Basic Bridge Terms


An important first step in understanding the principles and processes of bridge construction is learning basic bridge terminology. Although bridges vary widely in material and design, there are many components that are common to all bridges. In general, these components may be classified either as parts of a bridge superstructure or as parts of a bridge substructure.

SUPERSTRUCTURE

The superstructure consists of the components that actually span the obstacle the bridge is intended to cross and includes the following:

  • Bridge deck
  • Structural members
  • Parapets (bridge railings), handrails, sidewalk, lighting and some drainage features

The deck is the roadway portion of a bridge, including shoulders. Most bridge decks are constructed as reinforced concrete slabs, but timber decks are occasionally used in rural areas and open-grid steel decks are used in some movable bridge designs (bascule bridge). As polymers and fiber technologies improve, Fiber Reinforced Polymer (FRP) decks may be used. Bridge decks are required to conform to the grade of the approach roadway so that there is no bump or dip as a vehicle crosses onto or off of the bridge. The most common causes of premature deck failure are:

  • Insufficient concrete strength from an improper mix design, too much water, improper amounts of air entraining admixtures, segregation, or improper curing
  • Improper concrete placement, such as failure to consolidate the mix as the concrete is placed, pouring the concrete so slowly that the concrete begins the initial set, or not maintaining a placement rate.
  • Insufficient concrete cover due to improper screed settings or incorrect installation of the deck forms and/or reinforcement

A bridge deck is usually supported by structural members. The most common types are:

  • Steel I-beams and girders
  • Precast, prestressed, reinforced concrete bulb T beams
  • Precast, prestressed, reinforced concrete I beams
  • Precast, prestressed, concrete box beams
  • Reinforced concrete slabs

Secondary members called diaphragms are used as cross-braces between the main structural members and are also part of the superstructure. Parapets (bridge railings), handrails, sidewalks, lighting, and drainage features have little to do with the structural strength of a bridge, but are important aesthetic and safety items. The materials and workmanship that go into the construction of these features require the same inspection effort as any other phase of the work.

Componets of Bridge Plate ASUBSTRUCTURE

The substructure consists of all of the parts that support the superstructure. The main components are abutments or end-bents, piers or interior bents, footings, and piling. Abutments support the extreme ends of the bridge and confine the approach embankment, allowing the embankment to be built up to grade with the planned bridge deck.

When a bridge is too long to be supported by abutments alone, piers or interior bents are built to provide intermediate support. Although the terms may be used interchangeably, a pier generally is built as a solid wall, while bents are usually built with columns.

The top part of abutments, piers, and bents is called the cap. The structural members rest on raised, pedestal-like areas on top of the cap called the bridge seats. The devices that are used to connect the structural members to the bridge seats are called shoes or bearings. Abutments, bents, and piers are typically built on spread footings. Spread footings are large blocks of reinforced concrete that provide a solid base for the substructure and anchor the substructure against lateral movements.

Footings also serve to transmit loads borne by the substructure to the underlying foundation material. When the soils beneath a footing are not capable of supporting the weight of the structure above the soil, bearing failure occurs. The foundation shifts or sinks under the load, causing structure movement and damage.

In areas where bearing failure is likely, footings are built on foundation piling . These load-bearing members are driven deep into the ground at footing locations to stabilize the footing foundation. Piling transmits loads from the substructure units down to underlying layers of soil or rock.

Componets of Bridge Plate B


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Soil Improvement – Liquefaction


The main goal of most soil improvement techniques used for reducing liquefaction hazards is to avoid large increases in pore water pressure during earthquake shaking. This can be achieved by densification of the soil and/or improvement of its drainage capacity.

Vibroflotation

Vibroflotation involves the use of a vibrating probe that can penetrate granular soil to depths of over 100 feet. The vibrations of the probe cause the grain structure to collapse thereby densifying the soil surrounding the probe. To treat an area of potentially liquefiable soil, the vibroflot is raised and lowered in a grid pattern. Vibro Replacement is a combination of vibroflotation with a gravel backfill resulting in stone columns, which not only increases the amount of densificton, but provides a degree of reinforcement and a potentially effective means of drainage.

