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                        TECHNICAL PAPER # 67
                     UNDERSTANDING SMALL-SCALE
                          BRIDGE BUILDING
                          Robert J. Commins
                        Technical Reviewers
                       Dr. Luis Prieto-Portar
                           Alfred Samuel
                        Amde M. Wolde-Tinsae
                            Published By 
                  1600 Wilson Boulevard, Suite 500
                    Arlington, Virginia 22209 USA
               Tel:  703/276-1800 * Fax:   703/243-1865
              Understanding Small-Scale Bridge Building            
                         ISBN: 0-86619-306-5
            [C] 1990, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in Technical
Assistance to provide an introudction to specific state-of-the-art
technologies of intrest to people in developing countries.
The papers are intended to be used as guidelines to help
people chooe technologies that are suitable to their situations.
They are not intended to provide construction or implementation
details.  People are urged to contact VITA or a similar organization
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and illustrated
almost entirely by VITA Volunteer technical experts on a purely
voluntary basis.  Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time.  VITA staff included Patrice Matthews
handling production, and Margaret Crouch as project manager.
The author of the paper, Robert J. Commins, is a retired civil
engineer who has helped VITA answer technical questions throughout
the Third World.
The paper was reviewed by Dr. Luis Prieto-Portar, the Director of
Public Works for the City of Miami, Alfred Samuel, a retired
civil engineer specializing in water power, and Amde M. Wolde-Tinsae,
a professor with the Department of Civil Engineering at
the University of Maryland.
VITA is a private, nonprofit organization that supports people
working on technical problems in developing countries.   VITA
offers information and assistance aimed at helping individuals
and groups to select and implement technologies appropriate to
their situations.  VITA maintains an international Inquiry Service,
a specialized documentation center, and a computerized
roster of volunteer technical consultants; manages long-term
field proejcts; and published a variety of technical manuals and
                  by VITA Volunteer Robert J. Commins
Bridges are a part of the transportation system of a region. They
are used to span an obstacle like a stream or chasm.   Bridges make
the system more efficient either by saving travel distance or by
enabling vehicles or pedestrians to reach places that were previously
There are four basic types of free-standing bridges: beam, arch,
truss, and suspension.  In addition, pontoon bridges, which actually
float on the surface of the water, are used in some situations.
While all bridges are built from the basic structural
units of bending, tension, and compression members, the design of
suspension and pontoon bridges is highly specialized and their
construction is usually too costly for small-scale applications.
This paper, then, limits its discussion to beam, arch, and truss
bridges (Figure 1):

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o  The beam bridge is composed of members that flex or bend where
   transverse forces are applied.  The first bridge was probably
   this type of structure: a tree that fell across a stream was
   used to cross on foot.
o  The arch bridge was developed next, first appearing in Mesopotamia
   about 4000 B.C.  The arch bridge is primarily a compression
   member, subject to forces that tend to diminish its
   length.   This type of structure built of masonry was widely used
   by the Greeks and later by the Romans.  Arches continue to be
   built, but now reinforced concrete or steel is used.
o  The truss bridge is composed of both tension and compression
   members.   A tension member is subject to forces that tend to
   increase its length.  The truss bridge was first built in the
   16th century A.D. of wood; many of the covered bridges of the
   world are still built this way.  The development of iron, and
   later of steel, made truss bridges very popular for intermediate
   spans (12 to 30 meters).  At the same time, the construction
   of beam bridges became less costly for spans under 12 m.
   Eventually they, also, were used for very heavy, longer spans.
The site for the bridge should be selected on the basis of minimal
cost and maximal convenience for users.   Most bridge locations
are dictated by such obvious factors as shortest crossing between
banks of a river or gulley, the need to join roads of a town, and
replacement of an older structure or one that cannot be crossed
during floods.  Just as there are no low-cost materials of standard
quality, there is no such thing as low-cost bridge construction.
If funds are insufficient, a smaller structure should be
Several questions must be answered before choosing the type of
bridge to build:
o  Why is a bridge needed? The local people must answer this,
   since they will not only be the primary users but probably the
   financers, builders, and maintainers of the bridge.  Local
   involvement is vital in planning this kind of project.
o  What type of traffic will the bridge carry? The type of
   traffic--pedestrians or vehicles or both--determines the design
   loads for the structure.  Figure 2 shows design loads used in

