DEPARTMENT OF THE ARMYU.S. Army Corps of Engineers

CECW-ED Washington, DC 20314-1000 ETL 1110-2-340

Technical LetterNo. 1110-2-340 31 March 1993

Engineering and DesignSTRUCTURAL DESIGN OF REINFORCED CONCRETE U-SHAPED

CHANNELS, BASINS, AND DROP STRUCTURES

Distribution Restriction Statement

Approved for public release; distribution is unlimited.

DEPARTMENT OF THE ARMY ETL 1110-2-340U.S. Army Corps of Engineers

CECW-ED Washington, DC 20314-1000

Engineer TechnicalLetter No. 1110-2-340 31 March 1993

Engineering and DesignSTRUCTURAL DESIGN OF REINFORCED CONCRETE U-SHAPED

CHANNELS, BASINS, AND DROP STRUCTURES

1. Purpose . The purpose of this engineer technical letter (ETL) is tofacilitate the structural design of low-hazard, U-shaped, channels, basins,and drop structures. The ETL summarizes Soil Conservation Service (SCS)experience and criteria, discusses the differences between SCS and Corps ofEngineers practices, and provides guidance on the use of three computerprograms developed by SCS and one program developed by the Corps for thedesign of these structures.

2. Applicability . This ETL applies to all HQUSACE/OCE elements, majorsubordinate commands, districts, laboratories, and separate field-operatingactivities having civil works responsibilities for the design of civil worksprojects.

3. References . Corps programs may be requested from the U.S. Army EngineerWaterways Experiment Station (USAEWES), ATTN: CEWES-IM-DS (Engineer ComputerPrograms Library), Vicksburg, MS 39180-6199.

a. ER 1110-2-1150. "Engineering and Design for Civil Works Projects."

b. Hays, C. O. 1989. "Investigation and Design of U-Frame StructuresUsing CUFRBC; Volume A, Program Criteria and Documentation," TechnicalReport ITL-90-3, USAEWES, Vicksburg, MS.

c. Hays, C. O., and Ford, C. 1989. "Investigation and Design of U-FrameStructures Using CUFRBC; Volume C, User’s Guide for Channels," TechnicalReport ITL-90-3, USAEWES, Vicksburg, MS.

d. Hays, C. O., and Wright, T. 1989. "Investigation and Design ofU-Frame Structures Using CUFRBC; Volume B, User’s Guide for Basins," TechnicalReport ITL-90-3, USAEWES, Vicksburg, MS.

e. Price, W. A., and Alling, E. S. 1989. "CBASIN--Soil ConservationService Program SAFBASIN for Structural Design of Saint Anthony Falls StillingBasins; Adapted to Corps of Engineers Structural Criteria," TechnicalReport ITL-89-4, USAEWES, Vicksburg, MS.

f. Price, W. A., and Alling, E. S. 1989. "CCHAN--Soil ConservationService Program STRUCHAN for Structural Design of Rectangular Channels;Adapted to Corps of Engineers Structural Criteria," Technical Report ITL-89-5,USAEWES, Vicksburg, MS.

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g. Soil Conservation Service. 1977. "Structural Design of MonolithicStraight Drop Spillways," Technical Release No. 63, Engineering Division,US Department of Agriculture (NTIS No. PB85-175578/AS).

4. Summary .

a. Scope . This ETL is limited to the structural planning and design ofreinforced concrete U-shaped channel cross sections along with associatedhydraulic jump stilling basins and drop structures. Soil Conservation Servicedesign criteria for hydraulic structures are introduced, and the differencesbetween Corps of Engineers and SCS practices are examined. Engineering needsin the Reconnaissance, Feasibility, and Preconstruction Engineering and DesignPhases are recognized.

b. Background . The SCS has extensive experience in the planning, design,construction, and maintenance of hydraulic structures. Over time, the Servicehas developed and refined its design criteria and prepared a substantialvolume of engineering design tools and aids. The Corps of Engineers canprofit from this body of knowledge.

c. CCHAN and CBASIN. Two SCS mainframe computer programs for thestructural design of U-shaped channels and basins (STRUCHAN and SAFBASIN) havebeen modified and adapted to Corps of Engineers criteria for working stressdesign of hydraulic structures (References 3e and 3f). The resulting Corpsprograms, CCHAN and CBASIN, are microcomputer programs with on-line interac-tive input and screen output. They permit the selection of any desiredcombination of concrete working stress and reinforcing steel allowable stress.

d. CUFRBC. The Corps mainframe computer program CUFRBC was developed toinvestigate or design U-frame channel or basin structures (References 3b, 3c,and 3d). Comments on the selection choice between CCHAN or CBASIN and CUFRBCare provided. The program CUFRBC is listed and described in Hays (1989).

e. DROPSPIL. The SCS mainframe computer program DROPSPIL, for thestructural design of monolithic straight drop spillways, is available withinthe Corps in microcomputer version. The program has not been modified toCorps of Engineers criteria. It uses criteria from SCS Technical ReleaseNo. 63 (Reference 3g).

5. Objectives . The principal objective of this ETL and the enclosed designguidance and policy information (Enclosures 1-3) is to facilitate the struc-tural planning and design of U-shaped channels, basins, and drop structuresthrough the use of available computer programs and other engineeringresources. Specific objectives are as follows:

a. To reinforce the procedures, requirements, and responsibilities forengineering and design of civil works projects as described in EngineerRegulation 1110-2-1150 (Reference 3a).

b. To create an awareness of some of the available SCS materials pertain-ing to hydraulic structures in soil and water conservation engineering.

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c. To provide an understanding and recognition of the similarities anddifferences between Corps of Engineers and SCS criteria relating to reinforcedconcrete design and external stability requirements for hydraulic structures.

d. To present guidance for the use of a group of computer programswritten for civil works project design and development.

e. To make available a reference list of research and/or designliterature concerning U-shaped channels, basins, and drop structures.

