DESIGN GUIDELINES
Chart 3: Flow Chart for Forebay Design
3.2 Structural Design .1 Structural Design - General
3.2.1.2 Design Process Overview
Collection and Review of Available Information
Many elements are needed to come in the structural design from different and often incompatible sources to create a single whole. For example, hydrology provides loading data and gives flood and overflow information, while hydraulics set the constraints that influence layout and sizing.
Geotechnical information is important in determining allowable foundation pressure and in siting and layout. Also it determines the seismic characteristics of the site, defines on the location and characteristics of faults, and the specific geology of the site and region.
At present more than a hundred sites hydroelectric developments have been already studied in Nepal, but the level of study varies. Reports and studies available with different agencies contain a wealth of information that can be used by the structural designer(s) of the conveyance system. The preliminary design process can be greatly simplified if someone else has completed the process in the previous study.
Preparation of Design Criteria Memorandum
The design criteria memorandum is a document which describes physical data, testing and studies, methods of determining loading, assumptions used, and analysis and references used in the design.
There are many existing design criteria available for the design of hydroelectric structures. If the design falls under the jurisdiction of a governmental agency, the designer is encouraged to verify the design criteria with that agency. The US Army corps of engineers, the US Bureau of Reclamation, the Federal Energy Regulatory Commission, and state and local agencies typically have existing design criteria or have regulations which must be considered in developing the criteria. In Nepal, design based memorandum (DBM) is prepared for individual project and used for the detailed design of the project. DBM prepared for Kaligandaki “A” hydropower project could serve a ready reference.
Determination of Loading Loads are generally based on:
• Climate – ice and snow loads
• Geographic location – seismic and geotechnical loads
• Hydrology and hydraulic – normal and high water levels, wave and surge forces, hydrostatic forces, and uplift pressures
• Gate loads and transients
• Geotechnical soil and silt loads
• Structural systems
Preliminary Layout and Sizing of Structures
Coordinate with other disciplines to determine what the anticipated requirements will be. The final information may not be available, but best guess-estimates and assumptions allow the designer(s) to continue on a preliminary basis.
Hydraulic Considerations:
The primary function of the conveyance structure is to deliver required quantity of water to the power house for energy production. Hydraulic considerations make up a large portion of the criteria which contribute for the design. A good design should have:
• No vortices
• Smooth flow over the whole conveyance system
• Minimum flow separation
• Minimum head losses
Structural and Geotechnical Considerations:
Structural and geotechnical engineers can greatly influence the layout and sizing of structures with their early involvement. Their role is often that of making requirements of other disciplines work together as a coherent whole.
Geotechnical considerations can be put into two categories: (a) load producing, and (b) load resisting.
Load producing considerations include: silt load, active or at rest earth pressure, and rock loads. Load resisting considerations can be further subdivided into positive characteristics and limiting items.
Positive characteristics include soil bearing capacity, frictional resistance, and passive earth pressure.
Limiting items reduce foundation load capacity or make it unsuitable in other ways, such as deformation characteristics, creep potential, and deep-seated failure planes.
Construction Considerations:
Constructability (usually the structural engineer's responsibility) must be considered during the preliminary layout and structural design stage.
Mechanical Equipment:
The design and layout should accommodate the control equipment typically found in the conveyance system. That equipment may include gate controls, batteries, water level meters, ventilation, etc.
Maintenance Considerations:
The conveyance system layout should consider safe and easy access to all equipment. Control panel doors should be located so that the door swing does not create a safety hazard. Clear and unobstructed walkways should be identified early in the design as this "unused" space will greatly affect the size and orientation.
Preliminary Stability Analysis
Before too much time and effort are spent on a design, stability should be checked to determine whether additional measures are needed to achieve adequate stability. These measures include rock anchors or foundation drains, which can have a significant effect on project costs and scheduling of time required for design and preparation of documents. The preliminary base pressure and concrete stress should also be checked at this time. Planning for the further final analysis is also useful.
Stability analysis lends itself to automation in some degree by use of spreadsheets alone or in combination with other mathematical computer applications. By taking the time during the preliminary design to streamline the analysis process, the final design can become a much simpler task.
Layout
The layout proceeds concurrently with conveyance siting and the preliminary stability analysis phases of the design. Once preliminary stability has been completed and the workability of the conceptualized conveyance system is known, then the layout can be finalized, features identified, and the design can proceed ahead. The designer should categorize the assumptions which serves as a basis for the layout. As the design evolves these assumptions should be confirmed.
