Truss in truss structure refers toTruss beamIs a kind of lattice beam structure.Truss structures are commonly used inLarge spanFactory buildings, exhibition halls, stadiums, bridges, etcPublic buildingsMedium.Truss is also called roof truss because it is mostly used in roof structure of buildings.
The stress of each member is mainly one-way tension and compressionWeb memberThe reasonable layout ofbending momentandShearDistribution.becausehorizontal directionThe tension and compression internal forces of the structure achieve self balance, and the whole structure does not produce horizontal thrust on the bearing.Flexible structure layout,Scope of applicationVery wide.In terms of bending resistance, the truss beam and the solid web beam (that is, the beam we usually see), because the tension and compression sections are concentrated at the upper and lower ends, the internal force arm is increased, so that with the same material consumption, a largerBending strength。In the aspect of shear resistance, the shear force can be gradually transferred to the bearing by reasonably arranging the web members.In this way, the truss structure can makeMaterial strengthIt can be applied to the roof structure of buildings with various spans.More importantly, it will reduce the complexity of the internalStress stateIt is transformed into a simple tension and compression stress state in the truss member, so that we can intuitively understand the distribution and transmission of force, and facilitate the change and combination of the structure.
Historical evolution
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Historical evolution of truss
Only subjected to node loadEqual straight barThe ideal hinge system is called truss structure.It is formed by the intersection of some rod axes at a pointengineering structure It is abstracted and simplified.Truss was first used in the construction of wooden bridges and roof trusses.The ancient Romans used trusses to build spansthe danubeOf the Trey River BridgeSuperstructure(found in relief in Rome),The RenaissanceDuring this period, the Italian architect (Palladio) also began to adoptWood trussLang style, Tang style and Hao style trusses were built.The earliest metal truss in England was built in 1845, which was suitable for the lattice truss similar to the Tang style wood truss. In the next year, the triangular Warren truss was adopted.
Truss type
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Trusses can be classified according to different characteristics.
1、 According to the shape of the truss, it can be divided into:
threetriangleTruss (the distribution of rod force is more uneven, and the structural layout is difficult, butInclined planeMeet the needs of roof drainage).
2、 It is divided into:
1. Simple truss (increased from a basic hinged trianglediploidComposition);
2. Combined truss (composed of several simple trussesGeometric invariant systemThe simple composition rules of);
3. Complex truss (different from other statically determinate trusses of the first two types).
3、 According to the horizontal thrust:
1. Thrust free beam truss (compared with the corresponding solid beam structure, the hollowing rate is large, the upper and lower chords resist bending, the web members mainly resist shearing, the force is reasonable, and the material is economical);
2. Arch truss with thrust（Arch ringIntegrated with arch structureentiretyGood, easy to construct, strong spanning ability, saving steel materials).
Internal force characteristics
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The stress characteristic is that the internal force of the structure is onlyAxial forceWithout bending moment and shear force.thisOnce acceptedForce characteristicThe axial force is the main internal force of the truss, which reflects the main factors of the actual structure.Actual structure (e.greinforced concreteRoof truss, riveted (bolted) or weldedSteel truss bridge）Due to the non ideal hinge of the node and other reasons, there are also small bending moments and shear forces (idealArticulationNo), which also has a small impact on the axial force (due to the rigidity of the node and the truss rodCross sectional areaAndMoment of inertiaThe ratio varies from 5% to 0.1%), which is calledSecondary internal force。
Considering the balance of each node of the truss, the node bearsConverging force systemFunction to establish the projection of each node step by stepequilibrium equation , all unknown rod forces can be calculated, this method is calledNodal method, most suitable for simple trusses.The zero bar should be determined first according to the composition characteristics, and simultaneous equations should be avoided as far as possible.Sometimes the internal force of only a few members is required, or for joint truss and complex truss, when the node method cannot work, it is necessary to useSection method。The required axial force of the member can be obtained from the balance equation by selectively truncating the member (generally no more than three members), taking the local part of the truss as the balance object, and considering the balance of any part of it.For some trusses (such as K-type trusses), the joint application of node method and section method is more effective.For complex trusses with many members orSpace trussThe best choice should be the computer method.
Various comparisons
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Comparison of various beam trusses
Beam truss can be regarded as the evolution of beam. For beams with the same span and common beam trussUniform loadThe internal force under the action is compared as follows.The shape of truss has a great influence on the internal force distribution of members.Parallel chordtrussChordThe internal force of is decreasing from midspan to both ends;andtriangleThe internal force of the truss chord increases from the middle of the span to both ends.This is because the truss is formed by the internal forces of the upper and lower chordsbending moment, the internal force of chord can be expressed as:
F=±M°/r
Where, M°Same spanSimply supported beamcorrespondingtrussSection bending moment at node location,rIs the internal force of chord to the distance centerLever arm。Under the action of uniform load, the bending moment of simply supported beam isparabolaRegularly distributed, reachingMaximum。Since the force arm of the parallel truss chord is constant, the internal force decreases from the midspan to both ends;The force arm of triangular truss chord decreases linearly from midspan to both ends, faster than M°As the speed decreases according to the parabolic law, the chord internal force increases from the middle of the span to both ends.When the top chord node of the truss is located on a parabolaBottom chordAnd each upper chord levelComponent forceyesCentroidForce arm of and M°They all change according to the parabolic law, so the internal force of each lower chord andTop chordThe horizontal components are equal, so the internal forces of each upper chord are almost equal.
Internal force andDiagonal rodThe vertical component of is equal toShear, so it increases from the middle to both ends;The top chord of parabolic truss conforms toReasonable arch axisAt this time, the vertical force acting on the top chord node is completelyAxial forceBalance, soWeb memberInternal force is zero;The internal force of the web member of the triangular truss increases from the middle to both ends.
2. Truss bridges are commonly found in railways andexpressway；Divided intoTop chordStress andBottom chordThere are two kinds of stress.
3. The truss is composed of upper chord, lower chord and web member;The form of web member can be divided into diagonal web member and straight web member;Due to the large slenderness ratio of the members themselves, although the connection between the members may be "fixed connection", the actual bending moment at the end of the member is generally small, so the design analysis can be simplified as“Articulation”。When simplifying calculation, all members are“Bifilar rod”, under pressure or tension.
4. Due to the large span of the bridgetrussThe stiffness of "out of plane" is relatively weak, so "out of plane" needs to set support.When designing a bridge, "out of plane" is usually designed as a truss form, so that the bridge will form a whole with good stiffness in both directions.
5. Some bridge decks are set at the top chord, so the force is mainly transmitted through the top chord;Some decks are set at the bottom chord. Due to the requirement of out of plane stiffness, the top chord still needs to be connected to reduce the out of plane stiffness of the top chordCalculated length。
6. TrussChordIn the middle part of the span, the force is relatively large, and gradually decreases toward the support direction;The force of web member is the largest in the support attachment, and the force of web member in the middle of the span is relatively small, even there is a theoretical "zero member".