Vibroflotation Steps
Vibroflotation

Dynamic Compaction

Densifiction by dynamic compaction is performed by dropping a heavy weight of steel or concrete in a grid pattern from heights of 30 to 100 ft. It provides an economical way of improving soil for mitigation of liquefaction hazards. Local liquefaction can be initiated beneath the drop point making it easier for the sand grains to densify. When the excess porewater pressure from the dynamic loading dissipates, additional densification occurs. As illustrated in the photograph, however, the process is somewhat invasive; the surface of the soil may require shallow compaction with possible addition of granular fill following dynamic compaction.

Dynamic Compaction
Dynamic Compaction

Stone Column

As described above, stone columns are columns of gravel constructed in the ground. Stone columns can be constructed by the vibroflotation method. They can also be installed in other ways, for example, with help of a steel casing and a drop hammer as in the Franki Method. In this approach the steel casing is driven in to the soil and gravel is filled in from the top and tamped with a drop hammer as the steel casing is successively withdrawn.

Stone Column
Stone Column

Compaction Piles

Installing compaction piles is a very effective way of improving soil. Compaction piles are usually made of prestressed concrete or timber. Installation of compaction piles both densifies and reinforces the soil. The piles are generally installed in a grid pattern and are generally driven to depth of up to 60 ft.

Compaction Grouting

Compaction grouting is a technique whereby a slow-flowing water/sand/cement mix is injected under pressure into a granular soil. The grout forms a bulb that displaces and hence densifies, the surrounding soil. Compaction grouting is a good option if the foundation of an existing building requires improvement, since it is possible to inject the grout from the side or at an inclined angle to reach beneath the building.

Compaction Grouting
Compaction Grouting

Drainage Techniques

Liquefaction hazards can be reduced by increasing the drainage ability of the soil. If the porewater within the soil can drain freely, the build-up of excess pore water pressure will be reduced. Drainage techniques include installation of drains of gravel, sand or synthetic materials. Synthetic wick drains can be installed at various angles, in contrast to gravel or sand drains that are usually installed vertically. Drainage techniques are often used in combination with other types of soil improvement techniques for more effective liquefaction hazard reduction.

Verification of Improvements

A number of methods can be used to verify the effectiveness of soil improvement. In-situ techniques are popular because of the limitations of many laboratory techniques. Usually, in-situ test are performed to evaluate the liquefaction potential of a soil deposit before the improvement was attempted. With the knowledge of the existing ground characteristics, one can then specify a necessary level of improvement in terms of insitu test parameters. Performing in-situ tests after improvement has been completed allows one to decide if the degree of improvement was satisfactory. In some cases, the extent of the improvement is not reflected in in-situ test results until some time after the improvement has been completed

Image Credit: http://menardbachy.com.au


What is Liquefaction?


Liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. Liquefaction and related phenomena have been responsible for tremendous amounts of damage in historical earthquakes around the world.

Liquefaction occurs in saturated soils, that is, soils in which the space between individual particles is completely filled with water. This water exerts a pressure on the soil particles that influences how tightly the particles themselves are pressed together. Prior to an earthquake, the water pressure is relatively low. However, earthquake shaking can cause the water pressure to increase to the point where the soil particles can readily move with respect to each other.

Earthquake shaking often triggers this increase in water pressure, but construction related activities such as blasting can also cause an increase in water pressure.

When liquefaction occurs, the strength of the soil decreases and, the ability of a soil deposit to support foundations for buildings and bridges is reduced as seen in the photo above of the overturned apartment complex buildings in Niigata in 1964.

Liquefied soil also exerts higher pressure on retaining walls,which can cause them to tilt or slide. This movement can cause settlement of the retained soil and destruction of structures on the ground surface as shown below.

lateralspread3

Increased water pressure can also trigger landslides and cause the collapse of dams. Lower San Fernando dam (below) suffered an underwater slide during the San Fernando earthquake, 1971. Fortunately, the dam barely avoided collapse, thereby preventing a potential disaster of flooding of the heavily populated areas below the dam.

sanfernandodamairview3

Courtesy: http://www.ce.washington.edu


Design to Minimum Dimensions


By Javed B. Malik
First published in Concrete International Magazine, July 2007

Focusing on member size can defeat the purpose
Structural engineers generally strive to optimize the cost of structures, often by minimizing the sizes of structural members. An emphasis on minimizing the size of concrete members, however, can lead to unintended consequences that may defeat the global goal of minimizing the construction cost for the overall project. In short, it’s important to step back to consider how the individual components interact. Although this may seem rather basic, I’ve observed that problems occur often enough to warrant a reminder, especially for younger engineers and detailers.

