How should the expansion screw be designed?

First, the fixed principle of the expansion screw

The fixing of the expansion screw is to use the slanting inclination to promote the expansion to generate a frictional grip force to achieve the anchoring effect.

Second, the depth of the expansion screw

Generally, the buried depth of the expansion screw is calculated based on 4 times the diameter of the fixing bolt. Of course, the deeper the burial, the greater the tensile force and the shearing force it can withstand. However, due to factors that need to be considered in the design of the manufacturer, the material and anchoring are required. And other issues.

Third, the reference basis for the use of expansion screws

(1) Strength of concrete

(2) The strength of the fixing screw (calculated according to the material)

(three) the strength of the expansion screw (factory design)

Fourth, the strength of the expansion screw

The strength test of the expansion screw has been pressurized with a hydraulic press in the past. The maximum strength of the expansion screw is its tensile strength. The disadvantage of this test method is that it cannot detect the displacement of the screw away from the cement, that is, It is said that we cannot know that the elastic stress of the material of the expansion itself is within a few Newtons. Therefore, the new type of test instrument draws the tension and displacement in the coordinate drawing, the Y-axis is the pulling force, and the X-axis is the displacement (pictured). When the tension rises, the displacement increases until the cement breaks or expands the screw, pulling out or breaking. The highest point of this curve is the ultimate tensile force. In addition, when the tensile force rises to a certain point, if the tension is removed, the displacement can still return to the original position. This is the drop point of the material of the expansion screw itself. It is the proportional load we want in our design.

The displacement curve of the commonly used expansion screw can be divided into 5 clocks.

1. Chemical anchor bolt, SB high tensile expansion screw

2, NC type hammer type. H type. DR type

3, SH type casing type SHF type

4, nylon sleeve

5, cork

V. Adoption of safety rate

There are two general directions for safety:

(1) Ultimate Strength Method: This method is to pull the expansion into the concrete and use the breaking point as the benchmark, and then use the safety rate of 4-5 times. This law has been used abroad for decades.

(2) Proportional strength method: This method uses (1), but the key point is to find the deformation point (that is, the proportional load), so as to adopt the benchmark, and then consider the safety rate 2 times as the available strength, because it can It is more accurate and convenient for Luder's Line to know the "stress-strain" situation, but it is more difficult to use than the ultimate strength method because it wants to find the change point (proportional load). Therefore, it is generally used for research, and this method also complies with ASTME488-88.

The safety rate of the ultimate strength safety method is mainly based on the current domestic use of 4 times (according to the provisions of the building technical rules, the weight of the lifting parts is four times), but due to consideration of earthquakes and other factors, it is necessary to consider the more important projects or structures. Safety, life and other factors should be considered more than 5 times. The dynamic load condition of the dynamic load is such that the resiting force of the material is twice as large as the gradual returning external force, so the safety rate of the dynamic load is considered to be more than 8 times, if considered When the sudden force or vibration force is suddenly applied, it can be used 4-5 times according to the general safety rate, and the above-mentioned bar internal stress is limited to the proportional limit. In fact, the consideration of safety rate should be considered by the designer or engineer according to the actual needs of the design.

The safety ratio of the proportional strength method is relatively simple. Since the proportional load has been obtained, the safety load rate is generally 40%-60% of the proportional load. The general long-term load recommended by the company is 50% of the proportional intensity.

Sixth, the consideration of cement strength

In the process of grouting, due to the problem of sedimentation or effluent, the phenomenon of uneven strength of the cement itself is often caused. This is also the main reason for the on-site testing. In addition, the results tested in the laboratory should be corrected to apply. To the actual engineering location, so the correction factor is as follows:

The construction correction factor for ceiling and wall is 0.8-0.9

Ground construction, correction factor is 0.7-0.8

(1) Long-term static load The general long-term load is 1/2 of the correction value of the proportional load.

Long-term load = proportional load × K/2

(K: correction factor; 2: safety rate)

(2) Short-term static load The general short-term load is the corrected value of the proportional load.

Short-term load = proportional load × correction factor

The designer or engineer should consider the safety rate of the application.

Seven, two spacing and margin considerations

Two pitches, standard spacing, minimum margin (most edge distance)

The metal expansion screw is buried at a depth of 3.5 times or more, the buried depth is 3.5 times or more, and the buried depth is 3.5 times or more.

(The pull will be reduced)

Chemical anchor bolt 2 times 1.2 times 0.5 times (tension will be reduced)

Eight, other

The use of expansion screws often affects the quality due to other factors (such as concrete strength, construction deviation, etc.), so it is possible to obtain data at the site by instrument testing.

Related recommendation: Stainless steel expansion bolt supply topic Expansion screw supply topic

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