6. Action .

a. Where applicable, programs CCHAN, CBASIN, and DROPSPIL should be usedduring the Reconnaissance and Feasibility Phases of all channel projects andin the Preconstruction Engineering and Design Phase of low-hazard projects(where failure of the structure would not result in major property damage orloss of life). Typically, for projects that include dams, the subject struc-tures might serve as the outlet works of low-hazard, small dams (height lessthan 40 feet and storage less than 1,000 acre-feet) or might be situated inchannels downstream of such dams.

b. The use of SCS criteria will be on a project-by-project basis, withHeadquarters, USACE (CECW-E), approval, and should be identified in theproject’s reconnaissance report. Specific SCS criteria to be used and therationale for their use should be stated.

c. The program CUFRBC may be used for all projects.

FOR THE DIRECTOR OF CIVIL WORKS:

3 Encl PAUL D. BARBER, P.E.Chief, Engineering DivisionDirectorate of Civil Works

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GUIDANCE ON STRUCTURAL DESIGN OF REINFORCED CONCRETE U-SHAPEDCHANNELS, BASINS, AND DROP STRUCTURES

1. Introduction .

a. Scope of Guidance . This guidance capitalizes on the experience ofthe Soil Conservation Service (SCS), U.S. Department of Agriculture (USDA),during the life cycles of SCS hydraulic structures in both soil and waterconservation works and in watershed protection and flood prevention projects.Attention is focused on the structural design of U-shaped channels, basins,and drop structures. SCS computer programs and design tools can be of use inthe several phases of structural design of channel components.

b. References . The list of references to Enclosure 1 is contained inparagraph 7.

2. SCS Design Criteria and Practice for Hydraulic Structures .

a. Reinforced Concrete . The SCS participated in the final phase ofdevelopment of the Corps’ EM 1110-2-2104, "Strength Design for ReinforcedConcrete Hydraulic Structures" (Reference 7a) and may well be revising theircriteria accordingly. The use of SCS criteria should be verified with the SCSwhenever they are applied.

(1) Policy. SCS policy for reinforced concrete (Enclosure 2) permits con-crete compressive strength values of 2,500, 3,000, 4,000, and 5,000 psi(34,475 kPa) and steel yield strengths of 40, 50, and 60 ksi (415 MPa). Mosthydraulic structures are designed for the de facto standard combination of f c′= 4,000 psi and f s = 20 ksi or, for strength design, f y = 40 ksi. With fewexceptions, structural design in reinforced concrete can be accomplished byeither strength design (SD) or working stress design methods (WSD). The SDmethod is recommended. SCS policy establishes reinforced concrete designcriteria by structure environment class. Three general classes are specified.Only Soil Conservation Service hydraulic structures (SHY) are considered here-in. The strength design of these structures normally limits the design yieldstrength to 40 ksi for material yield strengths of 40, 50, or 60 ksi.

(2) Steel Ratios. The maximum steel ratio permitted for SHY may notexceed that allowed by SCS WSD criteria for the same material combination (SCSTechnical Release (TR) 67) (Reference 7l). Hence, the ratio of the maximumsteel ratio permitted for SHY to the steel ratio producing balanced strainconditions in SD accordingly varies from approximately 0.24 to 0.40 for thevarious material combinations (SCS Design Note (DN) 21) (Reference 7o). Forthe de facto standard, the ratio of the SHY ratio to the balanced strengthdesign steel ratio is 0.31. The maximum steel ratio permitted for SHY is

where n is given by

ρshy 0.40f c′f y

1.0

1.01.25 f y

nf c′

n 503.3

f c′

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ETL 1110-2-34031 Mar 93

The steel ratio for balanced strength design strain condition is

ρbal 0.85 β1

f c′f y

87,00087,000 f y

where, for f ′c less than 4,000 psi,

β1 0.85

and for f ′c equal to or more than 4,000 but not more than 6,000 psi,

β1 0.85 0.05

f c′ 4,0001,000

(3) Safety Factor. The SCS SD load factors basically follow the singleload factor concept with a value of 1.8. This load factor, plus a flexurestrength reduction factor of 0.9, produces a safety factor of 2.0 for purebending. Combined with the SHY steel ratio limitation, SD solutions tend tobe closely the same as those obtained from WSD.

(4) Design Aids. Design aids are available in several forms. Theseinclude graphs and charts in Engineering Standard drawings, preprepared struc-tural detail construction drawings in National Standard Detail Drawings(NSDD), and computer programs for specific structure types (SCS TRs 42, 45,50, 54, 63; References 7e-7i). Each TR documenting a computer program con-tains a reinforced concrete design criteria sheet (Figure 1-1). Mylar copiesof available NSDD (SCS DN 18) (Reference 7j) are obtainable from Unit Head,Cartographic Unit, South National Technical Center, SCS, PO Box 6567,Fort Worth, TX.

b. External Stability . Stability design provides structural dimensionsthat allow the structure to satisfactorily interact with its environment. Thefollowing features are considered:

(1) Load Combinations. The parameter values used in a load combinationattempt to encompass reasonably severe conditions for that loading. Possiblecritical combinations are recognized and investigated to ensure that appli-cable stability considerations are adequately tested.

(2) Loadings for Stability Computations. Loads computed for stabilityinvestigations are service values, i.e., they are not the factored loads usedin internal strength determinations.

(3) Lateral Earth Pressure Ratios. It is recognized that most hydraulicstructure proportions and configurations result in at-rest earth pressures.Walls of hydraulic structures tend to be quite stiff and are often restrained

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by their supports, so that wall movements are insufficient to reduce lateralpressures to the active case. Even lateral pressures that might originallyapproach the active case may, over time, increase to at-rest values. Further,compaction of fill against a wall can produce lateral earth pressures thatsignificantly exceed normally consolidated at-rest values.

(4) Bearing Pressures. The maximum intergranular contact-bearing pres-sures do not exceed allowable values for the site materials. Allowable valuesare based on experience with similar materials, bearing tests, or theoreticalanalyses. Except for some retaining walls, bearing pressures will seldomgovern structure proportions, for the types of structures contemplated herein,unless the foundation is relatively soft. Such material would often be unac-ceptable for other reasons and would thus be removed.

