Design of Rainwater Drainage System in Steel Workshop

The steel workshop mostly adopts the steel roof with purlin and steel beam below. Although the roof and gutter of the steel structure workshop are provided with roofing eaves at the joint, there are still overlapping seams. The roof and the gutter are generally not covered by waterproof coiled materials, thus rainwater will flood into the workshop from the overlapping seams when water height inside the gutter exceeds the lap joint.

Picture1- gutter installation display

This article will introduce the various forms of rainwater drainage systems for steel workshop buildings, and discuss the characteristics, application conditions and technical requirements of these rainwater drainage systems according to the actual conditions of the frame, roof and gutter’s structural performance, climate, and production process requirements. And finally will discuss some problems that should be paid attention to in the rainwater drainage system of steel workshop design.

The gutter’s depth of the steel workshop is limited by the height of roof purlin, which is generally about 200mm, and the effective depth is only around 150mm, so the gutter is very shallow. In addition, the roof slope of the steel factory is very large, and the rainwater flowing rapidly down from the roof and reaches the gutter to generate an impact surge, which causes the water level in the gutter to fluctuate greatly, so the actual effective depth of the gutter is shallower. Therefore, the waterproof capacity of the steel factory's roof and gutter is very weak. It is very prone to overflow once the rainwater system is poorly drained or the rainfall exceeds the recurrence period.

The roof rainwater drainage system of steel workshop can be divided into two types: external drainage system and internal drainage system. The former uses the roof gutter to drain the rainwater directly to outdoor storm sewer or road through the outdoor pipe. The later drainage uses the indoor rainwater pipe to discharge the rainwater to outdoor storm sewer.

The side span gutter of steel factory building with double-slope roof and other roof forms could use closed system directly to drain rainwater. The drainage effect is pretty good. Generally the overflow will not occur as long as the calculation is reasonable. There are many options for the internal gutter rainwater drainage system, such as long gutter external drainage, gravity flow suspension pipe and open internal ground pipe. The choice of rainwater drainage system should be determined after technical and economic comparison according to the structural form, roofs, gutters, climatic conditions and production process requirements. Their respective characteristics, applicable conditions and technical requirements are described below.

1.Rainwater Drainage System Classification

1.1 long gutter external drainage rainwater system

There is no rain pipe and no indoor leakage in long gutter external drainage storm water system. The project cost is low and the design and construction are convenient. The "Code for Acceptance of Roofing Engineering Quality (GB50207-2002)" stipulates that the longitudinal slope of the gutter and eaves should not be less than 1%, and the water drop at the bottom of the ditch should not exceed 200mm.

The gutter depth of the steel workshop is limited by the purlin height, generally within 200mm, while the thickness of the slope can not reach 200mm. Therefore, the long gutter external drainage should not be used.

1.2 gravity flow suspension pipe rainwater system

The gravity flow suspension pipe rainwater system is an internal drainage system, which connects the rainwater suspension pipe in the middle span with the outer wall, and drains down to the outdoor rainwater sewer. Suspension pipes are susceptible to vibration in the plant. The pipes should be tight and leak-free. Ductile iron pipes and PVCU drainage plastic pipes are generally used. Suspension pipes should not be placed above the production equipment, products and raw materials that can cause harm when encountered with water to prevent damage caused by condensation or leakage of the pipe. The suspension pipe system is also not suitable for use in clean workshops.

1.3 open internal ground pipe rainwater system

The open internal ground pipe rainwater system, that is, setting the buried rainwater pipe and the inspection well in the plant. However, there is a possibility of water leakage when the aerated water in the rainwater downpipe is separated from the water in the inspection well. In order to prevent rainwater leaking from indoor inspection well, it is recommended to calculate the rainwater amount according to the recurrence period of 5a≤p≤10a and the intensity of 5min storm rain. The buried rainwater pipe is calculated according to the gravity flow, and consideration should also be given to improving the rainwater flow from the downpipe to the buried pipe. Otherwise, due to improper connection of pipelines, it will aggravate the turbulence of the water flow in the inspection well, making the water flow in the well unfavorable, gas and water tumbling, and causing water escaping. The effective measures to improve the water flow state are as follows:

1. The drain pipe of the inspection well is connected to the downstream drain pipe to reduce the water drop and improve the water flow conditions from the facade.
2. The water flow turning angle is not less than 135° when connected, which improvesthe water flow condition from the plane.

2.Problems Should Be Noticed in Steel Workshop’s Rainwater Drainage System Design

The rainwater drainage system of steel workshop should be carefully considered and accurately calculated during designing.It is recommended to take a larger recurrence period, generally 10a. The gutter should have a slope of 1% according to the regulations, which is limited by the depth.

When determining the width of the gutter (not less than 600mm), it is necessary to satisfy the displacement and also consider the structural influence of inner bracket under the gutter. The bracket is close to the bottom of the gutter, and is set along the longitudinal direction of the gutter, which may affect the installation of rainwater buckets and pipes. The bracket at the gutter bottom should be set against the gutter’s side, or the width of the gutter should be enlarged to eliminate the influence of the bracket.

The roof of the steel factory is formed by the overlapping of steel plates. The lap joints are fastened to the purlin with a screw or in a 360° upright concealed connection. Generally there is no waterproof coiled material on the steel plates, and the roof’s waterproof capacity is weak. Every steel plate is composed of a wave crest and a wave trough, where the overlap is located on is wave crest, and rainwater flows in the wave trough. When there is too much rain on the roof, the rainwater may overflow the crest and enter the room from the overlap. For steel factory buildings with high and low span roofs, if the high span roof area is large, then a rainwater drainage system should be set up separately.

Picture 2-tile bracket

With the economic development, the safety requirements for the rainwater drainage system design of the steel factory are getting higher and higher. The design of the rainwater drainage system of the large-scale steel factory seems to be simple, but actually there are many factors that need to be considered, including the structural form, Climate conditions, production process requirements, and etc. Which require designers analyze them carefully, and determine a reasonable plan.

Hope this article will be useful for you. Thanks for your reading.