General Design Considerations

Notice: Unless otherwise noted, all load and design values presented herein are based on the provisions of ACI 349-97 or earlier. Design guidance based on ACI 318-05 and ACI 349-06 is being prepared and will appear here shortly. In the mean time you may wish to review the results of cracked concrete testing performed in accordance with ACI 355.2 as called for in the new codes.

The Maxi-Bolt Enables a Non-Slip, Ductile Design

The undercut geometry of the Maxi-Bolt's anchor head is inherently non-slip and the anchoring mechanism is sized so that the full steel capacity of the anchor stud is reliably achieved in 3,000 psi design strength concrete. While the anchoring mechanism provides these capabilities, it is up to the engineering professional to provide sufficient embedment, edge distance and anchor spacing to match the strength of the anchor's stud to the pullout strength of the concrete. Modern codes provide an incentive to do ductile anchor design because the load at which fracture of the steel occurs is very predictable and because plastic deformation of the steel between its elastic and ultimate strength limits is energy absorbing. By contrast, brittle designs, which are limited by concrete pullout strength, are less predictable in terms of load capacity and occur suddenly and without the energy absorbtion provided by steel failure. As a result these designs require higher safety margins. Regardless of the type of design, ductile or brittle, the Maxi-Bolt does not slip under load with resultant loss of embedment.

The Maxi-Bolt May Be Used in Vibratory and Seismic Environments

As has been shown by extensive testing, the non-slip characteristic of the Maxi-Bolt remains valid under vibratory and seismic load conditions. Contact Drillco Tech Support for detailed test information.

Safety Margins

The safety factor will be different depending on whether or not the design is a ductile one. Non-ductile design will usually have a safety factor of 4 (3 in commercial use), between the ultimate strength design load and calculated concrete pullout capacity. Ductile designs will have a smaller safety factor because of the increased confidence in a steel failure. With a ductile design there are two levels of safety factors to consider. The first level is to ensure steel failure over concrete failure. Usually a phi factor of .65 (occasionally .85) is multiplied by the calculated concrete pullout strength. This translates into a safety factor of 1.5. (1/.65) between bolt ultimate capacity, as determined by the actual steel area times the specified ultimate capacity of the steel, and calculated concrete pullout capacity. Once this criteria has been established, the steel design capacity, ultimate strength design method,  is determined as a function of steel area, specified steel yield limit and a phi factor of 0.81 or 0.9 depending on the code. Using a phi of 0.81 on the steel's yield strength, fy, the factor of safety between the ultimate strength design load and specified steel ultimate capacity, fut, may be computed as fut / (0.81 * fy). The ASTM A 193 B7 material typically used with the Maxi-Bolt has an fy of 105 ksi and an fut of 125 ksi with a resulting factor of safety of 1.47.

Concrete

Pullout Capacity

Concrete strength is usually determined based the pullout capacity of a 45 degree cone originating at the anchor head. This strength is a function of the lateral surface area of the cone and the tensile strength of the concrete which is usually taken as 2.8 times the square root of the concrete's design compressive strength. To simplify calculations, the lateral surface area of the cone may be projected onto the concrete surface where it becomes a circle with radius equal to anchor embedment. The concrete tensile capacity is adjusted accordingly (2.8 / sin(45 degrees)) to 4 times the square root of concrete compressive strength. Concrete strength, then, is dependent upon the concrete's design compressive strength (psi), anchor embedment (the radii of the projected circles), anchor center to center spacing, and edge distance. Closely spaced anchors, spacing less than 2 times anchor embedment, will limit concrete capacity and may be accounted for by increasing anchor embedment. Although any reinforcing in the concrete may also increase its strength, the added strength is usually neglected yielding a conservative result.

To aid in the calculation of projected concrete area, a java based Calculator is provided.

Edge Distance

Edge distance is an important factor affecting both anchor tensile and shear capacity. If your design cannot meet code requirements for edge distance we suggest you look at testing that Drillco has performed to estimate your capacity. Contact Drillco Tech Support for this information.

Depth of Concrete - Punching Shear

Depth of concrete is also a concern. Some anchor patterns in thin elements may be limited by punching shear. A good rule of thumb is not to embed an anchor any deeper than 2/3 the depth of the slab.

Shear - Tension Interaction

Most codes now use straight line interaction for combinations of tension and shear. Drillco also has testing available that may allow you to use an elliptical interaction with an exponent of 4/3. Contact Drillco Tech Support.

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