30772672 Steel Trusses
Transcript of 30772672 Steel Trusses
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Table of content
Introduction..3 Historical development..4
Truss Definition5
Characteristics of trusses..5
Modeling of trusses6
Behavior of truss..7 FULL-SCALETESTING..7
General design principles .8 Optimum depth of truss girder ..8 Design of compression chord members ..8 Design of tension chord members..8 Design of vertical and diagonal members.9
Lateral bracing for truss bridges..9
Trusses classifications11According the row materials ..11
y Timber truss11y Steel trusses.11
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According to plane .11y Plane truss..11y Space Truss12
Space grids..12 Double layer grids12
According Functions.13y Bridges truss..13y Roof truss system.14y Girder and valley truss system14
According s
hape .15
Advantage of trussesAdvantages of space truss.18Advantages of steel pipes.18Advantage of bridge truss..18
Components of the truss .....19
Connecting systems (joints ).....19 Nodular systems19Mero connector19
y Tuball..19y Octatube.19y Plate connector..20
References.....21
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Introduction:
Trusses are a structural frame usually fabricated from pieces of metal or timber
to form a series of triangles lying in a single plane. The linear members are subject
only to compression or tension. The horizontal pieces forming the top and bottom
of the truss are called the chords, and the sloping and vertical pieces connecting the
chords are collectively called the web.
The truss exerts no thrust but only downward pressure; supporting walls
require no buttressing or extra thickening. Trusses have been used extensively in
roofing and bridges. Wood trusses were probably first used in primitive dwellings
c. 2500 BC. Wood was replaced by iron, which in turn was succeeded by steel.
Trusses usually contain straight members that connected to form collection of
triangles to achieve required stability to the constructions; loads in truss applied at
the joints where the members connected and that are joints behave as a pin
connection. These characteristics mean that trusses are only axial member forces
and so all the members parts used in resisting loads and this cause efficiency in
using row material; and so the trusses are light structures.
Best sections for the trusses are the circular or rectangular sections since there
is no week axis like W shapes or other shapes
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Historical development:
The early trusses was simple and small , its function was to help people to
cross the rivers and valleys, trusses principle which the bridges was its first
application caused a commercial revolution in Europe and was the beginning of thenew life style.
Andrea Palladio, a Venetian architect (1518-1580), is usually credited as the
first to describe the form of structure we recognize as a truss, as presented in his
Four Books of Architecture, he was the first to publish information known to many
at that time, including examples constructed (and possibly still extant). In either
event, little attention was paid to his writings until the middle of the 18th
century.
Most trusses was not covered timber bridges, although the oft-cited Schaffhausen
Bridge over the Rhine River, constructed by the Grubenmann brothers in 1758,
which included an awkward and inefficient timber roof, was an impressive two-span (171-ft) and (193-ft) bridge. These early timber bridges consisted of piles
driven into the riverbed, with timber beams spanning longitudinally between pile
caps.
In the first half of the 19th Century, there were many designs and patents,
notably by Town (1820, Canfield (1833, iron truss bridge), Howe (1840), Whipple
(1841. Trusses allowed using relatively short elements, first timber and later iron
and steel, in order to construct much bigger overall span length. These trusses used
simple, axial tension and compression members, and the corresponding tension andcompression material properties rather than bending.
The iron material allowed to achieve much increased span lengths compared to
timber; for example Linville build a 320 ft. span over the Ohio River in 1864, and
a 519 ft. truss of the Cincinnati Southern bridge in 1876. However, the real
progress in building big and reliable truss bridges took place at the end of the 19th
Century and continued through the beginning of the 20th Century. It was related to
the developments in manufacturing of steel, and specifically the mass production
using the Bessemer process. The new material was approximately one quarter
stronger than iron and of better quality and homogeneity. At the same time, therewas rapid industrial development requiring increased transportation of materials,
goods, and people. Bigger, faster, and frequent trains with heavier and stronger
locomotives and numerous cars had appeared. Truss bridges were very well suited
to serve such traffic and to cross even the biggest rivers (including the Mississippi,
Missouri, Ohio, etc.) and other terrain and man-made obstacles. Especially the
simple and logically constructed truss followed the natural flow of internal forces
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such as the Pratt truss, initially pin-connected and later riveted, became the
preferred form and most common design between 1885 and 1920.
