Screw Anchor With Sand Dowel For Dynamic Load
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Solution Overview
Problem
Existing screw anchors, particularly one-piece concrete screws and self-tapping screws, face limitations in transmitting tensile forces effectively due to material properties and design constraints, leading to high installation torque, potential cracking in concrete, and restricted use to static or quasi-static loads, making them unsuitable for dynamic load applications and larger diameters.
Innovation Solution
A screw anchor design with a nominal diameter greater than 14 mm, featuring a thread depth of at least 2 mm and a thread pitch greater than 5 times the thread depth, which utilizes a sand dowel system to achieve greater anchor forces with lower installation torque and energy expenditure, allowing for deeper penetration and larger undercut surfaces without damaging the concrete.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If one-piece concrete screws with larger nominal diameters are used, then the load-bearing capacity is improved, but the screw-in resistance increases significantly requiring robust thread design and over-drilling
Solution Approach 1:
The invention divides the anchor system into two separate components: a screw and a sand anchor. The screw has a smaller core diameter optimized for easy insertion, while the sand anchor (a cylindrical sand pack) provides the load-bearing capacity through sand compaction and dilation effects. This segmentation allows each component to be optimized independently, resolving the contradiction between load-bearing capacity and screw-in resistance.
Solution Approach 2:
The invention changes the fundamental parameter of how anchoring force is generated. Instead of relying on thread cutting into concrete (which requires large screw diameters and high insertion forces), the system uses sand material parameters - specifically the dilation and compaction characteristics of sand under compression - to generate anchoring force. This allows small-diameter screws to achieve high load-bearing capacity through the sand anchor mechanism.
2Strength
If thread depth is increased to improve anchor force, then the positive connection is strengthened, but cracks can form in the concrete starting from the bottom of the milled thread groove
Solution Approach 1:
The sand anchor acts as an intermediary material between the screw and the concrete. Instead of the screw thread directly cutting into and potentially cracking the concrete, the sand pack serves as a buffer that compacts around the screw. The sand material absorbs and distributes stresses, preventing stress concentration at the thread groove bottom that would otherwise cause concrete cracks.
Solution Approach 2:
The sand anchor is inserted into the borehole before the screw is installed, creating a cushioning layer that protects the concrete from direct mechanical stress. This pre-positioned sand pack cushions the concrete against the threading action and subsequent tensile loads, preventing crack formation at critical locations.
3Force
If the screw core diameter is significantly smaller than the borehole diameter to reduce friction, then the screw-in resistance is reduced, but the load-bearing capacity through interlocking action is decreased
Solution Approach 1:
The invention changes the mechanism of load transfer from thread-to-concrete interlocking to screw-to-sand friction and sand-to-concrete confinement. The sand anchor material properties (friction coefficient, dilation angle, compaction characteristics) become the critical parameters instead of thread geometry. This allows small core diameters to achieve high load-bearing capacity through optimized sand-screw interaction rather than screw-concrete interlocking.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design enhances the load-bearing capacity and reduces installation resistance, enabling the screw anchor to handle dynamic loads and larger diameters effectively, allowing for independent anchoring depth without over-drilling, and supports both static and dynamic loads.
Implementation Method 1
The sand anchor (2) has a circular cross-section and is made of compacted sand material
Implementation Method 2
The load-bearing behavior of concrete screws is similar to that of undercut anchors. A positive connection is established via a large contact area between the screw's thread flank and the groove in the anchor base
Data Source
Figure 1~4
AI summary
The invention relates to a screw anchor comprising a screw for use together with a sand dowel in a drill hole in concrete, natural stone, or another solid building material, wherein the outer diameter of the threaded portion of the screw is slightly smaller than the inside diameter of the drill hole, and wherein the pitch p of the screw is larger than the thread depth t of the screw by a multiple. The screw anchor according to the invention improves the utilization of the sand material kinematics of the sand dowel and allows an effort-saving assembly of screw anchors together with sand dowels.