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Shear Wall and Core System in Tall Buildings

Shear walls are widely used for both tall buildings and low-rise buildings. They are important structural members used in the lateral resisting system. They work as a deep vertical cantilevered beam supported at the ground. They also carry vertical load together with columns.

Some structures may require coupled shear walls, where girders and the floor system join the two or more walls together as a coupled system to provide morestiffness.

In tall buildings, shear walls are generally located at the center of the building, normally in the form of core wall system to accommodate the vertical translation system such as lifts for the tall building. It is a very common form of lateral load support system in tall buildings.

1. Type of Cores

There are two major types of cores: concrete core and steel framed cores. Concrete walls are used widely in the tall building design; on certain occasions, steel core can be found in buildings built before 9/11, they being much lighter, can save the cost of the foundation. However, they are gradually abandoned after 9/11 attack.

1.2 Concrete Core

Fig.1 shows a concrete core for a tall building under construction. Reinforced concrete cores are a more standard option for tall buildings in general, as seen from the history, concrete structure is dominant in the market because they provide more stiffness than steel cores, and it is relatively cheaper to use a concrete core in certain countries such as China.

In certain countries such as China, the steel production was not sufficient in the past; therefore, most of the tall buildings were built in concrete. In addition, some codes require that the core of the building be constructed using reinforced concrete in case of fire and for emergency safety.

When designing a concrete core, there are several issues that must be considered:

Constructability and construction sequencing: The erection sequence for building with a concrete core will see the casting of the core proceed ahead of the steel framing. The core and elevator shafts can therefore allow for the use of a climbing crane rather than having to use separate tower cranes.
is also quite common to install the steel embedment such as steel plate in the core to further strengthen the core. One of the project example is the China Zun Tower in Beijing.
Due to the shortening effect, differential movement may incur especially in the structural system of the building, which is a combination of steel and concrete, the tendency of concrete to creep or shrink over time and thermal expansion of steel need to be considered.

Fig.1. Concrete core of a tall building under construction

1.2 Steel-Framed Cores

Steel core was quite common for tall building design before 9/11 attack. Most tall buildings in the United States at that time were predominantly using the structural steel for the cores. The twin towers in World Trade Center is one of the examples. It used the steel core at the center of the building. Another example is Swiss Re Tower in London, also called as 30 St Mary Axe or Gherkin.

The main reason for using steel core is that it provides a light weight structure solution. The total weight of the structure is quite important for tall building design, as it will directly affect the foundation design. Therefore, a light weight solution will make a cost-effective foundation design possible.

However, the investigation of NIST NCSTAR “Final Report of the National Construction Safety Team on the Collapses of the World Trade Center Towers,” shows that fire was the major cause of the collapse of the World Trade Center as majority of its structural members were steel. So for supertall buildings designed after 9/11, steel core is rarely used.

When designing the steel-framed cores, one need to check the lateral loads, fire protection, constructability, and erection sequencing. Among them, the most important issue is the fire protections, due to the lessons learnt from the twin tower. Different fire protection strategies can be considered, such as intumescent paint, board, spray, etc.

2. The Importance of Core Design

The structural system of Twin Towers is theso-called framed tube system. An internal steel core was used for TwinTowers, the floors were made of the steel composite truss floor system, under the fire, the truss starts sagging, which pulled inward on the perimeter columns: “This led to the inward bowing of the perimeter columns and failure of the south face of WTC 1 and the east face of WTC 2, initiating the collapse of each of the towers”.

As a result of 9/11 attack, more and more design engineers began to focus on how to design a tall building to be able to resist a similar attack. Therefore, a concrete core became one of the major choices for consideration. As discussed, core is also part of evacuation route when hazards happen,therefore, concrete core would also be a good option.

Different Types of movable Bridges

Introduction

One of the great beneficiaries of globalization is the transport sector, especiallymaritime transport. With cost between the Far East and Europe of about$2 for aDVD player and$30 for a television set, even the longest way pays off! This hasled to an explosionlike increase of container traffic (e.g., between 2004 and 2005 inShanghai by 24%, in Dubai by 17%, and in Hamburg by 17%) .

Consequently, the number and size of container ships has increased permanently (Fig.1).

Fig.1. Development of container ships.

In places with sufficient space for long-ramp bridges, normally high-level bridgesare built (Fig.2). In places with restricted space, road bridges may still be built ashigh-level bridges, but railway bridges as low-level movable bridges (Fig.3).

Because in many ports high-level bridges are unfeasible due to the very restricted space, movable bridges have experienced a veritable renaissance during the lastdecades.

Fig.2. High-level bridge for road and railway traffic: The Za ́rate-Brazo Largo Bridgesacross the Parana ́River, Argentina

Fig.3. A high-level bridge for long-distance road traffic and a low-level bridge for local roadand railway traffic: the Strelasund Crossing at Stralsund, Germany

Lift bridges

Lift bridges are suitable for great spans, but their clearance is limited by the lift towers, which have a great impact on the environment, even when the bridge is closed (Fig.4). The cables linking the bridge and the counterweights may suffer fromsignificant wear.