Truss Definition:
Truss, in architecture and engineering, a supporting structure or framework
composed of beams, girders, or rods commonly of steel or wood lying in a single
plane. A truss usually takes the form of a triangle or combination of triangles, since
this design ensures the greatest rigidity. Trusses are used for large spans and heavy
loads, especially in bridges and roofs. Their open construction is lighter than, yet
just as strong as, a beam with a solid web between upper and lower lines. The
members are known as tie-beams, posts, rafters, and struts; the distance over whichthe truss extends is called the span. The upper and lower lines or beams are
connected by web members.
External forces and reactions to those forces are considered to act only at the
nodes and result in forces in the members which are either tensile or compressive
forces. Moments (torsional forces) are explicitly excluded because, and only
because, all the joints in a truss are treated as revolutes.
Characteristics of trusses
A truss is composed of triangles because of the structural stability of that
shape and design. A triangle is the simplest geometric figure that will not change
shape when the lengths of the sides are fixed. In comparison, both the angles and
the lengths of a four-sided figure must be fixed for it to retain its shape.
The simplest form of a truss is one single triangle. This type of truss is seen in
a framed roof consisting of rafters and a ceiling joist. Because of the stability of
this shape and the methods of analysis used to calculate the forces within it, a truss
composed entirely of triangles is known as a simple truss.
A space frame truss is a three-dimensional framework of members pinned at their
ends. A tetrahedron shape is the simplest space truss, consisting of six members
which meet at four joints.
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Modeling of trusses:
Varoglu & Barrett made one of the first attempts to model roof truss systems
by developing a structural analysis program for roof systems (SAR) at Forintek
Canada Corp. Varoglu later used the results of the tests conducted by Wolfe &
McCarthy and Wolfe & LaBissoniere to verify the program. He found good
agreement (within 56%) between the vertical deflection predicted by SAR and
experimental results. Larger errors were observed in some trusses due to the
interaction between the supporting walls and the side trusses. He finally concluded
that system response is significantly better than individual truss performance.
Lam used SAR to assess load-sharing behavior of trusses in roof systems. He used
parallel chord trusses with one configuration and evaluated the performance of a
single truss inside and outside the roof assembly. He found an average system
factor of 1.111.31 for tension members and 1.131.27 for compression members,
using combined dead and snow.
Cramer & Wolfe developed a roof-truss system model using the program,
ROOFSYS, to study loadsharing effects in light frame wood roof assemblies. In
the model, simple hinged connections were used. Additionally, composite action
(T-beam action) and two-way action of the sheathing were also included.
To represent roof sheathing in the direction perpendicular to the truss span,
sheathing was modeled as a single continuous beam on each side of the ridge. The
sheathing beam was rigidly connected to each truss. The strong and weak axes of
bending of the sheathing beam were perpendicular and parallel to the truss slope,
respectively.
Cramer and Mtenga. developed the NARSYS program (Nonlinear Analysis of
Roof System) for determining the strength of roof assemblies. The program
included linear elastic three dimensional frame elements to represent the wood
truss members, nonlinear springs and rigid links to represent the joint connections,
and deep beams to represent the roof sheathing.
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Behavior of truss
A vast amount of literature has been accumulated on single trusses and metal-
plate-connected (MPC) joints, but the system behavior of truss assemblies has been
studied by only a few researchers. In the past few decades, a number of
investigators have studied the structural behavior of wood truss assemblies, using
both experimental testing and computer modeling. Experimental testing of truss
assemblies is expensive and therefore only simple truss assemblies have been
tested.
FULL-SCALETESTING
Research on full-scale testing of truss assemblies has been sporadic over the
last several decades. A few studies have been conducted on different types of
assemblies, mainly highlighting load sharing among various components of an
assembly. Wolfe & McCarthy provided an excellent review of the literature onfull-scale testing of roof assemblies conducted until the early 1980s. Their
conclusion was that most of the studies suggested load sharing and assembly
interaction, but failed to quantify it.
