DESIGN GUIDELINES
Chart 3: Flow Chart for Forebay Design
3.2 Structural Design .1 Structural Design - General
3.2.1.5 Construction Materials General
Ice Loads
The exposed conveyance system in high mountains where ice is formed in canal, desanding basin and forebay, designer should consider ice loads to the respective structure. Ice pressure is created by thermal expansion of the ice and by wind or current drag. Pressures caused by thermal expansion are dependent on the temperature rise of the ice, the thickness of the ice sheet, the coefficient of expansion, elastic modulus, and strength of the ice. Wind drag is dependent on the size and shape of the exposed area, roughness of the surface, and the direction and velocity of the wind. Ice loads are usually transitory. A second aspect of ice loading is its contribution to vertical live loads. If exposed decks or platforms ice-up, in addition to snow, the weight of an assumed amount of ice should be included in the design.
Wind and Snow Loads
Areas which experience extreme loading should use the locally accepted code loadings.
Equipment Loads
Items to consider for equipment loads are:
• Gate loads – partially open gates, fully closed gates, vibration, down-pull, opening and closing forces, and hoists.
• Trashrack loads
• Clogged trashracks and resulting differential head
• Crane loads – impact, crane weight plus load
3.2.1.5 Construction Materials
Differential settlement and the potential for seismic activity, although remote in some areas, making of un-reinforced concrete walls, beams, columns, slabs, etc is undesirable from a structural and water tightness perspective.
Shotcrete:
Shotcrete consists of cement, sand, pea gravel, water, and admixtures. In many respects, shotcrete is like concrete without the coarse aggregate. Shotcrete is pneumatically applied to a surface, usually rock or soil, to improve ground stability. For open excavation and tunnel portal work, shotcrete and mesh can support the in-situ material by locking blocks of adjacent rock masses together to reduce the potential or rock falls. It also protects rock materials from continued weathering and corrosion.
Shotcrete is applied by one of two processes- wet-mix or dry-mix. When the batch is dry-mixed, water is added at the nozzle along with any other admixtures. When the entire batch is mixed 'wet" and shot from the nozzle, the process is referred to as wet-mixed. The primary difference is when the water is applied to the other ingredients.
Masonry:
Masonry comes in many styles and configurations. Most common are concrete block and slay brick and river bed materials. Masonry, like concrete, has numerous structural applications like construction of canal, desanding basin and forebay and other components of hydropower structures. Its strength is primarily compressive in nature; however, by including fully grouted reinforcing steel, bond-beams, and structural diaphragms, lateral load resisting systems can be developed.
Structural Steel and Miscellaneous Metals:
a) Carbon Steel: Carbon steel is commonly used as a secondary structural material in structural design. Steel plate and shapes are manufactured to a variety of material and strength properties.
b) Stainless Steel: Stainless steel is similar to carbon steel with more chromium. A typical application is where coatings are uneconomic. Surfaces which are subject to mechanical wear, such as rolling or sliding surfaces, may be made of stainless steel. Gate sealing surfaces are almost always stainless steel and sometimes bronze. Where flows create excessive turbulence and/or cavitations, the designer should also consider stainless steel. Aggressive corrosion (water, soil) should be evaluated by a metallurgical engineer and the appropriate materials selected.
c) Rock Bolts, Rock Anchors and Soil Nails. Rock bolts, rock anchors and soil nails are used to support ground excavations and sometimes to anchor the structure to the foundation surface.
Timber
Timber construction is typically limited to structures where the exposure to environmental deterioration is slight. Roof framing of concrete or masonry buildings is the most common use of wood. Some species of woods have some natural resistance to "dry-rot" or animal consumers like termites and beetles. Otherwise, timber requires a more positive protection from deterioration such as pressure treating with a preservative, metal flashing, and/or paint.
Earth and Rockfills
Earth and rockfills are commonly used as backfill material. The backfill should be graded to meet the structural requirements. For example, if the bearing wall of the structure is designed to resist drained lateral earth pressures, then the backfill should be free draining or a drain system included. Typically the amount of material passing a #200 sieve should not exceed 11 percent to be considered free draining.
When the backfill is being placed and compacted, ensure that the backfill is brought up on small lifts consistently on both sides of the structure so as not to unbalance the lateral loads. If the designer considers 100 percent of the structures' mass to provide stability, there may be short-term constructions when the backfill is complete. But the incomplete structure cannot resist the full sliding forces. The designer is encouraged to monitor the construction during any critical phases to advice the contractor on appropriate sequences.
Plastic
Plastic trash rack bars may offer improved performance in cold weather climates. The thermal conductive characteristics of plastics reduce the potential for the formation of frazil ice on the rack but in general practice in Nepal, these materials are not used for the trash rack purposes.