(5) Rotational Stability. Rotational stability is usually not of directconcern. That is, an adequate margin of safety is obtained when structuresare founded on earth, contact-bearing pressures are held within normally con-servative values, and it is required that the line of action of the resultantof all applied forces lies within the middle third of the base. Some condi-tions, e.g., rock foundations, hurricane conditions, or earthquake loadings,can warrant relaxing the middle third rule.

(6) Sliding Stability. Acceptable factors of safety against sliding areselected based on the following considerations: the assumptions inherent inthe choice of lateral earth pressure coefficients, i.e., active, at-rest, orsome mobilized condition; the totality of forces included in the analysis(i.e., are all forces included, or have some been neglected?); the load combi-nation under investigation; the risk or hazard class (consequences of failure)of the structure; and the working definition of sliding factor of safety beingemployed (i.e., which forces are to be treated as driving forces (and summedwith the driving forces) and which are to be taken as resisting forces).Depending on these considerations, acceptable values of safety factor varyfrom about 1.0 to 1.7 (SCS National Engineering Handbook, Sections 6 and 11(References 7d, 7m, 7n, and 7r); SCS TRs 50, 54, 60, and 63 (References 7h,7g, 7p, and 7i).

(7) Flotation/Uplift. The numerical ratio obtained for the flotation fac-tor of safety depends upon the actual definition of safety factor beingemployed; the assumed effectiveness of any drains, if present; the confidenceplaced on assumed or computed uplift pressures (i.e., pressures may be basedon creep theory, on seepage analyses, on selected water table elevations, oron assumed crack locations); the permeability of the foundation; the complete-ness of the forces considered; and the load combination under investigation.Depending on such judgments, acceptable values of safety factor may vary fromabout 1.0 to 2.0 (SCS TRs 50, 54, 60, 63).

c. Site Investigation . In the past, most deficiencies of SCS structuresoccurred as the result of unknown conditions due to inadequate geotechnicalinformation. Geotechnical investigations vary widely in intensity and scope.In addition to required soil properties, factors that influence the degree ofstudy needed include complexity of the geomorphology in the site area, designrequirements that are independent of the soil materials in the study reach,hazard to life or property in the event of failure, and environmentalconcerns. Reconnaissance investigations provide the initial assessment of

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project feasibility and serve as the basis for future structure planning anddesign investigations.

(1) Channel Improvements. Investigations for channel improvements differfrom investigations at structure sites. Channels may extend for miles througha variety of materials. This requires that data from any one test pit ordrill hole be correlated (on as knowledgeable a basis as reasonable) with dataobtained at the next test stations, both upstream and downstream. It isimportant to determine what stratigraphic units exist and to identify theirrelationships.

(2) Structure Sites. Design data are usually obtained at three cross sec-tions at a structure site: an approach section, a transverse section throughmidstructure, and an exit section. Information obtained at these sections issupplemented at additional locations for either significant structures ordifficult foundation conditions. Design problems vary greatly with site con-dition. In locations where the ground water elevation is a considerable dis-tance below the foundation, the foundation is permeable, and the backfillaround the structure is normally dry, the problems of seepage, piping, anduplift are minimal and other dangers are fairly limited. However, where thewater table is high and/or the foundation is relatively impermeable, a quitedifferent situation prevails. Here sliding, piping, uplift, high lateralpressure on walls, and differential settlement may all require increased con-sideration. It is here that adequate geotechnical investigation is of primeimportance.

3. Differences in Corps of Engineers and SCS Practice .

a. Reinforced Concrete . The following differences should be recognizedand dealt with in the analysis or design of concrete components.

(1) Materials. SCS uses a default value of 4,000-psi concrete while theCorps default is 3,000 psi. SCS generally assumes 40-ksi steel yield whereasthe Corps specifies higher yields.

(2) Reinforced Concrete Design Methods. For WSD, SCS takes the ratio ofconcrete working stress to ultimate compressive strength as 0.40 while theCorps uses 0.35. For SD, SCS normally uses f y = 40 ksi even for higheryields, while the Corps designs for the higher values. For SD, SCS criteriapermit the steel reinforcement ratio to vary over the range 0.24 to 0.40 ofthe balanced reinforcement ratio, depending on the combination of concrete andsteel strengths selected. For the de facto standard, the ratio is 0.31.Corps criteria require that the steel reinforcement ratio be limited to amaximum of 0.25 of the balanced reinforcement ratio.

(3) Miscellaneous. Minor differences can be noted in the requirements forconcrete cover for steel. SCS normally requires 2 in. (5 cm) of cover, exceptfor bottom steel in bottom slabs where 3 in. of cover is specified. The Corpsoften requires greater cover. Steel ratio requirements for temperature andshrinkage also exhibit differences. SCS normally requires steel ratios of atleast 0.002 in an exposed face and at least 0.001 in an unexposed face. Whenthe distance between expansion or contraction joints exceeds 30 ft (9 m), theminimum steel ratios of the steel in the direction normal to the joints areincreased to 0.003 and 0.002, respectively. Slabs more than 32 in. thick are

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taken as 32 in. for purposes of temperature and shrinkage. Corps requirementsfor hydraulic structures can be found in EM 1110-2-2104 (Reference 7a).

b. External Stability . The primary differences between SCS and Corpsexternal stability criteria lie in the formalizing of load combinations to betreated, and in the differences in expressions for various safety factors.These differences include:

(1) Load Combinations. SCS normally treats only a few load combinations,e.g., channel empty and channel full. Corps references contain more formalloading lists, e.g., usual loadings, unusual loadings, and extreme loadings.

(2) Resultant Location. SCS usually requires that the resultant lieswithin the middle third of the base. Thus, since contact bearing is every-where compressive, a rather high margin of safety against overturning results.Corps criteria more readily permit, depending on loading, less than100 percent of the base area in compression.

(3) Sliding. The numerical ratio determined as the safety factor againstsliding is dependent on whether any forces of opposite sense to the main driv-ing force(s) are algebraically summed with the main driving force(s) or aretreated as part of the resisting force system. SCS references will show bothdefinitions used. Corps criteria are more consistent, as evidenced byEM 1110-2-2502 (Reference 7b).