In two major studies, Wolfe & McCarthy and Wolfe & LaBissoniere tested
four full-scale roof systems to improve design methods for light frame roof
systems. Their goal was to use the results of the tests in the development and
evaluation of analytical models capable of predicting roof system stiffness and load
capacity.
In the first study, Wolfe & McCarthy investigated the structural performance of
light frame roof assemblies with high truss stiffness variability by testing full-
scale, nine-truss assemblies.
Two-dimensional analysis can be attributed to the three-dimensional behavior
of the roof that is not considered in the simplifying assumptions. If the SDP is
used for analyzing and designing assemblies, both of the findings (concerns) of
Waltz may not be an issue.
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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. As per bridge
rules published by Railway board, the depth should not be greater than three times
width between centers of main girders. The spacing between main Trusses depends
upon the railway or road way clearances required.
Design of compression chord members
Generally, the effective length for the buckling of compression chord memberin 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.
Design of tension chord members
Tension members should be as compact as possible, but depths have to be large
enough to provide adequate space for bolts at the gusset positions and easily attach
cross beam. The width out-of-plane of the truss should be the same as that of the
verticals and diagonals so that simple lapping gussets can be provided without theneed for packing. It should be possible to achieve a net section about 85% of the
gross section by careful arrangement of the bolts in the splices. This means that
fracture at the net section will not govern for common steel grades.
In this case also, box sections are preferable for ease of maintenance but open
sections may well prove cheaper. For detailed design reader is referred to the
chapter on Design of Tension members.
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Design of vertical and diagonal members
Diagonal and vertical members are often rolled sections, particularly for the
lightly loaded members, but packing may be required for making up the rolling
margins. This fact can make welded members more economical, particularly on the
longer trusses where the packing operation might add significantly to the erection
cost.
Aesthetically, it is desirable to keep all diagonals at the same angle, even if the
chords are not parallel. This arrangement prevents the truss looking over complex
when viewed from an angle. In practice, however, this is usually overruled by the
economies of the deck structure where a constant panel length is to be preferred.
Lateral bracing for truss bridges
Lateral bracing in truss bridges is provided for transmitting the longitudinal
live loads and lateral loads to the bearings and also to prevent the compression
chords from buckling. This is done by providing stringer bracing, braking girders
and chord lateral bracing. In case of highway truss bridges, concrete deck, if
provided, also acts as lateral bracing support system.
The nodes of the lateral system coincide with the nodes of the main trusses.
Due to interaction between them the lateral system may cause as much as 6% of
the total axial load in the chords. This should be taken into account. Fig. 1 shows
the two lateral systems in its original form and its distorted form after axial
compressive loads are applied in the chords due to gravity loads.
The rectangular panels deform as indicated by the dotted lines, causing
compressive stresses in the diagonals and tensile stresses in the transverse
members. The transverse bracing members are indispensable for the good
performance of St. Andrews cross bracing system. In diamond type of lateralbracing system the nodes of the lateral system occur midway between the nodes of
the main trusses [Fig.1(c)]. They also significantly reduce the interaction with main
trusses. With this arrangement, scissors-action occurs when the chords are
stressed, and the chords deflect slightly laterally at the nodes of the lateral system.
Hence, diamond system is more efficient than the St. Andrews cross bracing
system.
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It is assumed that wind loading on diagonals and verticals of the trusses is
equally shared between top and bottom lateral bracing systems. The end portals
(either diagonals or verticals) will carry the load applied to the top chord down to
the bottom chord. In cases, where only one lateral system exists (as in Semithrough
trusses), then the single bracing system must carry the entire wind load.
Figure 1: Lateral bracing
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Trusses classifications
According the row materials:
- Timber truss:Wood trusses are the first trusses used by the human they are widely used in
single and multifamily residential, institutional, agricultural and commercial
construction. A truss is a structural frame relying on a triangular arrangement of
webs and chords to transfer loads to reaction points. This arrangement gives them
high strength- to-weight ratios, which permit longer spans than conventional
framing, and offers greater flexibility in floor plan layouts. They can be designed
in almost any shape or size, restricted only by manufacturing capabilities, shipping
limitations and handling considerations. Light frame wood trusses are
prefabricated by pressing galvanized steel truss plates into wood members that are
pre cut and assembled in a jig.
