Long Span Gate
Article 51. Application
" The following provisions are applicable to a long span gate.
Description:
Each provision herein specifies matters to be specifically considered in the designing of a shell type gate that has a long span.
In addition to those items stated herein, refer to each paragraph con-t cerned in 1. Fixed Wheel Gate, Section 3 of this Chapter for the auxiliary girder, the side girder, the main wheel, the guide roller, the gate guide, and the anchorage and operating loads.
These provisions should also be applied to a long span gate other than a shell type.
Article 52. Shape of the Gate Leaf
In cases such as overflowing, underflow discharging, or regulating the flow while submerged, the gate Idaf shall be of a shape which satisfies the purpose of its use and which does not produce an excessive uplift or down-pull force.
Description:
The shape of a long span gate shall be such that the overflow part is of a shape that does not produce a negative pressure.-It is desirable for the bottom shape of the gate leaf to incline at more than 20° to the horizontal plane when considering the case of bottom discharge (See Fig. 2.52-1)
Fig. 2.52-1 Bottom Shape
A water vent on the bottom plate and an air vent around the overflow part should be provided so as to minimize the effects of buoyancy and water weight inside on the operating load when the leaf is raised and lowered.
When the overflow covers the entire width of the span in a long span gate, voids trapped downstream by the overflowing water are generated and the pressure decreases as the air inside the voids are pulled downward by the water. Thus vibrations of the overflowing water arg generated. In order to avoid these vibrations, measures such as providing deflectors illustrated in Fig. 2.16-1, Article 16. of this chapter, providing spoilers suitably spaced downstream of the overflow plate or branching the overflow water by offsetting the downstream edges of the overflow plate are required to introduce air and also installing air vents at the back of the leaf is required.
It should be noted that long time discharge with a small opening is not recommended as vibrations may be induced for a long span gate.
When the gate regulates the How while it is submerged, it is desirable to determine the shape so as not to produce excessive uplift and downpull forces by running hydraulic tests. General trends are as follows:
Maximum down-pull force is produced at about half the depth of the design overflow water and the down-pull force decreases as the overflow depth increases.
The down-pull force decreases as the lip moves to the low pressure side.
As the lip height increases, the flow along the bottom plate is regulated and the pressure working of the bottom plate increases and consequently, the down-pull force decreases,
Down-pull force can be reduced by shaping the bottom plate so as not to separate the stream lines.
No effect by the water vent is reflected in the down-pull force as long as the opening position is relatively small, but as it gets larger the water level inside becomes lower and the water weight inside decreases. As this happens, down-pull force also decreases. The effect becomes larger as the water vent is installed closer to the rapid flow.
Fig. 2.52-1 Bottom Shape
A water vent on the bottom plate and an air vent around the overflow part should be provided so as to minimize the effects of buoyancy and water weight inside on the operating load when the leaf is raised and lowered.
When the overflow covers the entire width of the span in a long span gate, voids trapped downstream by the overflowing water are generated and the pressure decreases as the air inside the voids are pulled downward by the water. Thus vibrations of the overflowing water ar? generated. In order to avoid these vibrations, measures such as providing deflectors illustrated in Fig. 2.16-1, Article 16. of this chapter, providing spoilers suitably spaced downstream of the overflow plate or branching the overflow water by offsetting the downstream edges of the overflow plate are required to introduce air and also installing air vents at the back of the leaf is required.
It should be noted that long time discharge with a small opening is not recommended as vibrations may be induced for a long span gate.
When the gate regulates the How while it is submerged, it is desirable to determine the shape so as not to produce excessive uplift and downpull forces by running hydraulic tests. General trends are as follows:
Maximum down-pull force is produced at about half the depth of the design overflow water and the down-pull force decreases as the overflow depth increases.
The down-pull force decreases as the lip moves to the low pressure side.
As the lip height increases, the flow along the bottom plate is regulated and the pressure working of the bottom plate increases and consequently, the down-pull force decreases,
Down-pull force can be reduced by shaping the bottom plate so as not to separate The stream lines.
No effect by the water vent is reflected in the down-pull force as long as the opening position is relatively small, but as it gets larger the water level inside becomes lower and the water weight inside decreases. As this happens, down-pull force also decreases. The effect becomes larger as the water vent is installed closer to the rapid flow.
Article 53. Considerations for Gate Leaf Structure
A gate leaf shall have the required strength and rigidity in due consideration of the loads on its vertical direction and its distortion.
Description:
The loads working on a gate leaf include uplift and down-pull force in addition to horizontal force and as the effects of the above increase attention should be paid to.the stress by the vertical force and the distortion.
When the gate height is large as compared with the span, shell plates do not work effectively in some cases and thus stress and distortion should be checked on the basis of the clear width.
When a major axis of the gate leaf cross section is extremely inclined, attention should be paid to the increasing stress of the corners.
Article 54. Considerations for Gate Leaf Deformation
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When considering gate leaf deformation, the shape of the gate guides and the clearance between the leaf and the guides shall be decided by taking into account the horizontal and vertical deformation of the gate leaf by hydraulic pressure and by its own weight, and the deformation due to temperature changes and direct sunshine, etc.
Description:
Deflection of the gate leaf is specified in Article 22. of this chapter, but its maximum limit may be increased to 1/600 of the span provided that appropriate consideration is given to the stability of the watertight structure and vibration of a long span gate leaf.
It should be noted that a seal plate at the sill beam should be wide enough for a big deflection of the leaf.
Rubber seals and guide rollers should be arranged so as not to interfere with expansion of the gate leaf because of its large expansion due to temperature changes.
For a hydraulic gate whose height is small compared with its span and whose downstream water level is low or non-existent, :it is possible for ; the gate leaf to tend to warp and a leak may occur from the bottom center of the leaf due to the deflection of the leaf under direct sunshine. Therefore, it is desirable either to consider the shape of the bottom rubber seal, or to spray water on the upper surface of the gate leaf or to provide other means of alleviating this problem.
Article 55. Considerations for Gate Leaf Operations
The following shall be observed when the gate leaf is operating:
The gate leaf shall not be inclined sideways or fore and aft.
The gate leaf shall surely be operated at the required speed. :
Description:
Take extreirie care so as not to produce any inclination fore and aft or sideways particularly.for a long span gate. In order to avoid inclination of the gate leaf either forward or backward, a hanging point should be positioned close to the center of gravity line and guide rollers should be provided to prevent the fore and aft inclination even if there is no load.
Inclination to the left and right may be generated by elongation of the wireropes, or by the difference in speed when one gate leaf is hoisted up by two hoisting devices. Some measures should be provided to correct the inclination caused by the wirerope elongation such as the use of a wirerope adjusting device. In order to correct the inclination caused by the above mentioned difference in speed, an inclination adjusting device is normally provided to electrically correct left and right inclination.
When a hoisting device is placed on one pier and the wirerope on the other pier is wound through an access bridge or a gate leaf, it should be . noted that a horizontal force works on the piers or on the gate leaf.
In calculating the operating load of the gate leaf, the load is greatly influenced by the leaf shape and therefore it is necessary to bear in mind the matters in Article 52. of this chapter.