(4) Flotation. The numerical ratio determined as the safety factoragainst flotation is dependent on the treatment of various water loads. SCSuses the ratio of the sum of the downward loads to the sum of the upward loadsas the definition of flotation safety factor. The Corps follows ETL 1110-2-307 (Reference 7c), which subtracts the weight of any gravitational surchargewater from both the sum of the downward loads and the sum of the upward loads.

4. Availability and Selection of Computer Programs for Structural Design ofChannels and Basins .

a. Introduction . Many different channel cross sections are possible, anddiverse loading conditions are encountered in the design of concrete-linedchannels. The computer program CCHAN and its User Guide (WES Technical ReportITL-89-5) serve as the basic channel structural reference herein. Correspond-ingly, many basin configurations are dictated by the diversity of serviceconditions confronted. The computer program CBASIN and its User Guide (WESTechnical Report ITL-89-4) serve as the basic basin structural referenceherein.

b. Computer Programs CCHAN and CBASIN .

(1) Adaptations. CCHAN (Corps library program X0097) and CBASIN (Corpslibrary program X0098) are Computer-Aided Structural Engineering (CASE) Pro-ject programs. They were obtained by converting SCS programs STRUCHAN andSAFBASIN to Corps of Engineers criteria for working stress design of hydraulicstructures. The adaption was a task of the U-Frame Basins and Channels TaskGroup of the CASE project. CCHAN and CBASIN are microcomputer programs withon-line interactive input and screen output. An additional optional datainput line permits the use of any desired combination of f c , f c/f c′ , and

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f s . Also, both allowable bearing pressure and minimum allowable concretemember thickness may be specified. The capability to optionally compute andoutput moment, thrust, and shear values at each of the locations where steelarea and spacing are determined was added to both programs. Concrete coverfor steel is made a function of the ratio of concrete allowable working stressto concrete compressive strength. If the ratio is less than 0.38, cover isset to 3 in. If the ratio is 0.38 or more, cover is set to 2 in., except forbottom steel in bottom slabs where cover is made 3 in. The expression for theflotation safety factor has been modified to conform to the ETL 1110-2-307definition.

(2) Usage. These two programs fit in admirably with the structuralplanning and design needs of the Reconnaissance and Feasibility Phases ofproject development. Use of the programs will be of great value in obtainingthe needed cost figures. Additionally, both of these programs permit therapid performance of sensitivity analyses, which quickly lead to an apprecia-tion and understanding of the effect that various design parameters have onstructure proportions. Sometimes, depending on the hazard associated with astructure in the project, the programs may also have application to the Pre-construction Engineering and Design Phase. As stated in the abstracts of WESTechnical Reports ITL-89-4 (program CBASIN) and ITL-89-5 (program CCHAN), theprograms are for "use in obtaining preliminary structural designs of importantor unusual structures or complete designs of small, routine structures."

(3) Design Parameters. The concept of primary parameters and secondaryparameters is used. Seventeen independent parameters are identified for thechannels, and 22 for the basins. Values for primary parameters must be sup-plied by the user for each design. Secondary parameters will be assigneddefault values if values are not supplied by the user. The user is encouragedto evaluate and provide the secondary parameter values. Use of default valuesmay result in an overly conservative design, or possibly, because of designfunction interdependence, they may produce unconservative results. This isespecially true with the various water parameters. Also, the assumption ofearthfill unit weights in design that are heavier than those used at thestructure site can have adverse consequences.

(a) Primary Parameters. The primary parameters are usually associatedwith the hydraulic design requirements. Three primary parameters areidentified for the channels, and six for the basins.

(b) Secondary Parameters. Fourteen secondary parameters are given for thechannels, and 16 for the basins. The intent in providing default values is topermit a rough initial design to be obtained before site-specific parametervalues are available. Typical ranges of the secondary parameter values areprovided (see Figures 1-2 and 1-3).

(4) Design Modes. The computer programs operate, i.e., execute designs,in two modes controlled by the user. The computer output labels these modesas either preliminary designs or detail designs. These modes correspond wellwith the Reconnaissance Phase and the Feasibility Phase of Project Develop-ment, respectively. Preliminary designs will aid the designer in selectingthe type of structure desired for detail design. Trial concrete thicknessesand distances are determined for various critical dimensions, and preliminaryconcrete volumes are computed. The programs will execute the detail design of

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a specified type of channel or basin. Each detail design begins with the setof trial dimensions obtained in the preliminary design. Thicknesses are in-cremented and the design recycled, whenever it is determined that the capacityof any singly reinforced cross section being examined is exceeded. Required(minimum) steel area and maximum allowable steel spacing are computed at asufficient number of points in the structure to adequately define the steelrequirements of the channel or basin. Schematic steel layouts are shown forthe various design elements. Actual steel sizes and layouts are not selected.

(5) Types of Channels. Four types of channel cross sections are consid-ered (Figure 1-4). Each is assumed symmetrical about the channel center linein both loading and construction. Reinforced concrete design in CCHAN is byWSD. The channels are designed for two loading conditions (Figure 1-5) andmust satisfy flotation requirements.

(a) Type T1F. The walls and floor slab constitute a monolithic, rein-forced concrete, U-shaped rigid frame. The floor slab steel requirements arebased on analysis of the floor slab as a symmetrically loaded, finite-lengthbeam on an elastic foundation.

(b) Type T3F. The walls are designed as reinforced concrete, cantileverretaining wall stems. The stem base is similar to an inverted T-type canti-lever retaining wall base. The most advantageous base toe length, X , isdetermined in the design. The pavement slab, between the retaining wallbases, is independent of the bases except that it resists any thrust imposedon it by the retaining wall bases. Thus, the pavement slab may be subdividedas desired.

(c) Type T3FV. This is similar to type T3F except that the joints betweenthe pavement slab and the retaining wall bases are designed to transmit shearforces and the slab is monolithic between the two shear joints. Thus, pave-ment slab and retaining wall base deflect equally at the joints, and bendingmoment is induced in the pavement slab.

