Does 2 + 2 = 2?
Let’s compare the following two concrete base connections using ¼” Red Head Tapcons®.
At first glance, it might seem intuitive that the second connection is stronger, because it has more
fasteners.
However, with concrete anchors, a big consideration while determining the strength of anchors is the
spacing and edge distance.
![](https://flengineeringllc.com/wp-content/uploads/sites/2/2022/08/anchors-1024x366.jpg)
In general, if an anchor is installed too close to another anchor, their cones of influence overlap, causing the concrete in the overlapped region to be more susceptible to failure (See Figure 3 below). Similarly, if an anchor is installed too close to the edge of concrete, it’s cone of influence is intercepted by the edge and hence it cannot develop its complete strength (See Figure 4 below).
These strength reductions are usually represented in the form of load reduction factors, which are specified by the manufacturer.
A quick calculation might suffice to compare the two connections shown above.
![](https://flengineeringllc.com/wp-content/uploads/sites/2/2022/08/tension-failure-1024x392.jpg)
Shear value of (1) ¼” Red Head Tapcons® = 1670 Lbs. | (2) Tapcons® installed at a 3” edge distance and 4” spacing | (4) Tapcons® installed at a 1.5” edge distance and 2” spacing |
Load reduction factor for spacing | 1 | 0.82 |
Load reduction factor for edge distance | 1 | 0.59 |
Strength of connection | 2 X 1670 Lbs. X 1 X 1 = 3340 Lbs. | 4 X 1670 Lbs. X 0.82 X 0.59 = 3232 Lbs. |
So, as it turns out, a (4) Tapcon® connection is weaker than a (2) Tapcon® connection in this case.
As a general rule of thumb, for a concrete screw anchor should be installed at a spacing and edge distance of 16 times it’s diameter for full capacity, and never less than 8 times it’s diameter for reduced capacity.
At Florida Engineering LLC, we specialize in metal building design, along with foundation plans and anchor layout plans. Please contact orders@fleng.com for a quote.
REFERENCES
- ACI 318-14: Building Code Requirements for Structural Concrete
- RED HEAD Tapcon® Screw Anchor – Technical Data