Foundation types and selection criteria

Foundation types and selection criteria

 

There are a number of foundation types available for geotechnical engineers.

1. Shallow foundations

Shallow foundations are the cheapest and most common type of foundations (Fig.1).

Shallow foundations are ideal for situations, when the soil immediately below the footing is strong enough to carry the building loads. In some situations soil imme-diately below the footing could be weak or compressible. In such situations, other foundation types need to be considered.

Fig.1. Shallow foundations

 

2. Mat foundations

Mat foundations are also known as raft foundations. Mat foundations, as the name implies, spread like a mat. The building load is distributed in a large area (Fig.2).

Fig.2. Mat foundations

3. Pile foundations

Piles are used when bearing soil is at a greater depth. In such situations, the load has to be transferred to the bearing soil stratum (Fig.3).

Fig.3. Pile foundations

4. Caissons

Caissons are nothing but larger piles. Instead of a pile, a group few large caissons can be utilized. In some situations, caissons could be the best alternative (Fig.4).

Fig.4. Caissons

5. Foundation selection criteria

Normally, all attempts are made to construct shallow foundations. This is the cheapest and fastest foundation type. The designer should look into bearing capacity and settle-ment when considering shallow foundations.

The geotechnical engineer needs to compute the bearing capacity of the soil immediately below the footing. If the bearing capacity is adequate, settlement needs to be computed. Settlement can be immediate or long-term. Immediate and long-term settlements should be computed (Fig.5).

Fig.5. Different foundation types

The Fig.5 shows a shallow foundation, mat foundation, pile group, and a caisson. A geotechnical engineer needs to investigate the feasibility of designing a shallow foundation due to its cheapness and ease of construction.

In the previous situation, it is clear that a weak soil layer just below the new fill may not be enough to support the shallow foundation. Settlement in soil due to loading of the footing also needs to be computed.

If shallow foundations are not feasible, then other options need to be investigated. Mat foundations can be designed to carry large loads in the presence of weak soils. Unfortunately, cost is a major issue with mat foundations. Piles can be installed as shown in the figure ending in the bearing stratum. In this situation, one needs to be careful of the second weak layer of soil below the bearing stratum.

Piles could fail due to punching into the weak stratum (Fig.6).The engineer needs to consider negative skin friction due to the new fill layer. Negative skin friction would reduce the capacity of piles (Fig.7).

Due to the new load of the added fill material, weak soil layer 1 would consolidate and settle. Settling soil would drag the piles down with it. This is known as negative skin friction or down drag.

Fig.6. Punching failure (soil punching into the weak soil beneath due to pile load)

 

Fig.7. Negative skin friction

Different Types of movable Bridges

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

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 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.

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