Section 8.7 Building Storm Drainage Systems

Transcription

1 Building Storm Drainage Systems General Design Requirements a) A separate drainage system shall be provided for storm water from roof areas and to receive non-contaminated clear water atmospheric condensate. The building storm drain shall extend outside the building and connect to the campus storm sewer system. Storm drainage systems shall be conventional atmospheric pressure gravity drainage type that does not rely on storage of water on the roof, special drain weirs, or nonconventional system components. Any required storm water retention shall occur outside and downstream of the building in such a manner as to not allow water accumulation on the structure. b) The number and sizes of drains shall be determined to convey storm water from areas being drained at the same rate as water is collected in those areas. At least two drainage points shall be established for each roof or areaway drainage area, and no roof drain shall have a drain outlet smaller than 75 mm (3 in.) diameter. Roof and deck areas 46 square meters (500 square feet) and smaller may be provided with a single drain, provided overflow or other approved relief provisions are incorporated to prevent buildup of water in the event of failure of the primary drain. c) A dedicated secondary emergency roof drainage overflow system shall be provided to serve flat roof areas, except where such roof areas are provided with appropriately sized overflow scuppers. The overflow drain system shall consist of overflow drains installed alongside each roof drain, with a weir 50 mm (2 in.) to 75 mm (3 in.) above the roof low point. The system shall be piped independently to discharge through downspout nozzles spilling to the exterior, approximately 300 mm (12 in.) above grade. d) Stainless steel rain cap shall be specified over the top of the overflow roof drain dome grate to prevent intrusion of rainfall during normal conditions. e) Storm and overflow drainage systems above grade (non-buried) shall be provided with appropriate thrust restraint, joint rodding, and bracing in accordance with the recommendations of the CISPI handbook. Special bracing provisions are generally required for piping 125 mm (5 in.) and larger. f) The entire storm drainage systems (except foundation and under-slab subsoil drains) shall be tested for not less than 4 hours with a 3.0 m (10 ft) water head, or 60 minutes with 35 kpa (5 psi) air. g) Strict specifications and quality control are critical to ensuring contaminant free piping systems and preclude leaks, including detailed quality control requirements for specialized welded plastic (fusion) joint systems Sizing Criteria a) Either the primary storm drainage or secondary overflow drainage/ scupper system shall be sized based on the 10 year, 5-minute storm, which for the Bethesda campus Page 1

2 corresponds to a rainfall rate of 180 mm/hr. (7 in/hr.). The remaining system may be sized based on the 100-year, 60 minute storm [which for the Bethesda Campus corresponds to a rainfall rate of 85 mm/hr. (3.4 in/hr.) or as indicated by the latest IPC edition or 10-year, 5 minute storm at the A/E option. The clear water waste loads from air handling unit and other approved non-roof drainage discharging to the storm system shall be based on the peak load, and sizing for 20% overage of the total clear water waste load shall be included in the system design. Sizing of both primary and secondary systems shall incorporate horizontal roof surfaces as well as an allowance for adjacent vertical areas that may drain onto the roof structure where applicable. The A/E shall consider in the storm water calculations the following: 1. In cases of a single vertical wall adjacent to a lower roof, an allowance of 50% of the vertical area above the roof shall be included in the design load for the lower roof area. 2. In cases of two opposite walls of equal height, no additional vertical area shall be added. 3. In cases of two adjacent walls, 35% of the total wall areas above the lower roof shall be included in the design load. 4. Where adjacent walls are of differing heights, similar appropriate allowances shall be included in the design. 5. In limited cases where any overflow drain from a higher roof area is permitted to spill to a lower roof, both the primary and secondary drainage for the lower roof shall be sized for the load of both roof areas. This practice is generally undesirable, and shall not be utilized for primary drainage. b) Building storm drain slopes shall provide a minimum velocity of 0.9 m/s (3 fps) to keep sediment and debris in suspension; however 1% slope will be permitted for above ground lines 75 mm (3 in.) and larger and where required to meet sewer inverts. Underground storm sewers outside the building shall be sized to include a 20% additional capacity (based on rainfall rate) to allow for future expansion Distribution a) Storm drainage leaders (including overflow drains) shall be located in permanent shafts or at building columns. Vertical piping shall be routed as straight as practical, with minimal offsets. b) An expansion joint or acceptable horizontal offset (swing joint) shall be provided at connections to each roof and overflow drain. c) Drain leaders shall not be located in interior partitions. RATIONALE The installation of drain leaders or any piping risers in interior partitions will prevent efficient use of the space during future renovation of the space. d) The system design shall avoid placement of horizontal piping above conference spaces, offices, electrical rooms, or other critical areas. Lower roof areas shall not be connected to rainwater leaders within 600 mm (24 in.) of a horizontal offset, and then only with wyetype fittings. Page 2

