Truss bridges

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Fig.  some of the trusses that are used in steel bridges
       Truss Girders, lattice girders or open web girders are efficient and
economical structural systems, since the members experience essentially axial
forces and hence the material is fully utilised. Members of the truss girder bridges
can be classified as chord members and web members. Generally, the chord
members resist overall bending moment in the form of direct tension and
compression and web members carry the shear force in the form of direct tension
or compression. Due to their efficiency, truss bridges are built over wide range of
spans. Truss bridges compete against plate girders for shorter spans, against
box girders for medium spans and cable-stayed bridges for long spans.
        For short and medium spans it is economical to use parallel chord trusses
such as Warren truss, Pratt truss, Howe truss, etc. to minimise fabrication and
erection costs. Especially for shorter spans the warren truss is more economical
as it requires less material than either the Pratt or Howe trusses. However, for
longer spans, a greater depth is required at the centre and variable depth trusses
are adopted for economy. In case of truss bridges that are continuous over many
supports, the depth of the truss is usually larger at the supports and smaller at
midspan.
        As far as configuration of trusses is concerned, an even number of bays
should be chosen in Pratt and modified Warren trusses to avoid a central bay
with crossed diagonals. The diagonals should be at an angle between 50o and
60o to the horizontal. Secondary stresses can be avoided by ensuring that the
centroidal axes of all intersecting members meet at a single point, in both vertical
and horizontal planes. However, this is not always possible, for example when
cross girders are deeper than the bottom chord then bracing members can be
attached to only one flange of the chords.
      
General design principles

Optimum depth of truss girder
       The optimum value for span to depth ratio depends on the magnitude of
the live load that has to be carried. The span to depth ratio of a truss girder
bridge producing the greatest economy of material is that which makes the
weight of chord members nearly equal to the weight of web members of truss. It
will be in the region of 10, being greater for road traffic than for rail traffic. IS:
1915-1961, also prescribes same value for highway and railway bridges. As per
bridge rules published by Railway board, the depth should not be greater than
three times width between centres of main girders. The spacing between main
truss depends upon the railway or road way clearances required.

Design of compression chord members
     
  Generally, the effective length for the buckling of compression chord
member in the plane of truss is not same as that for buckling out-of-plane of the
truss i.e. the member is weak in one plane compared to the other. The ideal
compression chord will be one that has a section with radii of gyration such that
the slenderness value is same in both planes. In other words, the member is just
likely to buckle in plane or out of plane. These members should be kept as short
as possible and consideration is given to additional bracing, if economical.
       
The effective length factors for truss members in compression may be
determined by stability analysis. In the absence of detailed analysis one can
follow the recommendations given in respective codes. The depth of the member
needs to be chosen so that the plate dimensions are reasonable. If they are too
thick, the radius of gyration will be smaller than it would be if the same area of
steel is used to form a larger member using thinner plates. The plates should be
as thin as possible without losing too much area when the effective section is
derived and without becoming vulnerable to local buckling.

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Introduction

         The main types of bridges are beam bridges,arches, cable-stayed bridges, cantilever bridges and suspension bridges,and combinations.The cable of a suspension bridge is in tension, enabling it to be much narrower and cheaper than an arch of the same span. These are usually arches, beams or girders, or cantilevers, or they may be parts of bridges, for example the suspended span of a cantilever bridge, or the deck of a cable-stayed bridge or a suspension bridge. The towers hold up the cables.  They have to be rigid enough to act as struts between the downward forces from the cables and the upward forces from the foundations.The phrase "truss bridge", however, is sometimes reserved for those which act primarily as beams, while the others are discussed under the heading of the bridges of which they form a part. You could say that a truss, like a box-girder or a pre-stressed span, is more a type of construction than a type of structure.

At various places in this website there are sections which explain that the boundaries between the various types of bridges are not completely impervious, and that in principle at least, bridges can be built that are not obviously in a simple category. The reason that the types of most bridges are obvious is that these types have become popular because they are successful, and success is greatest in the broad central regions of the available variable-space. For example, if you make an extremely flat suspension bridge, you could put the wires in a concrete matrix, and you would have a pre-stressed beam requiring no anchorages. arches - an extremely flat arch would generate enormous thrust, and a beam would be a better solution.

The same difficulty applies to many other other human activities, and indeed of many natural groups of species: although there are many genera and species which tax the powers of biologists to classify them, the vast majority fall more easily into groups. On the other hand, where there are very many closely related species, there may be sporadic disputes between "lumpers" and "splitters".

This diagram shows the length of a bridge and two definitions of span.