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   the United States.  Local roads authorities should be consulted
   for loading requirements.  If a structure is for vehicles,
   consideration should be given to future growth of the region
   and to traffic that may be generated by a more efficient crossing.
o  What volume of traffic will the bridge carry? The volume and
   type of traffic will determine the width of the bridge.  For a
   bridge used for pedestrians, a width of two or three meters is
   adequate.   Vehicular traffic however requires at least one lane
   of 3 to 4 meters, plus an additional width for pedestrians.  If
   the bridge is to be used by motorized vehicles, a raised sidewalk
   or curbing should be used to separate vehicular and pedestrian
   traffic.   If the bridge is one way, adequate warning signs
   for motorized vehicles should be provided.
o  What span is required? If the obstacle spanned is a ravine, the
   answer is simply the width of the gap.  In the case of a river
   the answer is more complex.
A bridge crossing a river should be above the high-water elevation
to prevent the bridge from being washed out.   It must also
provide an adequate underclearance for boats or other river
traffic.  The needed high-water elevation can usually be determined
by examining the river bank and by asking local people the
highest water they have observed.   Figure 3a illustrates a typical

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river crossing.  Figure 3b illustrates the case of a wide

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floodplain.  In this instance a hydraulic study is necessary,
since the size of the floodplain is reduced and the waterway
narrowed by the combined widths of the bridge piers.   This condition
can result in flooding upstream and increased water velocity
under the bridge.  The increase in velocity can cause severe
erosion damage at the bridge site.
a.  Ideal situation: maintaining existing waterway area will not
    affect drive flow in flood stage.
b.  The floodstage waterway area is reduced by the crosshatched
    areas, causing the high water elevation to increase.  This increase
    could cause flooding upstream and erosion at bridge
After establishing the need, design loads, width, and length of
the bridge, the services of an engineer are required to design
the foundations and superstructure.   A discussion of types of
foundations and superstructure follows, including the information
that must be supplied to the engineer.
The superstructure of a bridge includes the roadway, the footpaths,
the railings, and the supporting structural members used
to span the required opening.   Figures 4 through 8 illustrate

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types of superstructure.
Wood Beams
Wood beams (Figure 4) require structural grade timber.   Since the

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strength of various types of wood varies widely, a source of a
structural grade timber of known strength characteristics must be
established before considering this type of structure.   The wood
must be treated with preservatives to prevent rotting.
A wood structure can be built by people with ordinary carpentry
skills and tools.  The only special equipment that might be needed
is some type of lifting device if the bridge beams are of excessive
Concrete Beams
Concrete superstructures can be of the flat slab or of the beam
and slab type (both shown in Figure 5).   Selection of the type to

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be used depends on the load and span requirements of the structure.
The materials required are wood for building forms, cement,
sand and gravel, clean (potable) water, and reinforcing steel.
Construction of the forms for this type of structure can be
complex, because they must be capable of supporting the weight
of the concrete until it is cured.
The dimensions shown in Figure 5 are based on the following properties
of construction materials:
o  Wood:   Allowable stress = 100 kilograms per square centimeter;
   allowable shear parallel to grain = 10 to 15 kg/sq cm
o  Concrete:   Allowable compressive stress = 200 kg/sq cm
o  Reinforcing steel:   Allowable stress = 1400 kg/sq cm
o  Structural steel :  Allowable tensile and compressive stress in
   bending = 1400 kg/sq cm
These properties are listed to help in estimating how much material
may be needed.  They may be used for preliminary design.
Building the forms requires ordinary carpentry skills.   Placing
the reinforcing steel and placing and finishing the concrete can
be done with unskilled labor, provided that the mixture is properly
vibrated to eliminate air spaces.   Technical skills are
required to design the formwork and determine the appropriate
mixtures for the concrete.
Required equipment includes carpentry tools, a concrete mixer,
shovels, wheelbarrows, and concrete-finishing tools (trowels,
floats, straight-edge, etc.)
To avoid the need to build complex forms, sections of the structure
can be precast on the ground near the site and then lifted
into place after curing.  The weight of these members may make it
necessary to use a lifting device to set them in place and means
must be provided to hold them in place after erection.   Precasting
and lifting are more complex and dangerous than pouring the
concrete into forms that have been built in place.   In this case,
the hazards arise from removing the forms before the concrete has
cured sufficiently to bear its own weight.
Two types of steel bridges are shown: a truss (Figure 6) and a

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beam (Figure 7) system.