Fig.4. Kattwyk lift bridge at Hamburg, Germany

The lift bridge has a free span of 50 m and a clearance above the low-water level of13.5 m when in service, and 40 m when opened. The lifting height, therefore, is 26.5 m.It consists of the bridge deck, a steel bridge with orthotropic plate, and four roundedtowers made of reinforced concrete (r.c.), which hoist (and hide) the concrete coun-terweights and machinery. Due to the graceful design of these towers, the often uglyappearance of lift bridges is avoide.

Swing bridges

Swing bridges are also suitable for great spans and do not limit the clearance. The biggest bridge of this type crosses the Suez Canal at El Ferdan, Egypt, with a free spanof about 300 m (Fig.5).

Fig.5.Swing bridge across the Suez Canal at El Ferdan, Egypt

The disadvantages of swing bridges include the following:

When opened, they occupy the embankment over a length of about their main span.
Due to geometrical reasons, it is impossible to have separate bridges for railways and high-ways in close vicinity.

Bascule bridges

Bascule bridges may have a single flap or two flaps and are also adequate for longspans without limiting the clearance. The connection between the two flaps may trans-mit shear forces only, or shear forces and bending moments. For great heights above the water, the counterweight may be attached to the reararm as a pendulum (Fig.6), for reduced heights it has to be integrated with it.

Fig.6. Sample of a bascule bridge with hang-on counterweight: Bridge across the Bay of Cadiz, Spain

Balance beam bridges (draw bridges)

Drawbridges, the precursors of bascule bridges, are most probably the oldest type ofmovable bridge (Fig.7). Compared to bascule bridges, they have the advantageof rather simple piers and a high architectural potential (Fig.8), but the disadvantage that they permit only rather reduced spans.

Fig.7. Vincent vanGogh – Langlois Bridge at Arles, France.Courtesy of Rheinisches Bildarchiv Köln

Fig.8. Diffené ́Bridge at Mannheim, Germany

Suspension Bridge Classification

In general, the suspension bridges can be classified according to theirspan numbers, the connection between stiffener girders, the layout of sus-penders, and anchoring conditions, etc.

1. According to Span Numbers

Based on the number of spans and towers, there are single-span, two-span,or three-span suspension bridges, as shown in Fig. 2. Among them,three-span suspension bridges with two main towers are the most commonlyused in engineering practice, like the Rainbow Bridge in Tokyo as shown in Fig. 3.

Fig.1 A suspension bridge in Izu, Japan

Fig.2. Suspension bridge classification according to span numbers. A) Single span.(B) Three-span. (C) Four (or multi) -span.

The Tsing Ma Bridge in Hong Kong and the Pingsheng Bridge in Guangdong are typical single-span suspension bridges, as shown in Figs. 4 and 5.

For multispan suspension bridges with more than two towers, the horizontal displacement of the tower tops due to live loads can be a concern and measures for controlling such displacement becomes necessary.

The Tamate Bridge built in 1928 in Japan is a typical multispan suspensionbridge, which is still in use now. Since then, several bridges were built inFrance (Pont de Château neuf-sur-Loire, 1932; Chatillon Bridge, 1951; and Bonny-sur-Loire Bridge, etc.), Switzerland (Giumaglio Footbridge).

Fig.3. The Rainbow Bridge, Tokyo

Fig.4. The Tsing Ma Bridge in Hong Kong

Fig.5. The Pingsheng Bridge Guangdong, China

Mozambique (Samora Machel Bridge, 1973), and Nepal (Dhodhara-Chandani Suspension Bridges, 2005). These bridges are generally built ina relatively short span except the Taizhou Yangtze River Bridge in China, which has three main towers and two main spans with a span length of1080 m, currently are the largest such suspension bridges.

2. According to Stiffening Girders

Based on the continuity, there are two types of stiffening girders, namelytwo-hinge or continuous types, as shown in Fig.6. Two hinge stiffening girders are commonly used for highway bridges, while the continuous stiffening girder is often used for combined highway-railway bridges to ensure the continuity between adjacent spans and to secure the smooth operation of the trains (Alampalli and Moreau, 2015).

The Akashi Kaikyo Bridge, the longest suspension bridge in the world, was designed with atwo hinged stiffening girder system.

Fig.6. Suspension bridge classification according to stiffener girders. (A) Two hinged stiffening girder. (B) Continuous stiffening girder.

3. According to Suspenders

In suspension bridges, suspenders (or hangers) can be designed as either ver-tical or diagonal, as shown in Fig.7. Vertical suspenders are more oftenused in suspension bridges, but diagonal hangers are sometimes used for the sake of increasing the damping and improving the seismic performance ofsuch bridges. For higher stiffness of a cable supported bridge, a combinedsuspension and cable-stayed cable system can also be used.

