Table of content

Introduction………………………………..3

Historical development……………………..4

Truss Definition……………………………………………5

Characteristics of trusses………………………………………..5

Modeling of trusses………………………………………6

Behavior of truss…………………………………..7

FULL-SCALE TESTING………………………………..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 classifications………………………11 According the row materials ……………………..11

Timber truss……………………………11 Steel trusses……………………………….11

According to plane ………………….11 Plane truss………………………..11 Space Truss…………………………………12

Space grids……………………………..12 Double layer grids…………………………12

According Functions………………………………….13 Bridges truss…………………………………..13 Roof truss system………………………………….14 Girder and valley truss system………………………14

According shape ………………………………….15

Advantage of trusses Advantages of space truss…………………….18 Advantages of steel pipes………………………………….18 Advantage of bridge truss…………………………………..18

Components of the truss …………………….....19

Connecting systems (joints )……………………….....19

Nodular systems……………………………………19 Mero connector…………………………………19

Tuball……………………………………..19 Octatube……………………………….19 Plate connector…………………..20

References……………………………….....21

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

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 the new 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 18 th 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 and compression 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, there was 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

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 which the 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.

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 ForintekCanada 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 5–6%) 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 1.11–1.31 for tension members and 1.13–1.27 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.

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-SCALE TESTING 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 on full-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.

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 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.

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 the need 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.

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. Andrew’s cross bracing system. In diamond type of lateral bracing 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. Andrew’s cross bracing system.

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

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– to–back 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 strength would 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.

Space TrussSkeleton, 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 doublelayer grid consist of two plane grids forming the top and bottom layers, parallel to each other and interconnected by vertical and diagonal members. A space truss 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

According Functions:

- Bridges truss 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 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

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 system A 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 system Buildings 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 roofs intersect, 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

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.)

- Mono This 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 the span.

Figure 10: Inverted truss

- Cut- off This 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

- 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

Advantage of truss

Advantages of space truss They are light, structurally efficient and use materials optimally. It can be

designed in such a way that the total weight comes between 15 to 20 kg/m2sign

It can be built up from simple, prefabricated units of standard size and shape The small size components simplify the handling, transportation and

erection. They allow great flexibility in designing layout and positioning of end

supports. Services such as lighting, air conditioning etc., can be integrated with space

structures. The use of complicated and expensive temporary supports during erection is

eliminated.

Advantages of steel pipes The load carrying capacity increases because of increase in moment of

inertia. Circular section may have as much as 30 to 40% less surface area than that

of an equivalent rolled shape and thus reduces the cost of maintenance, cost of painting.

There is no better section than the tabular one for torsional resistance. Tubes are of special interest to architect from an aesthetics viewpoint. 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

the truss bridge is fairly economical in the amount of material it uses truss bridges are more rigid than most other bridge types (this is usually an

advantage as it can help avoid problems with oscillation truss bridges are made where the bridge deck is between the tops of the

trusses (the trusses are under the bridge). This solves the two issues mentioned 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

Truss bridges are usually built mostly on-site and cheaper and easier in work.

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

Tuball It 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.

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

member is connected to a joint by two bolts. The plates are welded together to form the shape as nest figure.

Figure 15: Octatube connector

Plate connector It can be easily fabricated in any local workshop and it can take 13 members.It consists of a 9" x 9" square M.S base plate. Two rectangular plates with chamfered top corners are welded to the base plate perpendicular to each other across the diagonal. A solid piece with a slit is welded to the pipe ends. Web members are connected to the vertical plates while chord members are connected to the base plates.

Figure 16: Plate connector

References:

- Design of steel structure, Prof. S.R.Satish Kumar and Prof. A.R.Santha Kumar.

- 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 Don Sorgenfrei.

- System behaviour of truss assemblies, Rakesh Gupta, Oregon State University, USA.

- http://encyclopedia2.thefreedictionary.com/trusses.

- http://en.wikipedia.org/wiki/Trusses.

- http://www.tfhrc.gov/structur/pubs/04098/03.htm