17p07b.gif (600x600)

The truss type of structure requires
smaller steel members but needs
extensive fabrication by a local
specialist.  Because the needed skills
are not common, truss construction
may not be an available option.
The steel beam type of structure with a wood or concrete traffic
surface can be built locally.   Carpentry skills are required for
laying the wood deck, or for building forms for the concrete
deck.  It requires the same skills to build a concrete deck as to
build a concrete bridge, but the forming is much simpler.
The needed equipment includes a lifting device to set the steel
beams or trusses in place, and ordinary carpentry tools for
laying a wood deck.  A concrete mixer, wheelbarrows, and shovels
are needed to construct a concrete deck, in addition to hand
tools and wire that are needed to place and support reinforcing
A masonry or concrete arch type of structure (shown in Figure 8)

17p08.gif (540x540)

may be considered for short span lengths of 3 to 12 meters.  This
type of structure, if built of masonry, requires skilled masons
and a local quarry for a supply of stone.   The forming for an
arch is quite complex because curved forms are required to support
the weight of the masonry or concrete.
The tools and skills required to build a concrete arch bridge are
the same as those needed to build a concrete beam bridge.  Carpentry
and masonry skills and tools are required if a masonry arch
is chosen.
Table 1 gives guidelines for selecting the type of structure to
be used for vehicular traffic.   The span lengths noted are a
general guide for bridges from 3 to 25 meters; they vary depending
on design loads.
                                       TABLE I
                           TO BE USED FOR VEHICULAR TRAFFIC
MATERIAL          SPAN LENGTH,          SKILLS           TOOLS           COMMENTS
Beam Bridge
Wood               3 to 15            Ordinary         Carpentry       Wood of known strength
                                      carpentry        tool            characteristics and use
                                                                     of wood preservatives are
Concrete           3 to 10            Ordinary         Carpentry       Reinforcing steel of
(flat slab)                            carpentry       tools,         known strength character
                                      skills for       a concrete      istics is needed.  Regular
                                      forming;         mixer,          inspection of steel and
                                      design           wheelbarrow     concrete should be made.
                                      concrete         and shovels
                                      mixes of
Concrete           3 to 15            As under Con-    As under        As under Concrete
(beam)                                 crete (flat      Concrete        (flat slab)
                                      slab)           (flat slab)
Steel              3 to 25            Ordinary         Carpentry       Steel of known strength
                                      carpentry        tools.  See    characterics.
                                      skills for       also con-
                                      forming or       crete above
                                      placing the      if concrete
                                      deck.            deck is used.
                                                      Lifting device.
Truss Bridge
Wood              15 to 25            Carpentry        Carpentry       Structural grade timber
                                                      tools and      is required and skilled
                                                      a lifting       carpenters for fitting
                                                      device          and joining are needed.
Steel             15 to 25            Steel fab-       Drills,         Truss is made up of
                                      rication         wrenches,       angles or channels, and
                                                      cutting and    skill in fabrication is
                                                      /or welding    needed.
                                                      equipment for
                                                      steel, and a
                                                      Lifting device.
Arch Bridge
Concrete           3 to 10           See Con-          See Concrete    See Concrete (flat
                                     crete (flat       (flat slab)     slab).  In addition,
                                     slab)                            skilled carpenters
                                                                     are required to build
                                                                     curved forms.
Masonry            3 to 10           Carpentry         Carpentry       Skilled masons and
                                     and masonry       and masonry     carpenters are required
                                                                     to build curves and the
                                                                     forms to support the
                                                                      structure during con-
The maximal wheel loadings and the minimal spacing between vehicles
should be established by the community or the authority
requiring the bridge.  For this purpose, an impact figure should
be added to information obtained from vehicle manufacturers.
Sidewalk (footpath) flooring and supports should be designed for
a uniform load of 400 kg/sq m, unless a load concentration is
The cost of the structure is not covered in this discussion:  it
depends on material and labor costs, and these vary widely from
region to region.
These types of superstructure require minimal maintenance:
o  Wood structures require periodic reapplication of wood preservative.
o  Steel structures require periodic painting to avoid excessive
o  Concrete structures require patching of spalled (flaked or
   chipped) areas with cement grout if they occur.
Reinforced concrete structures can be difficult to maintain and
often impossible to repair.  The best defense against the need
for maintenance is extreme care in proportioning, mixing, and
placing the concrete.  Careful placement of reinforcing is equally
Broken and spalled concrete areas should be patched; worn roadway
surfaces should be given a suitable wearing and paving coat for
protection.  Cracks should be sealed with a commercial compound
recommended for this purpose.
The foundations of a bridge include those structural units that
transmit the loads from the superstructure to the underlying
soil.  There are two types:  piers and abutments.   Piers are the
intermediate supports for multispan structures.   Abutments are
the end supports.  The types of piers and abutments to be discussed
are shown in Figures 9 and 10.   Piers and abutments are