(d) Type T1S. This is similar to type T1F except that two reinforced con-crete struts are provided in each longitudinal span between transverse joints.Edge beams are provided along the top of the channel walls. Thus, walls arenot simple cantilevers as with the other types; instead, they are supported bythe edge beam and strut system and by the floor slab. The floor slab is de-signed as a beam on an elastic foundation. The design of this type isconsiderably more complex than any of the previous types (Figure 1-6). Thethree-dimensional nature of this channel type is recognized in its design.

(e) Application. The type T1S channel finds use where walls are high andthe channel width is relatively narrow. The type T1F channel is one of themost used, and finds application for medium channel widths. Next in order ofchannel width is type T3FV. The shear joints transfer forces that may assisteither the retaining wall bases or the monolithic pavement slab, sometimeswith resulting economy over the type T3F channel. The T3F channel is a much-used type since the T3FV pavement slab will require excessive thickness ifused for the wider channels. The most advantageous (requiring least concrete)base toe length is determined in the design of T3F and T3FV sections. Somedesigns of the T3F or T3FV will result in no pavement slab between retainingwall bases.

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(6) Types of Basins. Stilling basins depend on the hydraulic jump forenergy dissipation. Provision of adequate tailwater is critical to effectiveoperation. With low tailwater, the high-velocity jet leaves the structurewith little energy loss and hence aggravates downstream scour. Structurally,designs obtained from the program CBASIN need not adhere precisely to thehydraulic proportions determined for the Saint Anthony Falls (SAF) basin (Fig-ure 1-7). However, caution should be exercised if the layout departs greatlyfrom the norm. Advantages of the SAF basin are good hydraulic performance, ageneralized design procedure, small size compared to other basin forms, andeconomy. Three basin types are considered. Each is assumed symmetrical bothin construction and loading about the longitudinal center line of the basin aswell as about the vertical center line of any transverse cross section. Thesestructures are three-dimensional and are so treated in the various designelements. Reinforced concrete design in CBASIN is by WSD. The basins aredesigned for two loading conditions (Figures 1-8 and 1-9), and they must sat-isfy flotation requirements. A wingwall is designed for each of the basintypes (Figure 1-10). The wingwall is articulated from the basin wall. Hence,the wingwall stem acts as a simple cantilever. The layout shown in Fig-ure 1-10 is quite restrictive. Since the wingwalls and their footings are notincluded in the stability analyses of the basin proper, the wingwall propor-tions and orientation may be changed or the wingwalls may simply be deleted.

(a) Type A. This basin is a monolithic unit (Figure 1-11). The upstreamend section is normal to the plane of the top of the inclined floor slab. Thefloor slab thicknesses vary uniformly from the downstream end of the basin tothe break-in-grade, and from the break-in-grade to the upstream end.

(b) Type B. This basin has a transverse articulated joint at the break-in-grade (Figure 1-12). Some form of floor joint step is normally used atthis joint. The upstream end section is vertical, rather than normal, to theplane of the top of the inclined floor slab. The transverse articulationjoint makes the structural behavior of this type of basin differ from that ofthe type A.

(c) Type C. This basin has independent retaining wall portions and pave-ment slab (Figure 1-13). The pavement slab resists any thrust imposed on itby the retaining wall portions. Although the pavement slab is not subjectedto transverse bending, it does carry longitudinal bending because of the vary-ing loading along its length. The most advantageous base toe length isdetermined in design. The stability computations for the retaining wall por-tions are three-dimensional.

(d) Application. Basin types A and B are used for the narrower widthchannels. The type used may be based on the personal preference of thedesigner, or may be based on the design resulting with least volume of con-crete. In the type B basin, sidewall thicknesses, footing and floor slabthicknesses, and footing projections may be different on either side of thearticulation joint; thus, economies may sometimes be effected. The type C isused for the wider channels. Some designs of type C basins will result in nopavement slab.

c. Computer Program CUFRBC . CUFRBC is a U.S. Army Corps of Engineersmainframe time-sharing program for the interactive analysis and design of U-frame basin and channel structures. The program was developed under the CASE

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Project. A large number of options and features are available to thedesigner. The program models basins and channels as planar structures (trans-verse unit slices). Several optional methods of computing lateral earth pres-sures and forces are provided. Options are available for the determination offoundation bearing pressures. Foundations may include vertical tension-onlyanchors. Sections may be analyzed or designed by either WSD or SD methods.Graphic output of various members is optional. In addition to the normalloadings from earthfills, surcharges, water loads, and self weight, provisionis made for special distributed and concentrated loads. Basins may have one,two, or three bays. Structure geometry is symmetrical. In the design mode,loading is also symmetrical. In the investigation mode, loading and steelreinforcement may be unsymmetrical. Channels may have one or two bays. Inthe design mode, structure geometry and loading are symmetrical. In theinvestigation mode, structure geometry and/or loading may be unsymmetrical.

d. Comments on the Selection of CCHAN or CBASIN, or CUFRBC . The avail-ability of these three computer programs facilitates the design process ofchannels and basins. However, the designer must decide which is most appro-priate to project needs. The programs may be compared in two ways as follows.

(1) Program Capability Comparison. CCHAN can be used for preliminary anddetailed designs of single-bay, symmetrically loaded channels. CBASIN can beused for the design of single-bay, symmetrically loaded stilling basins thatare geometrically similar to the Saint Anthony Falls basin. Both programs arelimited to WSD and two load cases per run. CUFRBC can be used for most chan-nel or U-frame structures and must be used for multiple bays, multiple-loadcases, the strength design method, and structures with foundation anchors.Analysis runs using CUFRBC can be made for either symmetrically orunsymmetrically loaded conditions.

