# How to determine the number of effective anchors in a base plate?

The situation is a steel baseplate that has to be anchored to concrete to resist lateral loads.

The lateral load on the base plate is great enough that more than one or two anchor rods are required. The base plate has room for 8 anchor rods.

The holes in the base plate will be somewhat over-sized per AISC to aid in setting the base plate over over the anchor rods. The anchor rods have already been cast into the concrete. The nuts on the anchor rods will not be tightened enough to consider the connections to be slip-critical. There will be some movement of the base plate required before it engages the anchor rods.

Variations in the placement of the anchor rods and the hole locations will mean that not all of the anchor rods will be engaged at the same time.

In this situation, how do you determine how many anchor rods can be considered to be effective in resisting the load? The number of anchor rods will directly effect the size of each anchor rod.

• Is "experimentally" not the answer you're looking for? Because I see your issue, depending on the exact placement of the rods, you might engage 3/8, you might engage 7/8, but I don't see how you determine that without measuring the actual configuration. Feb 19, 2015 at 3:32
• @TrevorArchibald Experimenting on all of the legs of every tower and in every direction is frowned upon by the construction workers.
– hazzey
Feb 19, 2015 at 3:39
• Is this a purely theoretical question or a design question? If its design, then I'd just assume that every anchor rod is effective - factors of safety and plastic deformation would ensure that failure wouldn't occur. Feb 20, 2015 at 13:46
• @AndyT It is a real situation. I could see justifying that all of the anchors were effective is it was some smaller number (4?), but the chances of all 8 engaging at once is slim.
– hazzey
Feb 20, 2015 at 13:54

This report (large PDF warning) on base plate design says that the AISC Steel Design Guide I instructs the use of plate washers fillet welded to the top of the base plate, with the inner diameter of the washer being much closer to the rod diameter, to ensure initial engagement of the rod. Specifically, I found that information in Sections 2.3.2 (pp.8-10), 3.3.1.2(p.21), and a visual example in Figure 3.9.

While that doesn't directly answer your question, it seems the answer is "You can't determine that, so we put in other design elements to account for it."

• That is a good reference. It seems to cover all of the concerns. I just wish that the solution was something other than welding washers.
– hazzey
Feb 19, 2015 at 22:41
• Before finding that report, I had typed up a much longer answer where I posited that you might deform a couple bars, but in the course of that deformation, you'll engage the unengaged bars, and theoretically end up stabilizing the system. But I have no clue how safe that is. Feb 19, 2015 at 22:43
• I just learned of one other option to address this concern: hilti.ie/medias/sys_master/documents/h85/9111290937374/… Jun 27, 2015 at 15:51