Fig.7. Suspension bridge classification according to suspenders. (A) Vertical sus-penders. (B) Inclined suspenders.

4. According to Anchoring Conditions

Based on anchoring conditions, the suspension bridges can be classified intoexternally anchored or self-anchored types, as shown in Fig.8. For externally anchored suspension bridges, the anchorages need to be built on both ends ofthe bridges to sustain the tensile forces from the main cable, which is the mostcommon type of suspension bridges.

As for self-anchored suspension bridges,the anchorages are not necessary and main cables are connected directly to thestiffening girders. In this case, however, relatively large axial compressiveforces need to be carried by the main girder and this should be consideredin the design. The San Francisco Oakland Bay Bridge and Konohana Bridgein Osaka (Fig.9) are typical self-anchored suspension bridges.

Fig.8. Suspension bridge classification according to anchors. (A) Externallyanchored suspension bridges. (B) Self anchored suspension bridges.

Fig.9. The Konohana Bridge (self-anchored suspension bridge) in Osaka, Japan.

Steel Bridges Connecting Methods

Steel bridges, as well as other steel structures, are built of steel memberssuch as beams, columns, and truss members by connections or joints. Theuse of connections can affect the fabrication method, serviceability, safety,and the cost, thus they are particularly important in the steel bridge construction.

In general, the connection design should follow the principle thatshould be safe, reliable, simply in design and fabrication, easy installation, and should be able to save the materials and costs.

In steel bridges, the often used connecting methods include rivet connection, bolt connections, and welding connections, as shown in Fig.1.

Bolt connection is used earliest since the mid-18th century and stillis being used as one of the most important connections.

The rivet connection has been used since the early 19th century; there after the welding connection was also created and used in the end of 19th century.

The welding joint became very popular and gradually replaced the rivet connection in thesteel bridge construction. With the development of high-strength bolted connection at the mid-20th century, they are also widely used in the steel bridge construction.

Fig.1 Different connecting methods. (A) Welded connection. (B) Bolted connection.(C) Riveted connection

1. Bolted Connection

Bolted connection is more frequently used than other connection methods.They are very easy to operate and no special equipment is required. This is in particular due to the development of higher strength bolts, the easy to use and strong structural steel connections become possible.

In the bolt design, two kinds of forces including tension and shear forces should be considered.Bolted connection can be divided into ordinary bolted connection or high-strength bolted connection. Both of them are easy in installation, particularlysuitable for connection in the construction site.

Ordinary bolts are easy todisassemble and are generally used in temporary connections or those needto be disassembled. High-strength bolts are easy to disassemble, and theyhave higher strength and stiffness. However, the bolted connections alsohave some disadvantages because it is necessary to drill holes and adjustthe holes during the installation.

The cutting of the holes may weakenthe steel members and increase the use steel materials due to the memberoverlapping, and also this will increase the workload in the construction.There are many reasons that may result in the failure of the bolted connec-tions, such as overloading, over torquing, or damage due to corrosion.

2. Rivet Connection

From the mechanical behavior and design points of view, the rivet connectionis very similar to ordinary bolt connection. A rivet is a permanent mechanicalfastener, which was very popular for the early steel bridges due to their good performance in plasticity, toughness, integrity under statistic load, and fatigue performance under dynamic load.

Also, quality inspection of welded connection is also relatively easy than other connection methods.However, the rivet connection is rarely used in nowadays due to disad-vantages like complex in structure, high consumption of steel, high noiseduring the construction, etc., and gradually replaced by the bolted connec-tion and welded connection.

3. Welded Connection

Welding is another connecting method used to connect steel components inthe fabrication factory and on bridge construction site. Common types ofwelds are butt welds, fillet welds, and plug welds, as shown in Fig.2.

The work place (in a factory or on site) is an important criterion fordeciding whether to choose a bolted or a welded connection. If the connec-tion is performed in a factory, it is generally most economically achievedthrough welding. Although it is technically possible for site welding, theadditional cost for setting up welding and testing facilities as well as theincreased erection time usually makes bolted connections become moreefficient.

Fig.2 Welded connections. (A) Butt joint. (B) Longitudinal joint. (C) Butt joint.(D) Corner joint-1. (E) Edge joint. (F) Transverse fillet joint. (G) Transverse fillet joint.(H) Tee joint. (I) Corner joint-2.

Types of Shotcrete and Applications

Why Shotcrete ?

1. Importance of proper application.

Properly applied shotcrete is a structurally sound and durable construction material which exhibits excellent bonding characteristics to existing concrete, rock, steel, and many other materials. It can have high strength, low

absorption, good resistance to weathering, and resistance to some forms of chemical attack. Many of the physical

properties of sound shotcrete are comparable or superior to those of conventional concrete or mortar having the

same composition. Improperly applied shotcrete may create conditions much worse than the untreated

condition.

