Design Data 20M. Circular Precast Concrete Manholes

Transcription

1 Desgn Data 20M Crcular Precast Concrete Manholes Crcular precast concrete manhole sectons are versatle n the constructon of bured structures; they may be used as catch basns for stormwater dranage, manholes provdng access to bured ppelnes, juncton chambers at the ntersecton of sewers lnes, and used to construct reservors or wetwells for pump statons. The use of precast concrete manhole structures wll reduce constructon tme and labor costs, when compared to cast-n-place or masonry structures. Amercan Socety for Testng Materals (ASTM) C 478M, Standard Specfcaton for Precast Manhole Sectons, s the manufacturng of the components used n constructon of manhole structures, as well as the purchasng requrements of products used for the assembly of manholes. ASTM C 478M does not nclude desgn tables or a method for analyss and desgn for manhole components, but does permt acceptance of a component by proof-of-desgn testng. Test methods are found n ASTM C 497M, Standard Test Methods for Concrete Ppe, Manhole Sectons, or Tle. The typcal precast concrete manhole structure conssts of; grade or adjustng rngs, a or flat slab, sectons, a secton, and frequently a foundaton slab. In other varatons of manholes, sectons are stacked on top of precast tee sectons n the ppelne, or on a slab of a bured vault or juncton chamber. The sectons are usually,200 mm n dameter, but have been manufactured n nearly every larger ppe sze found n ASTM C76M Standard Specfcatons for Culvert, Storm Dran, and Sewer Ppe. Precast, or adjustng rngs, are stacked on top of the manhole or slab to provde the correct fnshed grade for the manhole castng and. When manholes are nstalled n roadways, desgners frequently specfy a stack of rngs at least 50 to 200 mm hgh so the grade may be easly lowered, f requred durng future constructon. In most precast manhole structures, the sectons are topped wth ether an or concentrc. Wthn the length of a secton, the dameter of the manhole s reduced from the sze of the manhole to the dameter of a cast-ron manhole and frame. Specal shallow sectons, or flat slabs, are used for very shallow manholes. Large dameter manholes are ed wth structural flat slab s desgned to resst the appled dead and lve loads. The of a manhole s smlar to a straght secton of concrete ppe wth ppe jonts at ether end. A ppe Fgure Typcal Precast Manhole Grade Rng Eccentrc Cone Secton Rser Secton Precast Base Secton

2 Fgure 2 Forces Actng on Manhole Department of Labor s Occupatonal Safety and Health (OSHA) Standards, and are publshed n ASTM C 478M and ASTM C 497M. FORCES ON CIRCULAR MANHOLE RISER SECTIONS A crcular precast concrete s the deal materal for constructng a vertcal bured structure. Manhole sectons have a thck hgh-strength concrete wall that easly ressts the compressve forces caused by lateral earth and hydrostatc pressure. The mass of the concrete gves the structure stablty n buoyant nstallaton condtons. See Desgn Data 4 Flotaton of Crcular Concrete Ppe, for addtonal nformaton on the effect of buoyancy on bured structures. H Lateral earth pressure. Because manhole s are manufactured n dameters wth the mnmum wall thckness drectly proportonal to the dameter, a general vertcal depth lmt for all szes may be calculated. The most severe loadng condton on a secton occurs when the ground water elevaton s the same as the surface of the ground. The forces actng on the secton are llustrated n Fgure 2. The total actve force conssts of two components; actve lateral earth pressure and hydrostatc pressure. Both components of the load act n a radal drecton and are dstrbuted unformly around the perphery of the manhole. Radal forces actng on a crcular cross-secton result n only compressve forces on the secton. There are no bendng forces n a secton unless there s a dscontnuty such as a hole for a sewer ppe connecton. Based on the radal load dstrbuton, the lateral earth pressure and hydrostatc pressure at any depth wthn the sol mass s gven by the followng equaton: w s HK s cos w w HK w or a stack of ppes s assembled to obtan the proper heght of the structure. Manhole s are fabrcated n lengths that are usually ncrements of the step spacng. The secton generally has openngs through whch water or sewer ppes are nstalled. Base sectons may be manufactured wth an ntegral floor, whch functons as a foundaton, or be placed on a precast or cast-n-place foundaton slab. Precast concrete tee ppelne sectons serve the same functon as a secton. The manhole structure may be furnshed wth attached steps or ladders. The dmensons, spacng and test strength of steps are specfed by Unted States p = w s HK s cos + w w HK w () where: p = total earth and hydrostatc pressure, - klonewtons/m 2 w s = effectve unt weght of backfll materal, kn/m H = depth of manhole, m K s = conjugate rato for sol = angle between backfll surface and the horzontal, degrees w w = unt weght of water, kn/m K w = conjugate rato for water In most cases, the ground surface s level and = zero degrees. Therefore cos =.0 and equaton () becomes: p = w s HK s + 9.8H (2) 2