3 e) Fittings specified for directional changes and branches in storm drainage systems shall be of the same long-radius type required for use in sanitary systems. f) The entire storm and overflow drainage system, (including vertical piping and drain bodies) inside the building shall be insulated. g) Wherever possible, cleanout requirements for horizontal piping in ceilings should be served by accessible roof and area drain inlets provided cleanout or access point spacing does not exceed 30 m (100 ft.) or 135-degrees aggregate directional change, and the roof area can be safely accessed with drain cleaning equipment, without necessity of hoisting. Roof and overflow drains shall include sump receivers, under-deck clamps, and aluminum or cast iron domes. h) Standpipes and clearwater waste receptors shall not connect to the storm drain system in locations as to be subject to backflow from storm water surges. Backwater valves on standpipe drains shall not substitute for properly sized piping systems with branch lines connected to wye-branches into rainwater leaders away from horizontal offsets and surge zones. i) Properly sized area drainage shall be provided for exterior walk ways, stairs, and as necessary to prevent accumulation of water or ponding with outlets not less than 75 mm (3 in.) diameter. Area drains should generally be sized to relieve peak rainfall with a maximum 7 mm (1/4 in.) head. Area drainage in area ways and otherwise subject to blockage shall be provided with dome tops. Area drainage in areas provided with pavers shall be arranged to discharge both surface drainage and accumulated flow below the paver through use of specially designed promenade-type perforated drains. The grate of all area drains shall be sufficient for the anticipated traffic loading Underslab/Subsoil Drains a) Underslab/sub-soil drainage piping shall be provided for slab on grade and buried structure where recommended by the geotechnical or structural consultant. The underslab/ subsoil drain system shall be designed and spaced as per the geotechnical engineer recommendations, using not less than 100 mm (4 in.) diameter perforated laterals and 100 mm (4 in.) or 150 mm (6 in.) mains as required, with filter fabric and a positive slope 5 mm/m (typically 0.5%). b) Where a sump pump is required for subsoil drainage, it should be located at the building exterior where possible and shall consist of two pumps sized for N+1 redundancy, on standby power. Areaway drains, rain leaders, downspouts, or other aboveground drainage points shall not be connected to subsoil drains. An exterior sand trap or catch basin shall be provided where subsoil drains connect to the storm drainage system. In addition, where subsoil drains connect to the storm drainage system without the use of pumps and is subject to backwater backflow, an automatic backwater valve shall be provided at the sand trap to prevent reverse flow of storm water into the subsoil drains. The backwater valve shall be provided at the inlet of the subsoil drain to the sand trap to permit access to the device. The cover of all interceptors shall be appropriately stamped to identify the interceptor type and system served. Whenever storm water vents are Page 3

4 required (such as for sealed sumps), they shall be piped independently of any sanitary vents. See Section 8.6 for additional requirements Storm Gravity Drainage and Backflow of Waste a) Building areas, which are sufficiently elevated above the storm drains, do not require discharge through a pumping system and shall be routed independently to discharge by gravity. Refer to Section 8.6 Drainage Systems for additional requirements. b) Plumbing systems shall be arranged so that a stoppage in the exterior storm sewer shall not result in storm water backflow into stairwell area drains, subgrade parking areas, or similar low-level storm water inlets that are not fully exterior of the building. Storm water shall be relieved outside the building through manhole covers, catch basins, or other exterior storm-drainage inlets. Refer to Chapter 1 for Environmental Management. c) Where such drains are located less than 230 mm (9 in.) above the elevation of the storm water relief point, automatic backwater valves shall be provided. Roof drains and other drains with flood level rim elevations above the reference point shall not discharge through the backwater valve. d) Drain openings with flood level rims that are not located above the crown level of the storm sewer shall be pumped. Pump systems serving underslab/subsoil drainage, critical applications, or any application serving more than one drain or multiple areas shall be provided with not less than two pumps for N+1 redundancy, and shall be on building standby power. Pump system shall be municipal duty grade, and capable of passing not less than 13 mm (1/2 in.) solids Applications a) Only clear-water, non-contaminated atmospheric drainage suitable for direct discharge into natural waterways shall be connected to the storm drainage system. Water such as clean atmospheric condensate from air-handler/ fan coil units is acceptable; however discharge of chemically treated, contaminated, and chlorinated potable water, as well as high temperature water violates provisions of the Clean Water Act, and shall be directed to sanitary. Discharge of unmetered storm water into the sanitary system is prohibited by WSSC and unnecessarily burdens sewage treatment systems. b) The waste discharge chart in Exhibit 8-7-A shall be used to determine where various services are piped. 187 Exhibit 8-7-A Storm Drain Discharge Type of Discharge Air conditioners: water cooled, non-contaminated Area well Bearing cooling water: reclaimed water on Storm Drain Discharge Page 4

