We are working on some equipment that is going to be attached to existing, 3,000 psi reinforced concrete floor slabs. In some locations, the best way to do this is to through-bolt and 'sandwich' the slab with a piece of threaded rod (5/8" rod in our case.) On the top side of the slab, our equipment has a large steel bearing surface. On the bottom side, we expect we'll need steel plate washers. We were thinking of using square washers cut from 1/2" A36 steel. What I don't know is how big of a square would be 'standard.'

Is there a chart or standard practice for what size plate washer to use on what size fastener? I couldn't find one in AISC, but may have missed it, or maybe it's in an ACI standard? If there's no prescriptive chart, what checks would typically be done to size it by calculation?


There are some standard square washer sizes. These may not specifically be for your configuration, but they may help to decide on a size to start with.

  • ASTM F 436 Square Washers are found in the AISC Steel Manual on the table that shows other bolt and washer diameters. These are specifically for beveled washers and the sizes given are only minimums. You can see the sizes here. For a 5/8" rod, the washers are 1.75" square.

  • Square washers are also used in timber construction. You can find some typical sizes here. These washers are 2.5" square for 5/8" rod.

  • Custom washers might be the way to go. Since none of the washer types listed above are specifically for bolting to concrete, a custom size might be required. The references above give a starting point, but the final size might be controlled by the allowable bearing strength on concrete.

Making a square washer is a simple machining task, so going the custom route may not add a lot of cost.

  • $\begingroup$ Thanks! Yes, I was assuming we would make them custom, although the ones for wood may well work off the shelf. Is checking the bearing strength on concrete as simple as 6.25 sq in * 3000 PSI = 18,750 lbs? (I know that's probably a naive question. I don't deal with concrete very often, and I'm just roughing in details that a PE is going to evaluate before anything is actually built.) $\endgroup$
    – Ethan48
    Mar 6 '15 at 15:22
  • 1
    $\begingroup$ @Ethan48 The allowable bearing strength on concrete is usually more like 25% of the concrete compressive strength, so in your case use 750psi. That conservative though. $\endgroup$
    – hazzey
    Mar 6 '15 at 15:25

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