3 K s s further defned as: ( sn ) K = () ( + sn ) Where µ = tan = coeffcent of frcton for the sol = angle of nternal frcton of the sol, degrees Table lsts the normal range of the angle of nternal frcton for varous types of sol. Table Table Backfll Materal Plastc Clay Wet, Fne Sand Dry Sand Gravel Compact Clay Normal Range of the Angle of Internal Frcton for Varous Sol Types Angle of Internal Frcton For desgn purposes, the average value of s assumed to be 0 degrees. Snce the tangent of 0 degrees s equal to, substaton of ths value nto equaton () results n K s =. If the saturated unt weght of sol s 8.86 kn/m, effectve or unt weght, because of the buoyant effect of the water, s = 9.05 kn/m. Substtutng the effectve unt weght for saturated sol of 9.05 kn/m and K s = nto equaton (2): p = 9.05 x H x / + 9.8H p = 2.8H (4) The radal pressure, p, s equally appled around the perphery of the manhole secton, placng the crosssecton n pure compresson unless there s an openng n the secton. The compressve stress n any porton of the manhole secton s found by usng the followng equaton. s = pd/2t (5) where: s = compressve stress n the concrete crosssecton, p = total lateral earth and hydrostatc pressure, D = nsde dameter of manhole, m t = thckness of manhole wall, m The mnmum wall thckness equals one-twelfth the nsde dameter of the manhole, when the thnnest wall thckness specfed n ASTM C76M s used. Substtutng the wall thckness nto terms of manhole dameter and, p, nto terms of manhole heght, and convertng the unts to pounds per square nch: s = 2.8H x D/ (2 x D/2) s = 76.98H (6) where: s = compressve stress n the concrete crosssecton, H = depth of manhole, m The allowable bearng stress of concrete, s, equals 45 percent of the mnmum requred compressve strength of 27.6 MPa for precast manholes or 2.42 MPa = 2,420. Substtutng 2.42MPa nto equaton (6) and solvng for H, fnds that the maxmum depth for a precast manhole s n excess of 50 m. 2,420 = 76.98H H = 6.4 m Vertcal dead load. Two other factors control the maxmum depth of a stack of manhole s; end bearng at the jonts and the foundaton capacty that wll depend on the ste condtons. Agan, usng the allowable bearng stress for concrete of 2.42 MPa, a,200 mm dameter manhole, wth mnmum wall thckness, could theoretcally support over 58 m of sectons. The effects of settlement n the adjacent backflled sols are not easly analyzed and are not ncluded n ths analyss. At the jont, only a small porton of the ppe cross-secton remans n contact wth the adjacent secton. Assumng that the end bearng area n a jont s only 25 percent of the full cross-secton, a stack of s 0 m deep could be constructed. The maxmum allowable depth for a bured precast manhole s, for all practcal purposes, unlmted. OTHER PRECAST MANHOLE DESIGN AND ANALYSIS CONSIDERATIONS Flat slab s. Flat slab s are frequently used n roadways and are desgned to support traffc loads. The method for the analyss and desgn of manhole slabs s not ncluded n ASTM C 478M, but the Amercan Assocaton for Hghway and Transportaton Offcal s (AASHTO) Standard Specfcatons for Hghway Brdges provdes gudance for desgnng renforced concrete slabs bured n shallow flls that are capable of supportng hghway loads.