ISSUES
Concrete members sized purely on the basis of applied loads may not be large enough to accommodate the required amount of reinforcing steel with the proper spacing between bars. Conflicts can be created by the reinforcement for the member in question, reinforcing bars from adjacent members, and embedded anchor bolts or headed studs. These conflicts can potentially lead to honeycombs and voids in the concrete, inadequate cover, and inadequate embedment. Designing individual members to minimum dimensions can also create a large number of similar, but not identical, members. This can significantly impact cost by limiting reuse of the formwork and reducing the efficiencies of workers and inspectors.

EXAMPLES
The following are some common examples where designing to minimum overall dimension can create problems. Addressing these and similar issues during the design phase saves time, reduces requests for information as well as change orders, and avoids headaches for both the contractor and the engineer.

Piers and pier caps
Sizing lightly loaded piers considering only the applied loads and allowable soil bearing capacity can result in relatively small piers. For piers supporting steel columns, this can create a conflict such as shown in Fig. 1(a), where the anchor bolts or bearing plates will not fit inside the steel cage for the pier. Obviously, this can be resolved by increasing the pier diameter as shown in Fig. 1(b), or a wider pier cap at top of piers can be installed to accommodate the anchor bolts as shown in Fig. 1(c). To minimize the number of pier sizes installed at a site, it’s preferable to change the shaft diameters in increments of at least 6 in. (150 mm) and the bell or under-ream diameters in increments of at least 12 in. (300 mm).

Spread footings
To minimize the number of different footing types, the length or width should be changed in minimum increments of 12 in. (300 mm). Before finalizing the footing thickness, the depth required to develop the column dowels or embed anchor bolts for steel columns should be checked because it may control the footing thickness (Fig. 2). An alternative to thickening the entire footing is to locally thicken it at the column location. For practical reasons, a minimum thickness of 12 in. (300 mm) is suggested.

Grade beams
To eliminate formwork, the sides of grade beams are often placed against earth, requiring a clear concrete cover of at least 3 in. (75 mm). To accommodate bend diameters at the corners of stirrups in grade beams, it’s good practice to use a minimum grade beam width of 12 or 15 in. (300 or 380 mm) as shown in Fig. 3. If the sides of the grade beams are formed, clear cover on the stirrups can be reduced to 1-1/2 in. (40 mm), and the grade beam can be made narrower. In these cases, a note should be added on the drawings requiring the contractor to increase the beam width by 1-1/2 in. (40 mm) on each side if the decision is made to eliminate forms.

Columns
It’s good practice to standardize column sizes on a job as much as possible. Ideally, all interior columns should be of one size and exterior columns of another size, if necessary. This will simplify the formwork and steel placement. It’s generally economical to keep the same column sizes for as many floors as possible and use higher strength concrete and more longitudinal reinforcement on the lower floors.

Beams
Beam dimensions, especially depth, should also be standardized on a job. It’s generally economical to use the same depth for all beams at a floor except for heavily loaded girders or spandrel beams. As shown in Fig. 4, making the beams slightly wider or narrower than the columns can help prevent interference between beam bars and vertical column bars. Although beams that are wider than the columns may be preferred to simplify formwork, the designer must also check the beam-column joint for any special reinforcement required in special moment frames for seismic applications.

Walls
Designing to the minimum thickness for walls can produce several problems. Walls are not only reinforced with vertical and horizontal steel, but sometimes have ties enclosing the vertical steel such as at boundary elements. In addition, bars from slabs, floor beams, and link beams terminate in the walls. As shown in Fig. 5, link beam bars placed in several planes can further complicate the placement and congestion of the reinforcement. If these issues are not carefully considered during design, the wall can become heavily congested at locations where several elements intersect and make it very difficult to place the bars and consolidate the concrete properly.

Tilt-up wall panels
For tilt-up walls, panel thickness is often set at about 1/48th the vertical span of the wall.[1] It’s important to note, however, the effect of architectural reveals on the net wall thickness. This is needed not only for design, but also for detailing. For example, to ensure that the wall is thick enough for embedment plates with headed studs, designers must verify that sum of the plate thickness, the stud length, and the cover on the end of the studs doesn’t exceed the net wall thickness (Fig. 6).