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supported by foundations, which are of two types:   spread footings
and piles.
A spread footing (Figure 9) is a shallow foundation and is the

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more economical of the two.  It can generally be used for small-span
bridges (less than 12 meters), provided that the soil can
bear the weight (at least 10 T/sq m. Piles (Figure 10) are required

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only if soft surface material is found to be incapable of
carrying shallow footing loads.   Piling is then used to carry the
footing loads to a deeper and firmer stratum.
The use of piling requires someone skilled in soil evaluation and
boring procedures.  This person performs a soil evaluation at the
site to determine what kind of piling would be the most economical
and what equipment would be required to install the piling.
Abutments carry vertical loads from the superstructure and
lateral loads from the retained earth on one side (Fig. 10a).

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Abutments are of two types:  gravity or cantilever.  A gravity
abutment carries its load through compression, and a cantilever
abutment through a combination of bending and compression.  Since
a gravity abutment is subject to compressive loads only, it can
be constructed of masonry or unreinforced concrete.   The cantilever
abutment requires the use of reinforced concrete to withstand
the stress caused by bending.
Piers carry spans between abutments in order to shorten the deck
lengths; they are subject to the following forces:   vertical
loads from the structure and from the traffic upon it; lateral
forces due to the expansion and contraction of the superstructure
and to the braking of vehicles on the bridge; lateral forces from
water or ice due to stream flow; and lateral forces due to wind
loads on the superstructure and to traffic loads.   In the case of
small-span bridges these forces are negligible except for the
vertical loads from the superstructure and the ice pressures in
deep rivers of cold-climate areas.   If we disregard all forces
except the vertical loads from the superstructure, the pier can
be considered a compression member and can be built of masonry or
unreinforced concrete.
If unreinforced concrete abutments or piers are used, a square
mesh of 1.25 cm-diameter reinforcing rods should be placed at
30-cm horizontal and vertical intervals to help control shrinkage
and surface cracking.  Should a crack develop due to settlement or
temperature stresses, the mesh will keep the faces of the crack
in contact.
Maintenance of substructure units is normally minimal, consisting
of patching of spalled concrete or masonry.   Major maintenance
occurs only if erosion undermines abutments or piers.   In this
case filling in the eroded area and placing rock protection to
prevent further erosion are required.   As prevention, substructure
units should be inspected yearly for erosion damage or immediately
after unusual run-off.
1.  Gidlow, B. Design of Suspension Footbridge, College Camp
2.  Strung, N. Your Own T (R)oll Bridge, December.  1990
3.  Weatherfrod, G.E., Bridge construction using Logs,
    Timbers, Stones and Soil, VITA Case No. 31977, 1980
4.  Small Footbridges:  Design and Construction, GPO, 1972