frame flange repair with a strap. In the example Flabel calculates the crippling capability of the strap and also the inter-rivet buckling capability of the strap.

My question is this: Once you calculate FIR, what area do you use to calculate the inter-rivet bucking load PIR of the strap??? The reason I ask is that intuitively I would expect the strap to buckle directly in between the rivets with the stress distribution being highest at that point and then going to zero at the free edges. At some point, increasing the strap width will not significantly increase the strap inter-rivet bucking capability.

Alex

 

[SuperStress]

I see what you're getting at, and I've seen this done a few different ways.

In this kind of situation I would normally draw a 80 or 90 degree cone (depends on what the company standard is...) originating at each fastener and growing toward the other. When the triangles intersect, you will have a diamond pattern. The height of the diamond is the effective width.

I will try to illustrate (crudely) below:

-------------------------
/|\ / \
| / \
| / \
| / \
Eff. width + +
| \ /
| \ /
| \ /
\|/ \ /
-------------------------

Anyway, this cone is MUCH too wide, but hopefully you get the idea. The angle you use to draw this cone will vary depending on who you work for and how conservative you need your analysis to be.

I think most well-designed repair straps will load up completely. Note that I haven't seen very many flat straps used in repairs like the one that Flabel shows. Almost always an angle is used in order to prevent inter-rivet buckling. You take a big hit on your compression allowable for having two edges free.

Hope that helps.

SuperStress

 

[hombre]

Just thought I would get my two cents worth in... we use flat fail safe straps on the upper chord of many different types of beams to carry the compression loads. I do not believe crippling applies in this case. Therefore, the allowable is the lower of Fcy or Fir.

Hombre

 

[CRACKKILZ]

SuperStress, thanks again, the method you suggest makes sense. I only employ repair straps for minor damages on the flanges of beams. If the damage removal involves a complete trimout (generally more than 50% material loss), or if the damage gets into the radius, an angle is the way to go.

As you mentioned, angles have greater compressive strength than straps. In addition, angles can restore the shear load path between the cap and the web.

Alex