- Steel trusses:Steel trusses are frequently used in industrial and residential buildings, mainly
as roof structures. The truss members are joined with bolts and screws, or using
multiple presses joined or Rosette type connections. For medium and large span
trusses, bolted connections are usually recommended. There are examples of cold-
formed steel trusses with built up back toback lipped channel sections in chords
and single lipped channels for diagonals, joined by bolts, able to cover spans until
60 meters. Concerning the joints behavior of this type of trusses, usually they are
with eccentrically connections, and this feature must be taken into account in the
global analysis.
According to plane:
Planar truss
A planar truss lies in a single plane. They are typically used in parallel to form
roofs and bridges.
The depth of a truss, or the height between the upper and lower chords, is what
makes it an efficient structural form. A solid girder or beam of equal strengthwould have substantial weight and material cost as compared to a truss. For a given
span length, a deeper truss will require less material in the chords and greater
material in the verticals and diagonals. An optimum depth of the truss will
maximize the efficiency.
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Space Truss
Skeleton, three dimensional frame works consisting of pin connected bars are
called space trusses. They are characterized by hinged joints with no moments
or tensional resistance. All members carry only axial compression or tension.
- Space gridsA grid may be defined as two or more sets of parallel beams intersecting each
other at any angle and loaded by an external loading normal to the plane.
They are characterized as two ways or three ways depending upon whether the
members intersecting at a node run in two or three directions.
- Double layer gridA space truss can be formed by two or three layers of grids. A double
layer grid consist of two plane grids forming the top and bottom layers, parallel
to each other and interconnected by vertical and diagonal members. A spacetruss is a combination of prefabricated tetrahedral, octahedral or skeleton
pyramids or inverted pyramids having triangular, square or hexagonal basis
with top and bottom members normally not lying in the same vertical plane.
Double layer flat grid truss, having greater rigidity allow greater flexibility in
layout and permit changes in the positioning of columns. Its high rigidity
ensures that the deflections of the structures are within limits. They are usually
built from simple prefabricated units of standard shape. Due to its high
indeterminacy, buckling of any member under any concentrated load may not
lead to the collapse of the entire structure.
Figure 2: Space truss
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According Functions:
- Bridges trussTruss 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 utilized. 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 sheer 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. Some of the most commonly used trusses suitable for both road and rail
bridges are illustrated in next figure.
Figure 3: Bridges trusses
For short and medium spans it is economical to use parallel chord trusses such
as Warren truss, Pratt truss, Howe truss, etc. to minimize 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
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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
mid span.
- Roof truss systemA standard gable roof is the simplest arrangement, with gable end trusses at
both ends and common trusses spaced in between. Gable end trusses sit on the
end walls and carry roof loads directly into the wall below. Common trusses are
designed to act as bending members spanning between the exterior walls.
Figure 4: Roof truss
- Girder and valley truss systemBuildings with intersecting ridge lines can be framed as shown below.
Valley trusses are supported on top of the common trusses to form the
intersecting ridge. If a clear span opening is required where the roofsintersect, a girder truss can be used to support the valley trusses and
common trusses at the intersection. The girder trusses usually are specially
made with heavier chords and plates and can consist of a number of trusses
laminated with nails or bolts.
Figure 5: Girder and valley truss
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According shapes:
- Howe
Figure 6:Howe truss
- Fink
Figure 7:Fink truss
- Triangular (Kingpost)
Figure 8: Triangular truss
These three trusses may be simple span, multiple bearing, or cantilevered.
Where the truss height exceeds approximately 3m (Height - Width restrictions vary
by location for shipping. Also plants can be limited by equipment. Some jobs may
be built one piece & shipped with an escort.)
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- MonoThis shape may be simple span, multiple spans, or cantilevered. Top
chord bearing is possible.
