What Is a Joist?

Joists, also called cover plates, are mainly found in single-story industrial plant structures. The joist is a key component of anchor support. It connects a single anchor to form an integrated bearing structure, which can significantly improve the overall effect of anchor support. There are three types of joists commonly used in coal mines: steel plate beams, reinforced beams and W-shaped steel strips.

A 4mm thick steel plate was cut into a strip with a width of about 10mm and a length of 1.0-1.8m by an arc gun.

In order to improve the stress condition of the steel belt and increase the overall rigidity and strength of the steel belt, some mining bureaus in China generally use W-shaped steel belts. W-shaped steel strips can be processed from steel plates, and can also be produced by rolling mills to produce special shaped products. Its cross-section is W-shaped, so it is called W-shaped steel strips. 6 times, the ability to control the rock mass between the poles is greatly increased, but its production is cumbersome and the cost is high.

The production of W-shaped steel belts is relatively troublesome, the distance between anchor bolts is strict, steel consumption is large, and the price is high. Therefore, many coal mines use steel bars to make reinforced beams ^[2]

There are two ways to calculate joists:

1. Regardless of the reaction of the foundation, the joist can be regarded as a continuous beam or simply supported beam supported on the pile. The calculation result is conservative.

2. Considering the reaction of the foundation, the intersection point of the pile and the joist is fixed. First, find the load acting on the beam, and calculate the bending moment and maximum shear force in the support and span according to the ordinary continuous beam or simply supported beam Then, the calculation is based on the elastic foundation beam. This method is closer to reality, but the calculation is more complicated. The calculation of joists can be generally divided into two cases according to the arrangement of the pile foundation according to the design of the continuous beam and the design of the simply supported beam suspended from the end ^[2]

1. Reasonably determine the segmented length of the retaining wall and the height of the retaining wall and joist according to the terrain and geological conditions. Mountain roads have large terrain fluctuations, and the length of the retaining wall section should not be too long. Generally, it should be controlled within 15m.

2. The determination of the wall height and section size, pile spacing, pile size, joist size, etc. of the pile foundation joist retaining wall should be comprehensively considered to achieve the principles of safety, reliability, economy and reasonableness.

3. The height of the retaining wall should meet the requirement that the joist base should not be suspended. If there is some suspension (the suspension should be controlled within 1m), it should be embedded. M7.5 mortar chipped stone can be used. The joist backfill should meet the requirements for the compactness of the fill.

4. When the backfill of the wall is fine-grained soil, the material of the back filter layer of the wall can be sand gravel, sand pebble or geosynthetics.

5. The vertical permanent combined load point of the retaining wall base should coincide with the centerline of the joist section and the centerline of the pile.

6. Joist beams can be set to continuous beams and simply supported beams according to the section length. The lower piles of continuous beams should be arranged at equal intervals, and simply supported beams are generally made of simply supported beams with hanging ends.

7. The distance from the outer edge of the joist to the outer edge of the bottom of the wall is not less than 75mm. The distance from the outer edge of the pile to the edge of the joist is not less than 75mm.

8. In addition to the design requirements of the joist main reinforcement, the minimum reinforcement ratio should also be met. The diameter of the main reinforcement should not be less than 12mm, the diameter of the stirrups should not be less than 6mm, and the thickness of the longitudinal main reinforcement protection layer should not be less than 60mm.

9. When the pile foundation uses a rectangular cross-section digging pile, the short-side dimension of the cross-section of the pile should not be less than 1.25m to facilitate construction.

10. The thickness of the protective layer of the main reinforcement of the pile foundation should not be less than 60mm, the diameter of the main reinforcement should not be less than 16mm, and the net distance should not be less than 12cm. It can be appropriately reduced in difficult cases, but it must not be less than 8cm. When using tendons, each bundle should not have more than three. When it is difficult to arrange a single row of steel bars, two or three rows can be set.

11. The pile foundation must be excavated in isolation to reduce disturbance to the soil.

12. When calculating the reinforcement of joists and pile foundations, attention should be paid to avoid the steel bars being reversed.

13. In the pile foundation joist retaining wall structure, the length of the pile top embedded in the joist is not less than 50mm. The reinforcement of the main beam deep into the joist should not be less than 35 times the diameter of the reinforcement.

14. Construction of locks and retaining walls: 1.55 meters above the entrance are locks. According to the geological situation, every 1m below is a retaining wall. After 1.55m of excavation of the pile well, the lock is immediately constructed. To ensure construction safety. During construction, the length of the retaining wall segment can be adjusted according to the actual situation ^[3] .