1. Selection of protection grade, form and stress mode of crash barrier
1. 1 Determination of protection level
Design speed of Wen Ming Avenue: 50km/h.
Road grade of Wen Ming Avenue: first-class urban trunk road.
According to document [1], a general accident or an A-class or Am-class major accident may occur on the main road of a first-class city with a design speed of 50km/h; Major accidents or minor accidents may occur in Grade A and Am; The possible secondary accidents are divided into SB and SBm. Considering the serious consequences of the car hitting the side wall of the overpass frame, the most unfavorable grade ss is adopted in this design.
1.2 Determination of guardrail form
The highest protection level of cable guardrail is Grade A, and the highest protection level of combined guardrail is Grade SAm, so these two guardrails are not considered.
Corrugated beam guardrail and metal column guardrail can meet the SS protection requirements and have beautiful forms. However, the construction cost is high, and the anti-rust paint of the guardrail is easy to fall off in the future operation, which will increase the life cycle cost of the guardrail. If the maintenance is not timely, the guardrail will be corroded and the appearance will be affected. Therefore, the construction cost is lower, the operation and maintenance cost is less, and the reinforced concrete crash barrier with beautiful appearance (Figure 1) becomes the first choice.
1.3 Determination of stress mode of guardrail
The reinforced concrete guardrail is a rigid guardrail, and its allowable deformation Z=0.
Force scope and action position: the distribution length of collision load of SS concrete guardrail is 5m, and its standard load value is q =104kn/m.
Collision load: collision load of bridge guardrail P= 104kN/m ×5m =520kN.
According to the specification, the point of impact force is located 5 cm below the top.
Preliminary draft of collision wall material: C30 concrete is adopted. The diameter of the stressed main reinforcement is HRB335 Ф18, and the structural reinforcement is R235 Ф10. Spacing of main reinforcement: 6 pieces per linear meter, with spacing of 20cm. According to the structural requirements, the spacing of structural steel bars should be ≤ 12cm.
Thickness of protective layer: according to the specification, the protective layer of reinforcement in reinforced concrete on collision surface is ≥4.0cm, and the distance from the center of main reinforcement on collision surface to the edge of concrete is a=5.0cm. Considering the backfill behind the crash barrier wall and safety requirements, the distance from the center of reinforcement on non-collision surface to the edge of concrete is a=5.0cm.
2, uplift calculation and section calculation
Calculation parameters: the design value of concrete strength under axial compression fcd =13.8mpa; The tensile design value of the stressed main reinforcement fsd = 280MPa, and the cross-sectional area of the stressed main reinforcement As=254.5mm? ; Structural importance coefficient r0 =1.1; Height coefficient ξ b = 0.56 of normal section relative compression zone; Section height h = 0.448m, section width b = 5m, number of reinforcement n = 25, effective section height h0 = h-a = 0.398m；; Bottom torque: l =110-5 =105cm.
2. 1 uplift pressure calculation:
PL=Nh0, P=520kN, L= 1.05m, h0=0.398m, then n =1371.86kn. When the number of steel bars is n=25, the stress of each steel bar is n' = n/n.
Resistance provided by reinforcement NK = fsdas = 71.26kn > 54.87kN, meet the requirements.
2.2 section calculation:
Because the concrete guardrail has variable cross-section, it is considered to calculate the stress of each cross-section according to the cross-section of a single steel bar to determine whether the proposed steel bar configuration meets the requirements.
Check the bottom of reinforced concrete guardrail.
Calculated bending moment of bottom section: m = r0pl = 600.6kn m.
Height of concrete compression zone x = (fsdas)/(fcdb) = 0.0258m ≤ ξ bh0 (= 0.223m). Therefore, it meets the requirements.
Reinforcement ratio μ = As/(H0b) = 0.32% > μmin(=0.223%). Therefore, it meets the requirements.
mu = fc dbx(h0-0.5x)= 686.04 kn·m & gt； M =(= 600.6 kNm). Therefore, it meets the requirements.
In the same way, check the section every 5cm along the height direction, which can meet the requirements.
According to the above calculation, it shows that the reinforcement of reinforced concrete crash barrier meets the requirements.
3. Concluding remarks
After the reinforced concrete crash barrier is completed, it can effectively avoid the damage caused by the direct impact of vehicles on the overpass frame. Moreover, the cushion material (such as sand) filled behind the crash barrier can also reduce the impact of car collision on the overpass frame. At the same time, it can ensure the safe operation of the overpass frame structure by cooperating with the height-limited viaduct and the sign line set on both sides of the overpass frame structure of Wen Ming Avenue.
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