4 AASHTO s Standard H and HS vehcles wth specfc wheel loads and spacng are commonly accepted as the bass for lve load desgn of hghway structures. For bured structures, AASHTO Brdge Specfcatons dstrbute the lve load as a concentrated force dstrbuted ether over a load strp wdth crossng the span for bural depths less than 0.6 m, or a square or rectangular shaped area on the top of a concrete slab for flls 0.6 m and greater. The dead load due to earth backflled on top of slabs may be found by multplyng the weght of the sol prsm over the slab tmes a sol structure nteracton factor. A factor of.5 s adequate for most cases. The Specfcatons provde several smplfed formulas that make reasonable approxmatons of lve load bendng moments n concrete slabs. Tabulated formulas that recommend mnmum slab thckness d on span are found n the Concrete Desgn secton of the Specfcatons. Impact must be added to the lve load for flls less than 0.9 m, and lve load may be excluded n the desgn of slabs that are bured depths greater than 2.44 m. When slabs have full bearng around the permeter of a crcular manhole, such as beddng n fresh Portland cement mortar, they may be desgned as a two-way slab wth a sgnfcant reducton n the flexural requrements. Slabs wth less than 0.6 m of must have dstrbuton renforcng placed transverse to the bottom flexural renforcng. Openngs. In conventonal nstallatons, only a small fracton of the strength of a manhole secton s requred to resst lateral and vertcal forces. Openngs n manhole sectons do reduce the strength of that secton, and may alter the load paths that carry the vertcal loads to the foundaton of the structure. When openngs are properly postoned, or supplemental renforcng s provded, there wll be no adverse affect on the structural performance of the secton. Three common methods of creatng an openng n a manhole secton are; molded wth a crcular form attached to the castng equpment, hand-cut or dugout of freshly placed concrete, and cored wth a large dameter crcular cutter. If the openngs are large, supplemental renforcement should be added to the prmary renforcng cage to redstrbute the load from the unsupported porton of the secton wall to the ntact porton. The locaton of the holes s known durng the producton process for the frst and second methods, so addtonal renforcng can be added to the prmary renforcng cage to enclose the openng before the concrete s placed. Cored openngs are usually cut nto a plan secton, so the openngs occur at random n the prmary renforcng. The dameter of cored openngs s usually lmted n sze, so supplemental renforcng s not requred. When an openng s made n a slab, renforcng Fgure Typcal Assembly Combnatons flat slab top concentrc concentrc transton secton transton secton Specfc nformaton concernng precast concrete manhole assembles to meet ndvdual project requrements s avalable from any member company of theamercan Concrete Ppe Assocaton. 4

5 steel wth an area equvalent to one-half the renforcng nterrupted by the openng shall be added to each sde of the openng. Many desgners enclose the openng wth an equal area of renforcng placed dagonally to the prncple renforcng. Dameter of juncton manholes. A juncton manhole s nstalled at the ntersecton of several sewer lnes to form a chamber n whch the nflow may be combned and drected to the outlet lne. The dameter of the juncton manhole must be large enough so that the dstances between adjacent openngs have enough strength to resst lateral and vertcal loads, as well as stresses caused by handlng. For non-cored openngs, the dameter of the openng should be 00 mm greater than the outsde dameter of the connectng ppe. To nsure structural stablty of the manhole secton, the mnmum dstance between adjacent openngs should be 00 mm. The dameter of a cored openng s generally selected for a propretary ppe-to-manhole connector, and the mnmum dstance between openngs should be 200 mm. The dameter of a juncton manhole wll greatly depend on the ncluded angle between adjacent ppelnes. Manhole jonts and ppe-to-manhole seals. Manhole jonts usually have a tongue and groove confguraton and may be made wth or wthout gasket seals. The sngle offset and recessed o-rng gasket are the most common type of jont seals. Gasketed jonts are popular because most of the labor requred to jon sectons can be done outsde the excavaton. Gasket jonts can greatly reduce the nfltraton nto the manhole structure. Non-gasketed tongue and groove jonts may be sealed wth pre-formed mastc cords, external wraps or Portland cement mortar. When low nfltraton rates are specfed for a ppelne system, ppe-to-manhole connectors may be effectve. Connectors may be cast nto the manhole wall or attached by an expanson rng n a crcular openng formed or cored n the manhole wall. Note: klo = 0 mega = 0 6 mll = 0 - kn/m = Newons per cubc meter x 0 = klo Pascals = Newtons per square meter x 0 MPa = mega Pascals = Newtons per square meter x 0 6 m = meter(s) mm = mllmeter(s) Techncal data heren s consdered relable, but no guarantee s made or lablty assumed. 5