This chart shows the relative lengths of the longest bridges of different types, in 2004. The completion of new bridges may mean that the diagram needs updating. The spans are measured on the vertical axis, while the horizontal axis merely counts the spans in order of length. The types of materials used are greatly dependent on the span. The designer of a small footbridge may have greater freedom of choice than the designer of a large cable-stayed bridge, for example, though economic principles always play a part.
       Truss Girders, lattice girders or open web girders are efficient and
economical structural systems, since the members experience essentially axial
forces and hence the material is fully utilised. Members of the truss girder bridges
can be classified as chord members and web members. Generally, the chord
members resist overall bending moment in the form of direct tension and
compression and web members carry the shear force in the form of direct tension
or compression. Due to their efficiency, truss bridges are built over wide range of
spans. Truss bridges compete against plate girders for shorter spans, against
box girders for medium spans and cable-stayed bridges for long spans.

If we look at the distribution of the longitudinal forces within a span, they can be summarized as follows.

Arch - the average line of the forces should be as near the centre line as possible, and certainly within the kern.

Cable - the forces will automatically be distributed across the cable.

Beam - the forces should be as far from the neutral axis as possible.

Cantilever - the forces should be as far from the neutral axis as possible.

This requirement leads to the use of constructions such as I-beams, truss girders and trusses.

BRIDGES

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 Introduction
       As discussed in earlier chapters the main advantages of structural steel
over other construction materials are its strength and ductility. It has a higher
strength to cost ratio in tension and a slightly lower strength to cost ratio in
compression when compared with concrete. The stiffness to weight ratio of steel
is much higher than that of concrete. Thus, structural steel is an efficient and
economic material in bridges. Structural steel has been the natural solution for
long span bridges since 1890, when the Firth of Forth cantilever bridge, the
world's major steel bridge at that time was completed. Steel is indeed suitable for
most span ranges, but particularly for longer spans. Howrah Bridge, also known
as Rabindra Setu, is to be looked at as an early classical steel bridge in India.
This cantilever bridge was built in 1943. It is 97 m high and 705 m long. This
engineering marvel is still serving the nation, deriding all the myths that people
have about steel. [See Fig.]

   Fig. Howrah bridge

       The following are some of the advantages of steel bridges that have
contributed to their popularity in Europe and in many other developed countries.



   · They could carry heavier loads over longer spans with minimum dead weight,
leading to smaller foundations.
    · Steel has the advantage where speed of construction is vital, as many
elements can be prefabricated and erected at site.
   · In urban environment with traffic congestion and limited working space, steel
bridges can be constructed with minimum disruption to the community.
   · Greater efficiency than concrete structures is invariably achieved in resisting
seismic forces and blast loading.
   · The life of steel bridges is longer than that of concrete bridges.
   · Due to shallow construction depth, steel bridges offer slender appearance,
which make them aesthetically attractive. The reduced depth also contributes to
the reduced cost of embankments.
   · All these frequently leads to low life cycle costs in steel bridges

Advanced structural forms

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    This section introduces more advanced types of structural forms
that are adopted in steel-framed multi-storeyed buildings taller than 60 storeys.
Framed -tube structures

   Fig. 3.14(a) Framed tube (b) Braced framed tube (c)Tube-in-Tube frame
   
  The framed tube is one of the most significant modern developments in
high-rise structural form. The frames consist of closely spaced columns, 2 - 4 m
between centres, joined by deep girders. The idea is to create a tube that will act
like a continuous perforated chimney or stack. The lateral resistance of framed
tube structures is provided by very stiff moment resisting frames that form a tube
around the perimeter of the building. The gravity loading is shared between the
tube and interior columns. This structural form offers an efficient, easily
constructed structure appropriate for buildings having 40 to100 storeys.
        When lateral loads act, the perimeter frames aligned in the direction of
loads act as the webs of the massive tube cantilever and those normal to the
direction of the loading act as the flanges. Even though framed tube is a
structurally efficient form, flange frames tend to suffer from shear lag. This results
in the mid face flange columns being less stressed than the corner columns and
therefore not contributing to their full potential lateral strength. Aesthetically, the
tube looks like the grid-like façade as small windowed and is repetitious and
hence use of prefabrication in steel makes the construction faster. A typical
framed tube is shown in Fig.3.14 (a).
Braced tube structures
        Further improvements of the tubular system can be made by cross bracing
the frame with X-bracing over many stories, as illustrated in Fig. 3.14(b). This
arrangement was first used in Chicago's John Hancock Building in 1969.
        As the diagonals of a braced tube are connected to the columns at each
intersection, they virtually eliminate the effects of shear lag in both the flange and
web frames. As a result the structure behaves under lateral loads more like a
braced frame reducing bending in the members of the frames. Hence, the
spacing of the columns can be increased and the depth of the girders will be
less, thereby allowing large size windows than in the conventional framed tube
structures.
        In the braced tube structure, the braces transfer axial load from the more
highly stressed columns to the less highly stressed columns and eliminates
differences between load stresses in the columns.
Tube-in-Tube structures
        This is a type of framed tube consisting of an outer-framed tube together
with an internal elevator and service core. The inner tube may consist of braced
frames. The outer and inner tubes act jointly in resisting both gravity and lateral
loading in steel-framed buildings. However, the outer tube usually plays a
dominant role because of its much greater structural depth. This type of
structures is also called as Hull (Outer tube) and Core (Inner tube) structures. A
typical Tube-in-Tube structure is shown in Fig. 3.14c.
Bundled tube
        The bundled tube system can be visualised as an assemblage of
individual tubes resulting in multiple cell tube. The increase in stiffness is
apparent. The system allows for the greatest height and the most floor area. This
structural form was used in the Sears Tower in Chicago. In this system,
introduction of the internal webs greatly reduces the shear lag in the flanges.
Hence, their columns are more evenly stressed than in the single tube structure
and their contribution to the lateral stiffness is greater.