Protective Coatings
Protective coatings are typically associated with steel construction; however, concrete sealants are becoming more prevalent. Coatings may consist of zinc rich paints, epoxies, urethanes, galvanic compounds, and coat tars. Environmental regulations are affecting the production and applications of many protective products. Products containing volatile organic compounds (VOCs) and carcinogens should be reviewed carefully with local, state and federal agencies before being specified on a project.
Serviceability Considerations
The performance requirements for the structure should be considered in the selection and configuration of the structural system and materials. While developing the design criteria, the engineer should evaluate the project's requirement for sustained operation following a design event such as a flood, snow storm, or earthquake. The structure should be designed to survive the least likely environmental event with an anticipated design life in excess of 50 years. Floods, design based earthquakes, windstorms, hail, and snow storms all contribute to the design criteria. Where the failure of a structure does not affect life safety, the engineer may consider designing to a lower return interval event after exhausting other relevant design guidelines.
Material Specifications
Concrete consists of the proportioned mixing of cement, aggregates, water, and admixtures to produce a concrete which is in conformance with the requirements of the design and the serviceability requirements of the structure. Normal Strength Gain Concrete achieves its minimum specified strength within 28 days and its other properties are in conformance with the requirements of the reference standards. High Early Strength Gain Concrete achieves its minimum specified strength gain at an accelerated rate, usually between three to ten days. Concrete can be designed by several methods, ACI's Reinforced Concrete Design for Buildings, Publication 318, presents the ultimate strength design method. The U.S. Army Corps of Engineers uses Strength Design for Reinforced- Concrete Hydraulic Structures, (EM 1110-2-2104 USCOE, 1992) as its concrete design specification.
Cement:
Various types of Portland cement for concrete construction could be selected. Each cement type has specific properties benefiting with the specific design conditions. For example, if the aggregates are slightly reactive, cement should counteract the reactive property. For mass concrete placements, the amount of cement should be minimized or "low heat" cement should be used.
Silica Fume:
Silica fume can be used to produce concrete with very low porosity, enhancing water tightness, and potentially increasing strength. Silica fume is a waste product from the Ferro-silicon manufacturing process, and reacts with the lime liberated during the hydration of Portland cement. Low porosity is achieved due to the smaller particle sizes of the silica fume (compared to Portland cement), and to the low water/cement ratio of the mix. A super plasticizer is typically used to obtain a suitable workability.
Pozzolan:
Pozzolan is often used to reduce the heat of hydration of mass concrete applications. Pozzolan materials are typically volcanic ash, pumicite, opaline shales and cherts, calcined diatomaceous earth, burnt clay, and fly ash. The most common artificial pozzolan is fly ash, or pulverized fuel ash, which is obtained by electrostatic or mechanical means from the flue gases of furnaces in coal-fired power stations. The pozzolan, is finely divided form and in the presence of moisture, will chemically react with the lime (liberated by hydrating Portland cement) at ordinary temperatures to form compounds possessing cementious properties.
Aggregates:
Both coarse and fine aggregates should be clean, sound, non-reactive and free of deleterious coatings, silts, clay wood and organic materials. Non-reactive aggregate quarry or source should not contain any materials that are deleteriously reactive with alkalis in cement in an amount sufficient to cause expansion in mortar or concrete.
Water:
Water used to batch concrete should be clean and free from deleterious amounts of silt, oil, acids, alkali, salts, organic substances and contain no chlorides.
Admixtures:
Admixtures are products added to the concrete to enhance the workability of plastic concrete, reduce water requirements, improve strength gain, improve durability and wear resistance (hardness), aid in curing, impart waterproofing properties while concrete is in its plastic state, enhance dispersion of the cement particles, and improve the entrainment of air. Admixtures include both products that are added and mixed prior to concrete placement as well as products that are added after the concrete has been placed and is still in a plastic state. Concrete bonding agents, sealants and waterproofing agents that are applied after the concrete has hardened are not defined as an admixture. All products should conform to applicable standards and with industry standards. All combinations of admixtures that will be added into any batch should be compatible, and when mixed in accordance with the manufacturer's instructions, should not produce any adverse side effects to the concrete.
Air Entraining Agents:
Air entraining agents may be used to improve concrete's durability when exposed to extreme weather variations.
Water Reducing Agents:
A water reducing agent may be used to increase the workability of the mix without decreasing the strength.
Retarder:
A water reducing/retarding agent may be used to retard to concrete set. The agent should be used in the amounts recommended by the manufacturer to obtain the desired retardation. Normal slump and air contents specified should be maintained during the use of the set retarder admixture.
Accelerators:
Accelerators may be used to decrease concrete cure times and/or provide early high strengths.
Accelerators should not contain calcium chloride and should not increase the drying shrinkage or be detrimental to the reinforcing material.
Super Plasticizers:
High range water reducing admixtures may be used to develop mix designs with improved workability.