(2) Design Process Comparison. CUFRBC is the obvious choice for thedesign or analysis of multibay structures. It is the early stages of designof single-bay structures that pose the selection decision. CCHAN or CBASINwould be the design program of choice, as appropriate to the structural func-tion and design phase, unless the site geometry requires an overly unsymmetri-cal layout, or if significant special loads must be accommodated. Both CCHANand CBASIN permit the use of structure types not recognized by CUFRBC. Thatis, CCHAN treats four types of channels and CBASIN treats three types ofthree-dimensional hydraulic jump stilling basins. CCHAN and CBASIN permitpreliminary and detailed design of the structure types for which the programswere written. They may be used to obtain preliminary designs of structuresthat are beyond their applicability (in either structure geometry, loading, orhazard potential) in order to obtain first guesses for input to CUFRBC, whichwould then be used for final design. CUFRBC performs both investigation anddesign. CCHAN and CBASIN only perform designs, but can also provide moment,thrust, and shear values at critical locations. Finally, where both CUFRBCand CCHAN (or CUFRBC and CBASIN) apply, it is designer’s choice. Either maybe selected. Perhaps best, both could be used, using one as a check on theother.

5. Guidance on the Design of Drop Structures . Preceding sections discuss SCSpractice and experience with design criteria, design aids, structure types,and computer programs relating to the structural design of channels and

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basins. This section summarizes available information on the use and designof drop structures.

a. Chute Spillways .

(1) Plain Chute Spillway. Plain chute spillways are sometimes used asdrop structures in channels. A typical chute spillway consists of an inlet, avertical curve section, a straight sloping section, and an outlet (Fig-ure 1-14). Reinforced concrete chute spillways, in addition to their uses inchannels, often function as emergency spillways for earth or rock dams wherethe usual vegetated earth spillway is inappropriate. The inlet portion cantake many forms. CCHAN can be used for the design of transverse sections ofthe inlet, vertical curve, and sloping channel. CBASIN will design the SAFbasin outlet as well as many other variations of hydraulic jump stillingbasins.

(2) Baffled Chute Spillway. Baffled chute spillways are used as overflowspillways and as drop structures in channel work (Figure 1-15). Baffledchutes are in many ways an economical answer to the problem of dissipatingenergy. They are interesting in concept. Flow impinges on the baffles, orpiers, arranged in a staggered pattern throughout the sloping channel, so thatthe flow velocity can never greatly exceed critical. Hence, the need for anormal stilling basin at the base of the chute is avoided. CCHAN can beemployed in the design of these chutes. The hydraulic design of baffledchutes has been generalized from tests on individual models, prototype experi-ments, and verification tests. They require no initial tailwater to be effec-tive, although local scour at the base of the chute and channel bed scourdecrease when tailwater is provided.

b. High-Drop Structures . Drop structures may be classified as high orlow drop. The high-drop structure is one in which upstream water levels arenormally unaffected by tailwater levels downstream. Thus, discharges over thehigh-drop structure weir are not a function of tailwater depths.

(1) Monolithic Straight Drop Spillway. Perhaps the most popular high-dropstructure is the straight drop spillway, which has received much hydraulicresearch effort. The Waterways Experiment Station, for example, has performedhydraulic investigations on inlet geometry and the use of shaped overflowweirs. Occasionally, the drop may use steel sheet piles for headwall, side-walls, and wingwalls. Cathodic protection may then be required. Morecommonly, the structure is completely reinforced concrete, and is designed andconstructed as a monolithic unit (Figure 1-16). The SCS computer programDROPSPIL was written to provide both preliminary and detailed designs of thesemonolithic reinforced concrete structures (SCS TR 63, Reference 7i). Thethree-dimensional behavior of these drop structures is recognized and treated.Three structural variations of the straight drop spillway apron may be re-quested. A microcomputer version of DROPSPIL is available for internal usewithin the Corps from the Engineer Computer Programs Library at WES.

(2) Articulated Straight Drop Spillway. The monolithic drop spillway islimited in the weir length and/or basin length that can be accommodated.Hence, for the larger structures, the drop spillway may be articulated invarious advantageous ways. Articulation may permit essentially unlimited weir

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length by constructing adjacent components that are essentially structurallyindependent elements.

(3) Box Inlet Drop Spillway. The box inlet drop spillway has a three-sided, rather than a straight, weir. The long crest permits large flows overthe crest with relatively low heads. The structure finds application as aninlet to a chute spillway, as the outlet structure for a low dam, and as adrop structure in a channel transitioning to a narrower width.

c. Low Drop Structures . Low drops, though important hydraulic struc-tures, have not been formalized to the extent of high drops. They are diffi-cult to design hydraulically with great assurance of their flow behavior.

6. Available SCS Resources . The Corps can use a number of SCS materials toadvantage, including preprepared NSDD for various types of standard two-waydrop inlet risers, standard concrete pipe conduits and, possibly, standardimpact basin outlets (SCS DN 18, Reference 7j). Alternate outlets might bepipe cantilever outlets with preformed plunge pools (SCS DN 6, Reference 7q)or perhaps SAF stilling basins with a transition between the circular pipeconduit and U-shaped basin. The SCS criteria for vegetated and earthemergency spillways (SCS TR 52, Reference 7k), in addition to those given inTR 60 (Reference 7p), would also be useful.

7. References .

a. EM 1110-2-2104. "Strength Design for Reinforced Concrete HydraulicStructures."

b. EM 1110-2-2502. "Retaining and Flood Walls."

c. ETL 1110-2-307. "Flotation Stability Criteria for Concrete HydraulicStructures."

d. SCS. 1968. "Drop Spillways," NEH Section 11, Engineering Division,USDA (NTIS No. PB-243645/AS).

e. SCS. 1969. "Single Cell Rectangular Conduits, Criteria and Proce-dures for Structural Design," Technical Release (TR) 42, Engineering Division,USDA (NTIS No. PB85-186021/AS).

f. SCS. 1970. "Twin Cell Rectangular Conduits, Criteria and Proceduresfor Structural Design," TR 45, Engineering Division, USDA (NTIS No. PB85-185130/AS).

g. SCS. 1974. "Structural Design of SAF Stilling Basins," TR 54, Engi-neering Division, USDA (NTIS No. PB85-179869/AS).

h. SCS. 1977. "Design of Rectangular Structural Channels," TR 50(Rev. 1), Engineering Division, USDA.

i. SCS. 1977. "Structural Design of Monolithic Straight Drop Spill-ways," TR 63, Engineering Division, USDA (NTIS No. PB85-186765/XAB).