2. Advantages of shotcrete

Shotcrete is used in lieu of conventional concrete, in most instances, for reasons of cost or convenience. Shotcrete is advantageous in situations when formwork is cost prohibitive or impractical and where forms can be reduced or eliminated, access to the work area is difficult, thin layers or variable thicknesses are required, or normal casting techniques cannot be employed. Additional savings are possible because shotcrete requires only a small, portable plant for manufacture and placement.

Shotcreting operations can often be accomplished in areas of limited access to make repairs to structures.

3. Strength of bonding.

The excellent bonding of shotcrete to other materials is often an important design consideration. The force of the impact of this pneumatically propelled material on the surface causes compaction of the shotcrete paste matrix into the fine surface irregularities and results in good adhesion to the surface. Within limits, the material is capable of supporting itself in vertical or overhead applications.

Applications

The selection of shotcrete for a particular application should be based on knowledge, experience, and a careful

study of required and achievable material performance.

The success of the shotcrete for that application is contingent upon proper planning and supervision, plus the skill and continuous attention provided by the shotcrete applicator.

The following paragraphs discuss the use of shotcrete in several of the more common applications.

1. Repair

Shotcrete can be used to repair the damaged surface of concrete, wood, or steel structures provided there is access to the surface needing repair.

The following examples indicate a few ways in which shotcrete can be used in repairs:

Bridges:

Shotcrete repair can be used for bridge deck rehabilitation, but it has generally been uneconomical for major full-thickness repairs. It is very useful, however, for beam repairs of variable depths, caps, columns, abutments, wingwalls, and underdecks from the standpoint of technique and cost.

Building:

In building repairs, shotcrete is commonly used for repair of fire and earthquake damage and deterioration, strengthening walls, and encasing structural steel for fireproofing. The repair of structural members such as beams, columns, and connections is common for structures damaged by an earthquake.

Marine Structures:

Damage to marine structures can result from deterioration of the concrete and of the reinforcement. Damaging conditions are corrosion of the steel, freezing and thawing action, impact loading, structural distress, physical abrasion from the action of waves, sand, gravel, and floating ice, and chemical attack due to sulfates. These problems can occur in most,marine structures such as bridge decks, piles, pile caps, beams, piers, navigation locks, guide walls, dams, powerhouses, and discharge tunnels. In many cases, shotcrete can be used to repair the deteriorated surfaces of these structures.

Spillway surfaces:

Surfaces subject to highvelocity flows may be damaged by cavitation erosion or abrasion erosion. Shotcrete repairs are advantageous because of the relatively short outage necessary to complete the repairs.

2. Underground excavations.

For the most part, shotcrete is used in underground excavations in rock; but on occasion, it has been successfully used in the advancement of tunnels through altered, cohesionless, and loose soils. Typical underground shotcrete applications range from supplementing or replacing conventional support materials such as lagging and steel sets, sealing rock surfaces, channeling water flows, and installing temporary support and permanent linings.

3. Slope and surface protection.

Shotcrete is often used for temporary protection of exposed rock surfaces that will deteriorate when exposed to air. Shotcrete is also used to permanently cover slopes or cuts that may erode in time or otherwise deteriorate. Slope protection should be properly drained to prevent damage from excessive uplift pressure.

Application of shotcrete to the surface of landfills and other waste areas is beneficial to prevent surface water infiltration.

4. New structures.

Shotcrete is not necessarily the fastest method of placing concrete on all jobs, but where thin sections and large areas are involved, shotcreting can be used effectively to save time. The following paragraphs describe some of the applications involved with construction of new structures.

Pools and tanks. Shotcrete has been used extensively to construct concrete swimming pools. More recently, large aquariums have been constructed using shotcrete.
Shotcrete floors and walls. Shotcrete floors in tanks and pools on well compacted subbase or on undisturbed earth have generally given excellent service. Vertical and overhead construction for walls, slabs, columns, and other structural members has been frequently shotcreted.
Shotcrete domes. Construction techniques using inflatable air-forming systems have made the construction of shotcrete shells or domes practical. These large structures have been used for residential housing, warehousing, bridge, and culvert applications.

Why do we use Large Stones in Construction

Stone has been used for many years to protect embankments, levees, river banks, and engineered features against erosion and in the construction of dams, breakwaters, and other large structures. Advantages over other materials and designs are often contingent on low cost of large-scale production and processing of stone and placement on the structure.

1. Slope Protection

a. Slope protection generally means the engineered feature composed of large-stone material constructed as a relatively thin overlay on a slope otherwise vulnerable to erosion. A bedding layer is usually included. The large-stone material is commonly called riprap. At the heart of some riprap design is the characteristic of physical flexibility. Riprap adjusts to minor flank erosion or undercutting and continues to function in its protective role.

b. One key consideration in slope protection is stone size, and the specifications for riprap should be detailed in regard to median size, gradation, and allowable tolerances. The cost advantage may be even greater where gradation requirements allow quarry-run material to be used. Even here, the importance of well defined specifications must be made clear since there are still limitations on oversize or undersize components that may require at least some separation processing.

