Adopting an inlet grate design requires an analysis of the functions the grate should perform. A designer must review the drainage requirements and then specify which functions will be necessary for satisfactory grate and all other performance requirements of each application. For example, if the only consideration is for the grate to efficiently intercept large quantities of storm water from the gutter - then a grate with the proper geometry and flow through area is the one to choose. Although capacity is an important function, there are other considerations that must be evaluated to satisfy the requirements of a functional, correctly located, safe grate. Some considerations are listed below. Designers must note that there may be other considerations in addition to those listed below that should be evaluated prior to making a choice.

CONSIDERATIONS FOR HANDICAPPED PERSONS, BICYCLES AND PEDESTRIANS

Design and placement of the grate must be suitable for use in areas where it is possible for handicapped persons, bicycles, and pedestrians to be present.  

THE ADOPTION OF THE AMERICANS WITH DISABILITIES ACT (ADA), THE PROMINENCE OF NARROW TIRED BICYCLES AND CONCERN FOR PEDESTRIAN SAFETY DICTATES THAT DESIGNERS CAREFULLY CONSIDER THE SAFETY OF THE TYPE OF GRATINGS SELECTED FOR VARIOUS APPLICATIONS. IF THE INSTALLATION IS CHANGED IN CONCEPT, SCOPE OR USAGE PATTERN AT SOME FUTURE DATE, DESIGNERS MUST AT THAT TIME RECONSIDER THE TYPES OF GRATINGS USED AND DETERMINE IF THEY STILL MEET THEIR SAFETY REQUIREMENTS. IF NOT, DESIGNERS MUST REPLACE THESE EXISTING GRATES WITH MORE APPROPRIATE STYLES.  

GRATES FOR BICYCLES

The designer must determine the appropriate grate design and inlet location for safe use by bicycle traffic, having in mind the specific details of location and foreseeable use. The following are some important considerations: 1. The style of grate employed should allow bicycle travel from all accessible directions. Grates located in open areas, or adjacent to driveways, or in other locations where they are not adjacent to barrier curbs, require special attention in this regard. 2. In general, we suggest consideration of our Type L vane style grates. We think certain grates shown in our catalog are generally not suitable for use where bicycle traffic is a consideration, and those grates are so designated in this catalog. The absence of such a designation, however, does not mean that a grate would necessarily be safe in any specific location, and the designer is responsible for making the appropriate grate selection.  3. If slotted grates or vane-type grates other than our Type L are considered, they should have sufficiently narrow openings, and/or should have appropriately spaced transverse (cross) bars, to ensure that any foreseeable width and diameter of bicycle tire cannot drop down into openings to an unsafe extent.  4. Gutter slopes should not be substantially swaled into the curb so as to create a disturbance in the roadway which might affect the ability of bicycles or other traffic to traverse them. 

GRATES FOR PEDESTRIANS AND HANDICAPPED

Various regulations or customs may dictate the use of specific types of grates: such as gratings with longitudinal slot openings no greater than 1/4” or 1/2”. Designers must carefully evaluate the location and placement of grates against applicable specifications to be sure they are using grates that will satisfy the specification's requirement and provide a pathway (accessible route) for pedestrians and handicapped persons. The Americans With Disabilities Act (ADA) provides grating specification guidelines that can be used in creating accessible routes for handicapped individuals.

HYDRAULIC EFFICIENCY

The ability of a grate to intercept storm water or its hydraulic efficiency is an important function. The main considerations in design are the geometry and the flow through area of the openings for each individual grate. We have recognized the designer's need for hydraulic information on individual grates. We studied individual grate performance by establishing our own hydraulic testing program and are now capable of supplying grate capacity information on most of the grates shown in this catalog.

SCREENING OUT HARMFUL DEBRIS

An inlet grate must act as a strainer and prevent harmful debris from entering sewer lines. Objects such as branches, large sheets of semi rigid material, sticks, chunks of wood, etc. which are easily passed by large curb openings (open throat) are conveniently prevented from entering the catch basin by a well chosen grate.  If large debris gets into the catch basin, it can often float or wash into the lines, clogging them at inaccessible locations and making the drainage system ineffective.  Should clogging of the grate occur at the upstream end, grates of longer lengths usually provide the extra flow through capacity necessary to accommodate the gutter flow as well as some of the side flow. 

ABILITY TO PASS UNOBJECTIONABLE DEBRIS

Organic material, such as grass clippings, leaves, small stones, scraps of paper and even small twigs should be passed into the catch basin. Because of their size and configuration they are not a hazard for the sewer lines.  Grates designed with closely spaced bars for strength and safety become easily clogged from very small but always present debris. The problem is magnified even more as this debris is packed into the openings by passing traffic creating a hard, semi solid surface. The velocity of the gutter flow is insufficient to dislodge the packed debris and so the grate becomes ineffective.  The answer then is to provide grate openings wide enough, of suitable length or of special design to pass this debris and still meet the other requirements.  We have many sizes and styles that can be implemented into your designs.  

STRENGTH

Inlet grates placed in roadways must be designed to withstand heavy traffic loads. A common designation for standard highway loads is described as H20-44 loading for single axle trucks and HS20-44 for tandem axle trucks (tractor semi-trailer units). The maximum axle loading in both cases is 32,000 lbs. Or 16,000 lb. for each set of dual wheels.  There may be cases where more extreme design loads are necessary such as airports, commercial applications, industrial sites or installations where extra heavy loads and/or extremely hard tires are present. (See page 4 for a list of published specifications available to assist you in your design.) 

PERMANENCY

A gutter inlet grate should be designed to match or exceed the expected life of the installation. The inherent rust resisting properties of unpainted cast iron and its time-tested performance insures long life. The strength of cast iron grates resists earth and pavement pressures-qualities one should question in other non-cast iron materials.  An important consideration in any storm water management plan is the proper selection and use of drainage grates. The line of inlet, area and drainage grates offers hundreds and hundreds of choices. Different shapes, sizes and geometry provide varying capacities which can match your inlet and outlet control requirements.  When runoff conditions overtax sewer lines or treatment capacity, engineers frequently detain excess storm water by temporarily impounding storm water in suitable depressed open space areas, parking lots or other available basin type structures. With capacity rated grates, ponded water can be slowly released at a controlled rate as sewer lines and treatment facilities are able to accept new discharges.