5 Type of Discharge Storm Drain Discharge individual determination based on contamination Condensation drains: AHU, cooler coil, refrigerated equipment (Atmospherically generated, noncontaminated condensate) Cooling water: industrial noncontact Grass areas Humidifiers, non-treated water, non-steam: Overflow from ponds: ornamental, utility; check for chemical treatment (if any contamination must route to san). Ozone treated with no other chemicals may be permitted to storm with prior approval. Parking Garage, Top Deck Exposed to Rainfall Roof drainage Subsoil drainage Note: This table may be used for discharge requirements for storm. Design shall include air gaps as necessary to prevent cross-connection between sanitary/storm systems and the water/air system Air Handler Unit Drainage a) Non-contaminated atmospheric condensate shall discharge to storm as clear water waste, and be fitted with a normally closed bypass arrangement with a valved connection and threaded cap near the AHU to permit extension of a hose to a sanitary drain during coil cleaning operations. b) The clean condensate drains for large AHU s shall discharge through an adequately sized insulated standpipe that is not less than 75 mm (3 in.) diameter and of sufficient length to preclude overflow, that passes through a sleeve (with water stop) embedded in the slab, projecting at least 50 mm (2 in.) above the slab and located near the AHU drain to preclude tripping hazards. c) Galvanized steel or copper condensate drain line, shall extend down into the standpipe approximately 50 mm (2 in.) (as an airbreak), and be fitted with not less than a 50 mm (2 in.) top access cleanout to permit the standpipe to be cleared, and the above noted bypass valve arrangement. See Exhibit 6-3-A in Chapter 6 for piping material. Near to the AHU, a sanitary floor drain shall also be provided, with floors sloped and sealed near the floor drain area to receive ancillary moisture. Page 5

6 d) Drainage for steam humidification shall be separately collected, and if discharging to the building drainage system, shall waste only through the sanitary system with an aftercooler, and discharging either through a standpipe receptor or floor sink. e) Special care shall be provided for the installation of any floor drains, standpipes, and floor sinks to preclude leakage around the drain perimeter to the floor below Garage Drainage System a) An independent garage drainage system shall be provided for parking garage drains below the top parking deck that is not directly exposed to rainfall. b) Garage drains shall be of the dry-pan type (connected without traps), have not less than 100 mm (4 in.) diameter outlets, and include a sediment bucket and ANSI special load class ductile iron secured grate and anchor flange. c) Except for the top deck exposed to rainfall, the garage drains shall collect to a common 150 mm (6 in.) collector line that discharges to an oil/sand separator designed to ensure discharge does not exceed 100 ppm oil. The interceptor shall incorporate a 150 mm (6 in.) deep submerged water trap seal at the inlet, and a 450 mm (18 in.) deep submerged seal on the outlet, as well as a dedicated vapor vent direct to the exterior terminating at least 2.1 m (7.0 ft) above the highest parking deck and away from any air intake or building opening. Interceptors shall be provided with internal ladders and non-slip rungs. d) The provision of dry pan drains eliminates requirements for freeze protection of traps and prevents accumulation of oil or flammable liquids in trap seals. An adequately sized automatic electrically actuated trap seal primer shall be provided to ensure continued maintenance of the interceptor trap seal. e) Garage drains shall be located at low points adjacent to ramp turnabout and at sufficient intervals to permit garage floor washdown and preclude puddling. f) Backwater valves shall be provided where necessary to protect parking garages where potential for flooding exists due to the elevation of the drains in relation to other points of relief, and where otherwise necessary to protect mechanical and electrical rooms that may be located in the garage from backflow of storm or sanitary sewers. g) The top deck of the parking garage exposed to rainfall shall be directed to the storm drainage system independent of the storm main serving occupied buildings. h) The requirements for an oil/sand interceptor to serve the top deck drainage shall be determined based on the most current state and county requirements. Currently Montgomery County requires an oil interceptor prior to release to the water way and it is typically necessary unless otherwise handled downstream prior to release to waterways or retention basins. i) Trench drains shall be provided at parking garage ramp entrances and exits as required to prevent water buildup. Heel-proof drains shall be provided at any stair landing exposed directly to rainfall from sides or above, and shall have outlets not less than 75 Page 6

7 mm (3 in.) diameter. Adequate washdown shall be provided (typically 38 mm (1-1/2 in.) outlets), drainable from a single point at the base of the system, with backflow protection, insulation with metal jacket, and heat tracing. Page 7