At the other extreme from quarry-run material is riprap of narrow size range for manual placement in a keyed or fitted-stone arrangement. This labor-intensive and costly method of protecting slopes is practiced only rarely today but may be encountered in maintenance of features constructed years ago.

2. Training Structures

River training structures are relatively short, linear features constructed near the bank of a channel to control the pattern and velocity of flow. Examples are spur dikes and groins perpendicular to the bank and vane dikes more or less parallel. Rock training structures may be unzoned or earth-cored but are always designed for low cost, sometimes at the sacrifice of longevity. Groins are also used along coastlines to control sea drift currents and sand deposition. Coastal jetties train the flow of river outlets and may secondarily function as groins and breakwaters.

3. Retention Dikes

Retention dikes are designed to impound saturated materials such as from dredging. Elaborate, zoned designs are sometimes used where the waste is contaminated and the stability and the control of leachates are of high priority. Where wave attack is predictable, two or more zones of large stone are commonly incorporated, with the most critical usually being the outer armor.

As designs prescribe larger and larger stones, the problems of quality and cost increase dramatically. Smaller sized stone materials are usually more than adequate to remain stable and support the superimposed layers. Emphasis may then be redirected from high-quality stone to quantity and the need to provide large volumes of core stone.

4. Breakwaters and Jetties

a. Large breakwaters and jetties provide the outstanding examples of construction with large stone. The term rubble mound, though somewhat entrenched in the technical literature, seems inappropriate to describe these engineered structures which are commonly designed with several massive zones of different materials.

The special demands for protection against ocean wave attack lead to the ultimate among numerous possible requirements, that is very large, dense, and durable individual stones. At some large size, different for each project, the cost of constructing armor with stones exceeds that of construction with man-made concrete units such as tetrapods and dolosse. For projects near this cutoff, an especially thorough investigation of stone sources is needed. Quarries at great distance are often included, and the cost of transportation and handling must be estimated carefully.

b. Structures in deep water present special construction problems. Placement is partly remote and obscure so that quality control, quality assurance, and measurement for payment are comparably more difficult than in construction above water.

5. Zoned Embankments

a. Construction of zoned rockfill dams has constituted the largest use of large stone in some CE districts. The demands regarding material quality are usually different than for rubble mounds or even for retention dikes, and
weak rock can be used. Instead, the focus is on the immense volumes required.

b. Much of the stone material can come from excavations required as part of the project, particularly, excavations for spillways and outlet works. Quarries or stone borrow areas may be needed at least as supplementary sources. An accurate estimate of processing requirements is essential to assure sufficient volumes where suitable deposits are thin or otherwise in short supply or the material quality is marginal.

The attendant blasting, handling, and placement sometimes degrade some of the material and can cause shortfall in acceptable stone.

6. Other Uses

Other uses of large-stone material are subordinate as a group to those identified above. Nevertheless, special uses can be significant.

a. Energy Dissipators. Energy dissipators may be constructed from derrick stone placed in plunge pools to reduce erosion of foundation soil or bedrock. The high-energy environment may preclude long service life without
additions of fresh stone.

b. Structure Protection. Stone placed around the upstream end of a bridge pier is an example of protection of structures. Stone placed to stabilize and protect dikes or breakwaters primarily composed of concretefilled caissons is another example.

c. Masonry. Masonry construction is included in this manual for completeness. The high cost of this labor-intensive method largely disfavors its use today, but stone masonry still exists in CE structures and facilities and occasionally can be repaired with essentially new masonry construction.
Dimensional and cut stone for masonry are produced in a relatively few, specialized quarries.

d. Landscaping Stone. Landscaping stone is not part of an engineered structure or feature as such, but it essentially completes some CE projects. Guidance in selecting stone and in estimating or inspecting stone work for landscaping is potentially useful on a broad scale.

Spillway Function and Classification

Project functions and their overall social, environmental, and economic effects may influence the hydraulic design of the spillway. Optimization of the hydraulic design and operation requires an awareness by the designer of the reliability, accuracy, sensitivity, and possible variances of the data used.

The ever-increasing importance of environmental considerations requires that the designer maintain close liaison with other disciplines to assure environmental and other objectives are satisfied in the design.

Spillway Function

The basic purpose of the spillway is to provide a means of controlling the flow and providing conveyance from reservoir to tailwater for all flood discharges up to the spillway design flood (SDF). The spillway can be used to provide flood-control regulation for floods either in combination with flood-control sluices or outlet works, or in some cases, as
the only flood-control facility.

A powerhouse should not be considered as a reliable discharge facility when considering the safe conveyance of the spillway design flood past the dam. A terminal structure to provide energy dissipation is usually provided at the downstream end of the spillway. The degree of energy dissipation provided is dependent upon the anticipated use of the spillway and the extent of damage that will occur if the terminal structure capacity is exceeded.