Using double mats of reinforcing can significantly increase the moment capacity as well as the cracked moment of inertia (and thus, the axial capacity of slender wall elements). For panels thinner than 6 in. (150 mm), however, double mats of reinforcement are not preferred as they will be located nearly on top of each other. Finally, note that a standard hook may not fit well in a thin wall panel, so it may be necessary to place the hook in the plane of the wall or use welded-bar mats.

STEPPING BACK
Although the size of structural members must be appropriate for the applied loads and material properties, this should only be considered the starting point. By simply taking a step back and looking at how various elements interface with one another, the issues discussed in this article and other, similar issues can often be easily found and corrected. Making this a continuous process during design and detailing can help avoid having to redesign elements when conflicts are found, and it can lead to a better understanding of how the structural elements interact as a whole.

Acknowledgments
The author is thankful to the members of ACI Committee 315-B, Details of Concrete Reinforcement—Constructibility, for their valuable suggestions and contributions.

References
“Tilt-Up Construction and Engineering Manual,” 6th Edition, Tilt-Up Concrete Association, Mount Vernon, IA, Aug. 2006, p. 9-2.


 

Factors Affecting Selection of Foundation


Factors affecting selection of foundation for a building can be many from the soil conditions to the type of structure and loads from the building. All the factors are considered during selection of foundation for durable building construction.

Factors Affecting Selection of Foundation are:

1. Loads from Building

The first factor considered is loads from building on the foundation. This load is a combination of dead load and imposed loads on the buildings. Other loads such as wind loads, earthquake loads, snow loads etc. are also considered based on location.

loads-from-buildings

The quantity of loads depends on the type of structure, number of floors and material of construction. As the number of floors increases, the dead load and imposed loads also increase. Choice of material for construction such as reinforced concrete or steel construction also has impacts on foundation. Reinforced concrete buildings exert more loads on the foundation compared to steel structures.

Based on the safe bearing capacity of structure and quantity of loads on foundation, type of foundation and its base area is calculated.

2. Type of Soils

Soil is a mixture of solid particles, moisture and air. Soil can be of many types such as clayey soil or expansive soil, sandy soil or loose soils etc. The soil near surface is called as top soil and below a depth of 300mm is called as sub soil. Generally subsoil is used as base for foundation for small buildings.

However, soil investigation should be carried out to know the nature of soil, depth of water table, type of soil, depth of different layers of soil and to know the bearing capacity of soil at different levels for large structures.

When the load is transferred from the structure to soil through foundations, the soil tends to consolidate and settlement of foundation occurs. This consolidation process can be quick in case of non-cohesive soils such as sands and can even take years for other soils. The complete settlement of foundation in sandy soil may occur even before the building construction has been completed. Clayey soil can hold the water for longer time and thus settlement is very slow and can take years. Soil clayey holds large amount of water, and thus settlement of foundation is large in such soils.

type-of-soils

The settlement of foundation causes cracks in building walls, beams, slabs etc. and building can even fail in case of large settlement.

settlement-of-foundation.gif

The soil investigation is necessary when the loads from the building are large and the bearing capacity cannot be estimated based on type of soil condition at site.

The soil investigation should be carried out for following information:

  • The nature and thickness of made-up ground/top soil above the sub-soil
  • The nature, thickness and stratum depth of sub-soil
  • An assessment of allowable bearing pressure
  • Groundwater levels, chemicals in the ground, etc.
  • Existing structures or hazards in the ground.

3. Type of Structure in Neighborhood:

The selection of foundation for building construction can also be done based on the type of foundation selected for the buildings in the neighboring buildings for the same types. Based on the success or failure of foundations for such buildings, decision can be taken for the selection of foundation.

4. Types of Foundations:

Types of foundation such as isolated foundations, combined footings, pile foundations and raft or mat foundations etc. based on the type of soils and loads from the buildings can be selected based on suitability and requirement.


READ ORIGINAL ARTICLE HERE:

http://theconstructor.org/geotechnical/factors-affecting-selection-of-foundation-for-buildings/10504/