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j. SCS. 1977. "’Unattached’ ES Drawings," DN 18, Engineering Division,USDA.

k. SCS. 1980. "A Guide for Design and Layout of Earth Emergency Spill-ways As Part of Emergency Spillway Systems for Earth Dams," TR 52, EngineeringDivision, USDA.

l. SCS. 1980. "Reinforced Concrete Strength Design," TR 67, Engineer-ing Division, USDA (NTIS No. PB85-181576/AS).

m. SCS. 1980. "Structural Design," National Engineering Handbook(NEH), Section 6, Engineering Division, USDA (NTIS No. PB-243890/AS).

n. SCS. 1980. "NEH Notice 6-4" (update concerning ACI 318-77 WorkingStress Alternate Design Method), Engineering Division, USDA.

o. SCS. 1983. "Considerations on the Substitution of Higher StrengthSteels in Reinforced Concrete Construction," DN 21, Engineering Division,USDA.

p. SCS. 1985. "Earth Dams and Reservoirs," TR 60, Engineering Divi-sion, USDA (NTIS No. PB85-243681/AS).

q. SCS. 1986. "Riprap Lined Plunge Pool for Cantilever Outlet," DesignNote (DN) 6, Engineering Division, USDA.

r. SCS. 1986. Circular No. 1, NEH Section 11, Engineering Division,USDA.

8. Bibliography .

a. Beauchamp, K. H. 1969. "Chapter 6, Structures," Engineering FieldManual , SCS, USDA.

b. Blaisdell, F. W. 1948. "Development and Hydraulic Design, SaintAnthony Falls Stilling Basin," Transactions, American Society of Civil Engi-neers , 113.

c. Blaisdell, F. W. 1959. "The SAF Stilling Basin," Agriculture Hand-book No. 156, Agricultural Research Service, USDA.

d. Blaisdell, F. W. 1973. "Model Test of Box Inlet Drop Spillway andStilling Basin Proposed for Tillatoba Creek, Tallahatchie County, Missis-sippi," ARS-NC-3, Agricultural Research Service, USDA.

e. Blaisdell, F. W. 1981. "Chapter 5, Engineering Structures for Ero-sion Control," Tropical Agricultural Hydrology , John Wiley and Sons, New York.

f. Blaisdell, F. W., and Donnelly, C. A. 1956. "The Box Inlet DropSpillway and Its Outlet," Transactions, American Society of Civil Engineers ,121.

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g. Blaisdell, F. W., and Donnelly, C. A. 1966. "Hydraulic Design ofthe Box-Inlet Drop Spillway," Agriculture Handbook No. 301, AgriculturalResearch Service, USDA.

h. Blaisdell, F. W., and Moratz, A. F. 1961. "Erosion Control Struc-tures," Agricultural Engineers’ Handbook , McGraw-Hill, New York.

i. Bureau of Reclamation. 1973. "Design of Small Dams," 2d ed.,US Department of the Interior (USDI).

j. Bureau of Reclamation. 1974. "Design of Small Canal Structures,"USDI.

k. Bureau of Reclamation. 1977. "Design Criteria for Concrete Retain-ing Walls," USDI.

l. Donnelly, C. A., and Blaisdell, F. W. 1965. "Straight Drop SpillwayStilling Basin," Journal, Hydraulics Division; Proceedings American Society ofCivil Engineers , 91.

m. EM 1110-1-1804. "Geotechnical Investigations."

n. ETL 1110-2-236. "Design Criteria - Paved Concrete Flood ControlChannels."

o. ETL 1110-2-322. "Retaining and Flood Walls."

p. Frevert, Richard K., et al. 1955. Soil and Water Conservation ,John Wiley and Sons, New York.

q. Hayes, R. B. 1974. "Chapter VI, Energy Dissipators; Baffled ApronDrops," Design of Small Canal Structures , Bureau of Reclamation, USDI.

r. Hoffman, C. J. 1973. "Chapter IX, Spillways," Design of Small Dams ,2d ed., Bureau of Reclamation, USDI.

s. Palmer, V. J., Law, W. P., and Ree, W. O. 1954. "Handbook of Chan-nel Design for Soil and Water Conservation," Technical Paper (TP) 61, SCS,USDA.

t. Peterka, A. J. 1964. "Hydraulic Design of Stilling Basins andEnergy Dissipators," Engineering Monograph No. 25, Bureau of Reclamation,USDI.

u. Portland Cement Association. 1969. Concrete for Hydraulic Struc-tures , PCA IS0012.03.

v. Rhone, T. J. 1977. "Baffled Apron as Spillway Energy Dissipator,"Journal, Hydraulics Division; Proceedings, American Society of CivilEngineers , 103.

w. SCS. 1968. "Some Comments on Flexural and Anchorage Bond Stresses,"Design Note (DN) 5, Engineering Division, USDA.

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x. SCS. 1970. "Gated Outlet Appurtenances for Earth Dams," TechnicalReport (TR) 46, Engineering Division, USDA (NTIS No. PB85-189173/AS).

y. SCS. 1970. "Single Cell Rectangular Conduits, Catalog of StandardDesigns," TR 43, Engineering Division, USDA.

z. SCS. 1977. "Chute Spillways," NEH Section 14, EngineeringDivision, USDA (NTIS No. PB-279759/AS).

aa. SCS. 1977. "Design of Open Channels," TR 25, Engineering Division,USDA.

bb. SCS. 1977. "National Handbook of Conservation Practices," Engi-neering Division, USDA (NTIS No. PB85-177137/AS).

cc. SCS. 1980. "Input Data for Design Unit Programs," DN 19, Engineer-ing Division, USDA.

dd. SCS. 1982. "Seismic Analysis of Risers," TR 68, Engineering Divi-sion, USDA (NTIS No. PB85-174241/AS).

ee. SCS. 1984. "Engineering Field Manual," Engineering Division, USDA(NTIS No. PB85-175164/AS).

ff. SCS. 1985. "Considerations on the Use of General Purpose Structu-ral Analysis Programs," DN 23, Engineering Division, USDA.

gg. SCS. 1986. "Specifications for Construction Contracts," NationalEngineering Handbook, Section 20, Engineering Division, USDA.

hh. SCS. 1987. "National Engineering Manual," Engineering Division,USDA.

ii. USDA. 1962. "After a Hundred Years," The Yearbook of Agriculture ,87th Congress, 2d Session, House Document No. 279.

jj. Young, R. B. 1974. "Chapter VI, Energy Dissipators; Baffled Out-lets," Design of Small Canal Structures , Bureau of Reclamation, USDI.