The standard project flood is a minimum value used for terminal structure design discharge. The designer must keep in mind that damage to the dam structure that compromises the structural integrity of the dam is not acceptable. Acceptance of other damages should be based on an economic evaluation of the extent of damage considering the extremely infrequent flood causing the damage.

Spillway Classification

Spillways are classified into four separate categories, each of which will serve satisfactorily for specific site conditions when designed for the anticipated function and discharge.

1. Overflow Spillway

This type of spillway is normally used in conjunction with a concrete gravity dam. The overflow spillway is either gated
or ungated and is an integral part of the concrete dam structure.

Figure 1 : Chief Joseph Dam overflow spillway

2. Chute Spillway

This type of spillway is usually used in conjunction with an earth- or rock-filled dam; however, concrete gravity dams also employ chute spillways. In these cases the dam is usually located in a narrow canyon with insufficient room for an overflow spillway. The chute spillway is generally located through the abutment adjacent to the dam; however, it could be located in a saddle away from the dam structure.

Figure 2 : Mud Mountain Dam

Figure 3 : Wynoochee Dam

3. Side Channel Spillway

This type of spillway is used in circumstances similar to those of the chute spillway. Due to its unique shape, a
side channel spillway can be sited on a narrow dam abutment. Side channel spillways generally are ungated; however, there is no reason that gates cannot be employed.

Figure 4 : Townshend Dam side channel spillway

4. Limited Service Spillway

The limited service spillway is designed with the knowledge that spillway operation will be extremely infrequent, and when operation occurs, damage may well result. Damage cannot be to the extent that it would cause a catastrophic release of reservoir water.

Types of Concrete Gravity Dams

Introduction

Gravity dams are solid concrete structures that maintain their stability against design loads from the geometric shape and the mass and strength of the concrete. Generally, they are constructed on a straight axis,
but may be slightly curved or angled to accommodate the specific site conditions.

Gravity dams typically consist of a nonoverflow section(s) and an overflow section or spillway. The two general concrete construction methods for concrete gravity dams are conventional placed mass concrete and RCC.

1. Conventional concrete dams

Conventionally placed mass concrete dams are characterized by construction using materials and techniques employed in the proportioning, mixing, placing, curing, and temperature control of mass concrete (American Concrete Institute (ACI) 207.1 R-87). Typical overflow and nonoverflow sections are shown on Figures 1 and 2.

Figure 1 : Typical dam overflow section

Figure 2 : Nonoverflow section

Construction incorporates methods that have been developed and perfected over many years of designing and building mass concrete dams. The cement hydration process of conventional concrete limits the size and rate of concrete placement and necessitates building in monoliths to meet crack control requirements.

Generally using large-size coarse aggregates, mix proportions are selected to produce a low-slump concrete that gives economy, maintains good workability during placement, develops minimum temperature rise during hydration, and produces important properties such as strength, impermeability, and durability. Dam construction with conventional concrete readily facilitates installation of conduits, penstocks, galleries, etc., within the structure.

Construction procedures include batching and mixing, and transportation, placement, vibration, cooling, curing, and preparation of horizontal construction joints between lifts.

The large volume of concrete in a gravity dam normally justifies an onsite batch plant, and requires an aggregate source of adequate quality and quantity, located at or within an economical distance of the project.

Transportation from the batch plant to the dam is generally performed in buckets ranging in size from 4 to 12 cubic yards carried by truck, rail, cranes, cableways, or a combination of these methods. The maximum bucket size is usually restricted by the capability of effectively spreading and vibrating the concrete pile after it is dumped from the bucket. The concrete is placed in lifts of 5- to 10-foot depths. Each lift consists of successive layers not exceeding 18 to 20 inches. Vibration is generally performed by large one-man, air-driven, spud-type vibrators.

Methods of cleaning horizontal construction joints to remove the weak laitance film on the surface during curing include green cutting, wet sand-blasting, and high-pressure air-water jet. Additional details of conventional concrete placements are covered in EM 1110-2-2000.

The heat generated as cement hydrates requires careful temperature control during placement of mass concrete and for several days after placement. Uncontrolled heat generation could result in excessive tensile stresses due to extreme gradients within the mass concrete or due to temperature reductions as the concrete approaches its annual temperature cycle.

Control measures involve precooling and postcooling techniques to limit the peak temperatures and control the temperature drop. Reduction in the cement content and cement replacement with pozzolans have reduced the temperature-rise potential. Crack control is achieved by constructing the conventional concrete gravity dam in a series of individually stable monoliths separated by transverse contraction joints.

2. Roller-compacted concrete (RCC) gravity dams

The design of RCC gravity dams is similar to conventional concrete structures. The differences lie in the construction methods, concrete mix design, and details of the appurtenant structures. Construction of an RCC dam is a relatively new and economical concept.