System Analysis
System analysis describes analysis that takes the individual conveyance system as a single unit. This consists of stability analysis and resultant stresses in the concrete and foundation. This type of analysis is considered mostly for the exposed conveyance system like canal, desanding basin, forebay, aqueduct, etc. but does not apply to the underground conveyance system like tunnel.
Stability:
Loading cases and factors of safety used for stability will depend on whether or not the structures are a water retention structure and the applicable requirements for stability are set.
Foundation Pressure and Frictional Resistance:
Allowable foundation pressure and frictional resistance are usually set by a geotechnical engineer.
The foundation pressure, which results from the stability analysis, must be within the allowable limits.
Structure-ground interaction is a critical aspect of the design. If a zero settlement criteria is established, then extensive foundation analysis and preparation may be required. However, the structure could be designed to accommodate a determined amount of settlement by creatively using expansion and contraction joints.
Concrete Stress:
The most common approach is to use the two-dimensional gravity method or limit equilibrium method to determine the stresses in the concrete and foundation simultaneously. Finite element modeling provides highly accurate results as it can more accurately simulate the effects of geometry. Concrete stress determined by a simple analysis method may not provide an accurate representation, especially for desanding basin and forebay with complicated geometry.
Component Analysis
Component analysis considers individual items that make up a conveyance system, such as gates, hoists, gate slots, trashracks, air vents, concrete walls and slabs roof system, valves, and bypass piping.
Detailed Design
During detailed design, structural members are sized and reinforcing steel requirements are calculated. Structural construction joints and water stops are located and added to the drawings.
Specifications are prepared, describing materials and methods.
Drawings and Specifications
Drawings and specifications are the main tools a designer uses to direct the contractor in the construction of the structures in the project. As with any form of communication, success depends on clear understandings between parties. Organizing the specifications using a conventional method like the Construction Specification Institute's format or government specification will improve the reader's understanding.
The following list is taken from a variety of projects, and is intended to provide guidance as to the breakdown and type of features presented on drawings.
• Location and layout plan
• General plan
• Individual structure’s plan and location
• Individual structure sections
• Reinforcing details
• Trashrack
• Trashrack frame
• Gates
• Miscellaneous metal
• Mechanical equipment and details
• Electrical equipment and arrangement
• Instrumentation and control Coordination:
The drawings and specifications for the conveyance system must be integrated with other principal project features.
Design Changes:
Design changes should be documented and approved to ensure that changes are recorded consistent with design intent.
Construction Completion Drawings:
After the project is completed drawings are prepared to provide a record of the structure as it was constructed. These are referred to as "record drawings” (formerly referred to as "as built" drawings).
These drawings include actual foundation levels and any changes in the design or construction from the bid or released for construction drawings.
Quality Control and Assurance
A program of quality control and assurance reduces risks to public and employee safety, reduces professional liability, and reduces possibility of costly wasted time and revisions due to calculation or drawing errors.
Check Calculations Checker should evaluate:
− Codes, standards, and regulatory requirements are correctly selected, referred to and applied
− Assumptions are based on sound engineering principles and are all adequately documented and ultimately confirmed.
− Applicable construction and operating experiences are considered
− Appropriate calculation methods are used
− Load combinations are reasonable and foreseeable
− Conclusions are reasonable compared to assumptions
− Mathematical accuracy and safety margins
− Appropriateness of any computer programs used
− Correctness of computer input and reasonableness of output
Check and Revise Drawings Checker should evaluate:
− Completeness: Whether the drawing contains or refers to everything that the contractor will need?
− Compatibility: Are the objects shown on different views compatible with the other views of the same object? Other portions of the project? Do they properly reflect the design calculations?
− Clarity: Does the object shown convey the information in a way that the contractor can understand easily? Are cross references to applicable drawings and sections of specifications clear and up to date?
Design Review:
The review should be performed by people familiar, but not directly associated with this portion of the project. They may identify conflicts in the design like the reinforcing steel congestion at a column- girder-slab connection that will not allow the concrete's maximum size aggregate to pass through the cage and so on.
Constructability Review:
The most elegant design is useless if it can not be constructed economically. The construction process is examined step by step considering access, equipment operation, seasonal restriction, permitting, scheduling interferences, ease of water, safety, and economics. It is often useful to have someone experienced with construction on similar projects, evaluate constructability. Total construction time may be evaluated for successful completion of the project's cost and schedule objectives.
3.2.1.3 Design Criteria