However, super plasticizers should only be used on a case-by-case basis.
Pigmentation:
Where required by environmental concerns, concrete structures can be "painted" to blend with the surroundings. A pigment can be added to the concrete to produce the desired coloring. Colored concrete typically fades with exposure to ultra violet rays (sunlight) and may even appear blotchy or uneven. Determining the appropriate color may prove to be an involved and lengthy negotiation with the involved parties. Unless very specific quality controls are used, the seasoned color of the concrete may not be the same color originally anticipated. If a uniform color is required, then tight controls on the pigment and the cement must be anticipated. Typically only white cement has a uniform color because of ASTM requirements; hence adding controlled pigment to white cement ensures a uniform concrete pigment.
Finishes and Sealants Joint Filler and Joint Sealer:
Expansion and isolation joint fillers should be pre-molded strips of Styrofoam or other specific joint filler. There are a variety of commercial products available to create seal and maintain expansion joints.
Waterproofing:
Generally, conveyance system structures are submerged and the occasional dewatered condition does not require completely dry interior conditions. However, where water-tightness may be a concern, all exterior surfaces of the structure should be applied with a waterproofing compound.
Placing the waterproofing compound on the exterior surface of the concrete will minimize the amount of water which permeates into the concrete and reinforcing steel. Interior placement of the compound will improve moisture resistance and reduce "sweating" of the walls.
Roughening Agent:
For concrete steps, stairs, and other traffic areas subject to slippery conditions, an abrasive agent abrasive, such as aluminum oxide, can be applied to the concrete surface during finishing improving traction.
Curing Agent:
A curing compound which dries clear should be used for all exposed surfaces. No wax-based product or other type of product that would prevent the proper bonding of patches, paint, mortar, or plaster should be used.
Reinforcing Steel or Rebar
Reinforcing steel or rebar (sizes, shapes, corrosion resistance), comes in many different sizes (diameters) and strengths. Reinforcing steel can be deformed billet-steel bars (ASTM A615-S1), welded wire fabric cold drawn wire (ASTM A 82); welded sire steel (ASTM A185); and welded deformed steel (ASTM A497), The selection of a particular type of reinforcing steel should consider the complexities of the reinforcing steel patterns, accessibility, and ease of installation, in addition to strength requirements.
Structural steel
Steel design and fabrication should conform to the requirements of the Code referred for the structural design.
Timber
Timber design and fabrication should conform to the requirements of the Code referred for the structural design.
Masonry
Like timber, masonry is used throughout the building industry and has many available design codes which are applicable. Masonry construction uses many of the specifications of cement, water, reinforcing steel, and fine aggregates typically found in concrete construction.
Rock Bolts, Rock Anchors, and Soil Nails
Rock bolts, rock anchors, and soil nails are used in surface excavations of rock and soil reinforcement and may be tensioned or intentioned (dowels). All bolts should be steel, whether smooth or deformed, and grouted in a tensioned or intentioned state. Rock bolts, rock anchors, dowels, cement grouts, and resins should be produced by a manufacturer regularly engaged in the production of the specified items with a history of successful installations.
Formwork
The following items should be considered when laying out the structure and presenting the design requirements on the engineering drawings.
Chamfer Strips:
Chamfer strips should be placed in forms to bevel all edges and angles of concrete. The lack of sharp 90-degree corners will minimize the amount of broken edges and irregular "vertical" lines of the structure. Chamfer sizes vary by orientation – exterior, vertical wall corner chamfers are typically larger than those for penetrations or openings which have no embedded steel framing and for small equipment pedestals. Chamfer sizes range from ¾ inch by ¾ inch to 1-1/2 inch (approximately).
Metal Forms:
Reusable metal or steel forms are sometimes used where lifts of the same height and configuration and placed repeatedly. The designer should consider repeatedly using cross sections and wall sizes where forms might be economically re-used.
Architectural Form Liners:
Where the finished concrete surface will be textured, the designer may consider using form liners.
Rigid plastic liners are attached to the formwork prior to placing the concrete. Many of the same considerations as for metal forms should be used during design.
Leave-In-Place Forms at Construction Joints:
In complicated structures, there are sometimes benefits to staging concrete placement for blockouts, first and second stage concrete, pedestals, and where a vertical formed construction joint is used. In lieu of using a smooth form, brush finish or "green cutting" a leave-in-place form can be considered.
The material resembles light gauge expanded galvanized metal which s easily molded and cut with sheet metal shears. When concrete is placed against the leave-in-place form, cement paste and sand extrudes through the form to create a rough texture with approximately 1/4 –inch to 3/8- inch profile.
The leave-in-place form can remain in place or be removed.
Material Testing Requirements
All material and fabrication should be of the highest quality. Any damaged material should be rejected, removed, and replaced; the material and fabrication should meet the governing acceptance standards.
3.2.1.6 System Analysis