Economic advantages are achieved with rapid placement using construction techniques that are similar to those employed for embankment dams. RCC is a relatively dry, lean, zero slump concrete material containing coarse and fine aggregate that is consolidated by external vibration using vibratory rollers, dozer, and other heavy equipment.

In the hardened condition, RCC has similar properties to conventional concrete. For effective consolidation, RCC must be dry enough to support the weight of the construction equipment, but have a consistency wet enough to permit
adequate distribution of the past binder throughout the mass during the mixing and vibration process and, thus,
achieve the necessary compaction of the RCC and prevention of undesirable segregation and voids.

Load Test On Piles Foundation

Pile load test are usually carried out that one or some of the following reasons are fulfilled:

To obtain back-figured soil data that will enable other piles to be designed.
To confirm pile lengths and hence contract costs before the client is committed to over all job costs.
To counter-check results from geotechnical and pile driving formulae
To determine the load-settlement behaviour of a pile, especially in the region of the anticipated working load that the data can be used in prediction of group settlement.
To verify structural soundness of the pile.

Test loading: There are four types of test loading:

compression test
uplift test
lateral-load test
torsion-load test

the most common types of test loading procedures are Constant rate of penetration (CRP) test and the maintained load test (MLT).

CRP (constant rate of penetration)

In the CRP (constant rate of penetration) method, test pile is jacked into the soil, the load being adjusted to give constant rate of downward movement to the pile. This is maintained until point of failure is reached.

Failure of the pile is defined in to two ways that as the load at which the pile continues to move downward without further increase in load, or according to the BS, the load which the penetration reaches a value equal to one-tenth of the diameter of the pile at the base.

Fig.1 – Test being carried out

Fig.1, In the cases of where compression tests are being carried out, the following methods are usually employed to apply the load or downward force on the pile:

A platform is constructed on the head of the pile on which a mass of heavy material, termed “kentledge” is placed. Or a bridge, carried on temporary supports, is constructed over the test pile and loaded with kentledge. The ram of a hydraulic jack, placed on the pile head, bears on a cross-head beneath the bridge beams, so that a total reaction equal to the weight of the bridge and its load may be obtained.

MLT (the maintained increment load test)

Fig.2, the maintained increment load test, kentledge or adjacent tension piles or soil anchors are used to provide a reaction for the test load applied by jacking(s) placed over the pile being tested. The load is increased in definite steps, and is sustained at each level of loading until all settlements has either stop or does not exceed a specified amount of in a certain given period of time.

Fig.2 – Test load arrangement using kentledge

Classification of piles foundation

Classification of pile with respect to load transmission and functional behaviour

End bearing piles (point bearing piles)
Friction piles (cohesion piles )
Combination of friction and cohesion piles

End bearing piles

These piles transfer their load on to a firm stratum located at a considerable depth below the base of the structure and they derive most of their carrying capacity from the penetration resistance of the soil at the toe of the pile (see figure 1.1).

The pile behaves as an ordinary column and should be designed as such. Even in weak soil a pile will not fail by buckling and this effect need only be considered if part of the pile is unsupported, i.e. if it is in either air or water. Load is transmitted to the soil through friction or cohesion. But sometimes, the soil surrounding the pile may adhere to the surface of the pile and causes “Negative Skin Friction” on the pile.

This, sometimes have considerable effect on the capacity of the pile. Negative skin friction is caused by the drainage of the ground water and consolidation of the soil. The founding depth of the pile is influenced by the results of the site investigate on and soil test.

Fig.1.1 – End Bearing Piles

Friction or cohesion piles

Carrying capacity is derived mainly from the adhesion or friction of the soil in contact with the shaft of the pile (see fig 1.2).

Fig.1.2 – Friction Pile

Cohesion piles

These piles transmit most of their load to the soil through skin friction. This process of driving such piles close to each other in groups greatly reduces the porosity and compressibility of the soil within and around the groups. Therefore piles of this category are some times called compaction piles.

During the process of driving the pile into the ground, the soil becomes moulded and, as a result loses some of its strength. Therefore the pile is not able to transfer the exact amount of load which it is intended to immediately after it has been driven. Usually, the soil regains some of its strength three to five months after it has been driven.

Friction piles

These piles also transfer their load to the ground through skin friction. The process of driving such piles does not compact the soil appreciably. These types of pile foundations are commonly known as floating pile foundations.

Combination of friction piles and cohesion piles

An extension of the end bearing pile when the bearing stratum is not hard, such as a firm clay. The pile is driven far enough into the lower material to develop adequate frictional resistance.

A farther variation of the end bearing pile is piles with enlarged bearing areas. This is achieved by forcing a bulb of concrete into the soft stratum immediately above the firm layer to give an enlarged base.