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Figure 1-1. Typical SCS design criteria sheet

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Figure 1-4. Structural channel types

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Figure 1-5. Channel loadings

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Figure 1-6. Definition sketch, type T1S channel

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Figure 1-7. Saint Anthony Falls stilling basin(SCS ES-86), rev. October 1977)

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Figure 1-8. Basin load condition 1

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Figure 1-9. Basin load condition 2

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Figure 1-10. Wingwall Layout

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Figure 1-11. Type A basin

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Figure 1-12. Type B basin

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Figure 1-13. Type C basin

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Figure 1-16. High-drop structures

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SOIL CONSERVATION SERVICE (SCS) REINFORCED CONCRETE POLICY

Copied from SCS National Engineering ManualPart 536 - STRUCTURAL ENGINEERING

536.20 Design criteria for reinforced concrete.

(a) The structural design of reinforced concrete structures is commonlyguided by the ACI Standard, Building Code Requirements for Reinforced Concrete(ACI 318) developed by Committee 318 of the American Concrete Institute. Thiscode covers the design and construction of buildings. The code provides mini-mum requirements and contains several precautions about special attentionneeded when corrosive environments or other severe exposure conditions exist.SCS uses reinforced concrete in hydraulic structures for components of waterresource projects. These structures are often subject to severe exposure.Because of the type of structure usually involved, design must often exceedthe minimums required by building codes.

(b) Concrete is to be designated by class. The class corresponds to thecompressive strength assumed in the design and specified in construction. Theclass selected for use is to be determined by evaluating the requirements forstrength and durability. The availability of materials and construction qual-ity control must also be recognized in making the determination. The strengthvalues normally used are 2,500, 3,000, 4,000, and 5,000 pounds per square inch(psi).

(c) With one exception contained in the criteria for waste storage struc-tures, structural design in reinforced concrete may be carried out by eitherstrength design or working stress design methods.

(1) For waste storage structures , design is to be in accordance withPractice Standard 313, Waste Storage Structure, contained in the NationalHandbook of Conservation Practices.

(2) For Service hydraulic structures , the design yield strength, f y , isto be taken as 40 kips per square inch (ksi) for grade 40, grade 50, or grade60 steels. The only exception to this general requirement is for a specialdesign at critical locations where higher yield strengths will reduce exces-sive congestion of reinforcement and the potential for accelerated deteriora-tion due to increased flexural cracking is acceptable.

(i) The strength design method is to be in accordance with the require-ments of Technical Release No. 67, Reinforced Concrete Strength Design.

(ii) The working stress design method is to be in accordance withrequirements of NEH Section 6, Structural Design, subsection 4, ReinforcedConcrete, as updated by National Engineering Handbook Notice 6-4.

(3) For other structures -- with uncontrolled environments , the designyield strength, f y , may be taken in accordance with the grade of steel spec-ified for construction.

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(i) The strength design method is to be in accordance with the require-ments of Technical Release No. 67, Reinforced Concrete Strength Design, exceptthat temperature and shrinkage steel may be in accordance with ACI Standard,Building Code Requirements for Reinforced Concrete (ACI 318-77).

(ii) The working stress design method is to be in accordance with the ACIStandard, Building Code Requirements for Reinforced Concrete (ACI 318-77),Appendi x B - Alternate Design Method, except that the allowable extreme fiberstress in compression is to be f c = 0.40 f’ c and the Z factor controllingflexural crack widths is not to exceed 145.

(4) For other structures -- with controlled environments , design is to bein accordance with the ACI Standard, Building Code Requirements for ReinforcedConcrete (ACI 318-77).

(d) The following additional criteria are to be used in the design ofService hydraulic structures .

(1) Reinforcing steel is required in both faces and in both (orthogonal)directions in all concrete slabs and walls, except that only one grid of rein-forcing is required in:

(i) Concrete linings of trapezoidal channels, and

(ii) Structures of Class V or less, as defined in par.501.04 of thismanual, if authorized by the state conservation engineer (SCE). If authorizedby the SCE under this exception, a single grid of steel reinforcement is per-mitted in slabs and walls having a maximum thickness of 8 inches, provided thesteel is positioned approximately in the middle of the wall and strength anddurability requirements are satisfied.

(2) Redistribution of moments in continuous members is not permitted ineither:

(i) The strength design method when grade 50 or grade 60 steels are spec-ified for construction and the design yield strength, f y , is taken as40 ksi, or

(ii) The working stress design method.

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ABBREVIATIONS AND ACRONYMS

ACI American Concrete Institute

ARS Agricultural Research Service (USDA)

CASE Computer-Aided Structural Engineering

DN Design Note (SCS)

EFM Engineering Field Manual (SCS)

ES Engineering Standard [Drawing] (SCS)

HQUSACE Headquarters, US Army Corps of Engineers

NEH National Engineering Handbook [Sections] (SCS)

NEM National Engineering Manual (SCS)

NSDD National Standard Detail Drawings (SCS)

NTIS National Technical Information Service (USDC)

PCA Portland Cement Association

SAF Saint Anthony Falls

SCS Soil Conservation Service (USDA)

SD Strength Design [Reinforced Concrete]

SHY Service Hydraulic Structure (SCS)

TP Technical Paper (SCS)

TR Technical Release (SCS)

USA US Army

USACE US Army Corps of Engineers

USDA US Department of Agriculture

USDC US Department of Commerce

USDI US Department of the Interior

WES Waterways Experiment Station

WSD Working Stress Design [Reinforced Concrete]

Enclosure 3 3-1