Figure 9: Mono truss
- InvertedThe inverted truss is used to provide a vaulted ceiling along a portion of thespan.
Figure 10: Inverted truss
- Cut- offThis shape may be used where a triangular truss will not fit. Usually
stubbed at jogged exterior or at change to vaulted ceiling in opposite
direction.
Figure 11: Cut-off truss
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- Dual SlopeThis truss provides an asymmetric roof slope.
Figure 12: Dual slope
- Vierendeel truss
The Vierendeel truss is a truss where the members are not triangulated but form rectangular openings, and is a frame with fixed joints that are
capable of transferring and resisting bending moments. Regular trusses
comprise members that are commonly assumed to have pinned joints with
the implication that no moments exist at the jointed ends.
The utility of this type of truss in buildings is that a large amount of the
exterior envelope remains unobstructed and can be used for fenestration and
door openings. This is preferable to a braced frame system, which would
leave some areas obstructed by the diagonal braces.
Figure 13: Vierendeel truss
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Advantage of truss
Advantages of space truss
y They are light, structurally efficient and use materials optimally. It can bedesigned in such a way that the total weight comes between 15 to 20kg/m2sign
y It can be built up from simple, prefabricated units of standard size and shapey The small size components simplify the handling, transportation and
erection.
y They allow great flexibility in designing layout and positioning of endsupports.
y Services such as lighting, air conditioning etc., can be integrated with spacestructures.
y The use of complicated and expensive temporary supports during erection iseliminated.
Advantages of steel pipes
y The load carrying capacity increases because of increase in moment ofinertia.
y Circular section may have as much as 30 to 40% less surface area than thatof an equivalent rolled shape and thus reduces the cost of maintenance, cost
of painting.
y There is no better section than the tabular one for torsional resistance.y Tubes are of special interest to architect from an aesthetics viewpoint.y Under dynamic loading the tube has a higher frequency of vibration than any
other cross section including a solid round bar.
Advantage of bridge truss
y the truss bridge is fairly economical in the amount of material it usesy truss bridges are more rigid than most other bridge types (this is usually an
advantage as it can help avoid problems with oscillation
y truss bridges are made where the bridge deck is between the tops of thetrusses (the trusses are under the bridge). This solves the two issuesmentioned above, but is generally only used to cross gorges where there is
plenty of room to put the trusses under the bridge without having to raise the
bridge deck high in the air
y Truss bridges are usually built mostly on-site and cheaper and easier inwork.
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Components of the truss:
- Elements (members)Axial members which may be tubes or any other shapes. Connect
between the joints and resist tension or compression forces only.
- Connectors which join the members together- Bolts connecting members with nodes.
Figure 14: Components of truss
Connecting systems (joints)
- Nodular systemsThey consist of members and nodes.
- Mero connectorThe space frames successfully due to factory mass production of standard
components and easy field assembly. It can accept as many as 18 members
y TuballIt consists of 14 of hollow sphere as cap and 3/4 as cup. It is made of steroidal
graphite. The ends of members are fitted with treated solid props by welding. It is
lighter, less expensive. Each end of a member has a cast end piece with a threaded
boring to receive a bolt. There are also other type connectors such as triodetic,
nodus, schkul etc.
y OctatubeIt is a plate connector and developed in 1973. It can be fabricated at any well
equipped workshop. The joint consist of three plates an octogonal base plate and
two half octagonal plates. Each member end is pressed to form a flat shape. A
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References:
- Design of steel structure, Prof. S.R.Satish Kumar and Prof. A.R.SanthaKumar.
- BEHAVIOUR OF COLD FORMED STEEL TRUSS BOLTED JOINTS,R. ZAHARIA and D. DUBINA, University of Timisoara.
- Deterioration of Pin-Connected Bridge Trusses, Jan Jarosz and DonSorgenfrei.
- System behaviour of truss assemblies, Rakesh Gupta, Oregon StateUniversity, USA.
- http://encyclopedia2.thefreedictionary.com/trusses.- http://en.wikipedia.org/wiki/Trusses.
- http://www.tfhrc.gov/structur/pubs/04098/03.htm