A similar effect is produced with bored piles by forming a large cone or bell at the bottom with a special reaming tool. Bored piles which are provided with a bell have a high tensile strength and can be used as tension piles (see fig.1.3)

Fig 1.3. Under-reamed base enlargement to a bore-and-cast-in-situ pile

Classification of pile with respect to type of material

Timber
Concrete
Steel
Composite piles

Timber piles

Used from earliest record time and still used for permanent works in regions where timber is plentiful. Timber is most suitable for long cohesion piling and piling beneath embankments. The timber should be in a good condition and should not have been attacked by insects.

For timber piles of length less than 14 meters, the diameter of the tip should be greater than 150 mm. If the length is greater than 18 meters a tip with a diameter of 125 mm is acceptable. It is essential that the timber is driven in the right direction and should not be driven into firm ground.

As this can easily damage the pile. Keeping the timber below the ground water level will protect the timber against decay and putrefaction. To protect and strengthen the tip of the pile, timber piles can be provided with toe cover. Pressure creosoting is the usual method of protecting timber piles.

Concrete pile

Precast concrete Piles or Pre fabricated concrete piles : Usually of square (see fig 1.4 b), triangle, circle or octagonal section, they are produced in short length in one metre intervals between 3 and 13 meters. They are pre-caste so that they can be easily connected together in order to reach to the required length (fig 1.4 a) .

This will not decrease the design load capacity. Reinforcement is necessary within the pile to help withstand both handling and driving stresses. Pre stressed concrete piles are also used and are becoming more popular than the ordinary pre cast as less reinforcement is require.

Fig 1.4a – Concrete pile connecting detail

Fig 1.4b – Squared pre-cast concert pile

The Hercules type of pile joint (Figure 1.5) is easily and accurately cast into the pile and is quickly and safely joined on site. They are made to accurate dimensional tolerances from high grade steels.

Fig 1.5 – Hercules type of pile joint

Driven and cast in place Concrete piles

Two of the main types used in the UK are: West’s shell pile : Pre cast, reinforced concrete tubes, about 1 m long, are threaded on to a steel mandrel and driven into the ground after a concrete shoe has been placed at the front of the shells. Once the shells have been driven to specified depth the mandrel is withdrawn and reinforced concrete inserted in the core. Diameters vary from 325 to 600 mm.

Franki Pile: A steel tube is erected vertically over the place where the pile is to be driven, and about a metre depth of gravel is placed at the end of the tube. A drop hammer, 1500 to 4000kg mass, compacts the aggregate into a solid plug which then penetrates the soil and takes the steel tube down with it. When the required depth has been achieved the tube is raised slightly and the aggregate broken out.

Dry concrete is now added and hammered until a bulb is formed. Reinforcement is placed in position and more dry concrete is placed and rammed until the pile top comes up to ground level.

Steel piles

Steel piles: steel/ Iron piles are suitable for handling and driving in long lengths. Their relatively small cross-sectional area combined with their high strength makes penetration easier in firm soil. They can be easily cut off or joined by welding. If the pile is driven into a soil with low pH value, then there is a risk of corrosion, but risk of corrosion is not as great as one might think. Although tar coating or cathodic protection can be employed in permanent works.

Fig 1.6 – Steel piles cross-sections

It is common to allow for an amount of corrosion in design by simply over dimensioning the cross-sectional area of the steel pile. In this way the corrosion process can be prolonged up to 50 years. Normally the speed of corrosion is 0.2-0.5 mm/year and, in design, this value can be taken as 1mm/year.

Composite piles

Combination of different materials in the same of pile. As indicated earlier, part of a timber pile which is installed above ground water could be vulnerable to insect attack and decay. To avoid this, concrete or steel pile is used above the ground water level, whilst wood pile is installed under the ground water level (see figure 1.7).

Fig 1.7 – Protecting timber piles from decay:
a) by pre-cast concrete upper section above water level.
b) by extending pile cap below water level

Classification of pile with respect to effect on the soil

A simplified division into driven or bored piles is often employed.

Driven piles

Driven piles are considered to be displacement piles. In the process of driving the pile into the ground, soil is moved radially as the pile shaft enters the ground. There may also be a component of movement of the soil in the vertical direction.

Fig 1.8 – Driven Piles

Bored piles

Bored piles(Replacement piles) are generally considered to be non-displacement piles a void is formed by boring or excavation before piles is produced. Piles can be produced by casting concrete in the void.

Some soils such as stiff clays are particularly amenable to the formation of piles in this way, since the bore hole walls do not requires temporary support except cloth to the ground surface. In unstable ground, such as gravel the ground requires temporary support from casing or bentonite slurry. Alternatively the casing may be permanent, but driven into a hole which is bored as casing is advanced.

A different technique, which is still essentially non-displacement, is to intrude, a grout or a concrete from an auger which is rotated into the granular soil, and hence produced a grouted column of soil.

There are three non-displacement methods: bored cast- in – place piles, particularly pre-formed piles and grout or concrete intruded piles.

The following are replacement piles:

Augered
Cable percussion drilling
Large-diameter under-reamed
Types incorporating pre caste concrete unite
Drilled-in tubes
Mini piles