Introduction
Prying action is defined as a phenomenon (in bolted construction, for connections with tensile bolt forces) whereby the deformation of a connecting element under a tensile force increases the tensile force in the bolt above that due to the direct tensile force alone.
Proper Design for Prying Action
To properly design for prying action, a bolt diameter and a fitting thickness (t) must be selected such that there is sufficient stiffness and strength in the connecting element and strength in the bolt.
Factors to Consider when Designing for Prying Action
Designing bolts and fittings for prying action requires a detailed understanding of several key factors to ensure safety, efficiency, and durability:
Lever Arm Length: The distance from the load application point to the bolt (lever arm) influences the prying force. Longer lever arms generally increase the prying effect.
Strength of Materials: The materials used in both the bolt and the connected components must withstand the amplified forces without yielding or failing.
Use of Stiffeners or Reinforcements: Stiffeners can be added to reduce prying action by transferring loads more directly and reducing leverage effects.
Geometry of Connection
Connection Configuration: The arrangement of the plates, flanges, and bolts affects prying. Connections with large overhangs or cantilevers are more susceptible to prying.
Edge Distance: The distance from the bolt to the edge of the connected component should be sufficient to prevent tearing or failure under prying action.
Practical Applications in Design for Prying Action
In practice, it can save engineers time to simply aim to design the fitting thickness (t) to be large enough such that the effects of prying action may be neglected and the bolt can be designed for the direct tensile force alone. The geometry of the connection may also be altered to give more desirable values for the dimensions b and p, thus reducing the required thickness tₙₚ to neglect the effects of prying action. This can be accomplished by rearranging the bolt configuration or adding stiffeners.
Important Details to Be Aware of in Prying Action
Note that the dimension b is not exactly the same for tee profiles and angle profiles.
For tee profile fittings, b is taken to be the distance from the center of the bolt to the face of the tee stem, or web.
However, for angle profile fittings, b is taken to be the distance from the center of the bolt to the centerline of the angle leg.
Below are two images that illustrate the dimensional inputs and equations for determining the necessary fitting thickness (t) to mitigate prying action in tee and angled profiles.
Example Problem (Solutions Provided Using CalcBook): Problem Statement: Consider an L3x3x1/4 angle with bolts 5/8"Ø spaced at 6" on-center, 1.5" from the face of the vertical angle leg. The LRFD factored tensile load on the bolt is 2.5 kips. Determine if the thickness of the angle is sufficient to neglect the effects of prying action.
Step 1: Design Inputs
Step 2: Determine Geometric Parameters
Step 3: Perform Prying Action Check
Because the factored LRFD load of 2.5kips is less than the maximum force required to ignore prying action, the bolts can simply be designed to resist the direct tensile force alone.
Conclusion:
Addressing prying action in bolted connections is vital to ensuring structural integrity. By carefully considering factors such as lever arm length, material strength, and connection geometry, engineers can effectively design fittings that withstand amplified forces due to prying. Optimizing fitting thickness and adjusting bolt configurations can simplify the design process and allow for the effects of prying to be minimized or even neglected, focusing instead on direct tensile forces. These strategies not only enhance safety and performance but also contribute to the longevity and reliability of structural designs.