Loads

Share Dead load
        Dead load on the roof trusses in single storey industrial buildings consists
of dead load of claddings and dead load of purlins, self weight of the trusses in
addition to the weight of bracings etc. Further, additional special dead loads
such as truss supported hoist dead loads; special ducting and ventilator weight
etc. could contribute to roof truss dead loads. As the clear span length (column
free span length) increases, the self weight of the moment resisting gable frames
(Fig. 2.2b) increases drastically. In such cases roof trusses are more economical.
Dead loads of floor slabs can be considerably reduced by adopting composite
slabs with profiled steel sheets as described later in this chapter.
Live load
        The live load on roof trusses consist of the gravitational load due to
erection and servicing as well as dust load etc. and the intensity is taken as per
IS:875-1975.     Additional special live loads such as snow loads in very cold
climates, crane live loads in trusses supporting monorails may have to be
considered.
Wind load
        Wind load on the roof trusses, unless the roof slope is too high, would be
usually uplift force perpendicular to the roof, due to suction effect of the wind
blowing over the roof. Hence the wind load on roof truss usually acts opposite to
the gravity load, and its magnitude can be larger than gravity loads, causing
reversal of forces in truss members. The calculation of wind load and its effect on
roof trusses is explained later in this chapter.
Earthquake load
       Since earthquake load on a building depends on the mass of the building,
earthquake loads usually do not govern the design of light industrial steel
buildings. Wind loads usually govern. However, in the case of industrial buildings
with a large mass located at the roof or upper floors, the earthquake load may
govern the design.    These loads are calculated as per IS: 1893-2002. The
calculation of earthquake load and its effect on roof trusses is explained later in
this chapter.

TRANSMISSION TOWERS

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       In every country, developed and developing, the elastic power
consumption has continued to rise, the rate of growth being greater in the
developing countries on account of the comparatively low base. This in turn had
led to the increase in the number of power stations and their capacities and
consequent increase in power transmission lines from the generating stations to
the load centres. Interconnections between systems are also increasing to
enhance reliability and economy. The transmission voltage, while dependent on
th quantum of power transmitted, should fit in with the long-term system
requirement as well as provide flexibility in system operation. It should also
conform to the national and international standard voltage levels.
       In the planning and design of a transmission line, a number of
requirements have to be met. From the electrical point of view, the most
important requirement is insulation and safe clearances to earthed parts. These,
together with the cross-section of conductors, the spacing between conductors,
and the relative location of ground wires with respect to the conductors, influence
the design of towers and foundations. The conductors, ground wires, insulation,
towers and foundations constitute the major components of a transmission line.

Steel used in bridges

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Steel used for bridges may be grouped into the following three categories:
(i)     Carbon steel: This is the cheapest steel available for structural users
        where stiffness is more important than the strength. Indian steels have
        yield stress values up to 250 N/mm2 and can be easily welded. The
        steel conforming to IS: 2062 - 1969, the American ASTM A36, the
        British grades 40 and Euronorm 25 grades 235 and 275 steels belong
        to this category.
        High strength steels: They derive their higher strength and other
(ii)
        required properties from the addition of alloying elements. The steel
        conforming to IS: 961 - 1975, British grade 50, American ASTM A572
        and Euronorm 155 grade 360 steels belong to this category. Another
        variety of steel in this category is produced with enhanced resistance
        to atmospheric corrosion. These are called 'weathering' steels in
        Europe, in America they conform to ASTM A588 and have various
        trade names like ' cor-ten'.
(iii)   Heat-treated carbon steels: These are steels with the highest
        strength. They derive their enhanced strength from some form of heat-
        treatment after rolling namely normalisation or quenching and
        tempering.
   
The physical properties of structural steel such as strength, ductility, brittle
fracture, weldability, weather resistance etc., are important factors for its use in
bridge construction. These properties depend on the alloying elements, the
amount of carbon, cooling rate of the steel and the mechanical deformation of the
steel.