"Pratt TrussTruss" is a list of types of steel girders.
A truss is a system of interdependent structural elements.In buildings, support to roofs, the floors and internal loading such as services and suspended ceilings are readily available.There are a number of reasons for using trusses.
The article introduces design considerations and describes alternative forms of truss.There is discussion of pin jointed and rigid-jointed Vierendeel trusses.
An open web girder is a triangulated system of straight interdependent structural elements.The connections are often assumed to be pinned.The reactions at the supports are applied to the system by external forces.The system is a plane if all the members and applied forces are in the same plane.
There is a requirement for very long spans in airports, aircraft hangars, sports stadia roofs, auditoriums and other leisure buildings.Transfer structures are sometimes used to carry heavy loads.trusses are widely used to serve two main functions in single storey industrial buildings.
The figure below shows two types of general arrangement of a single storey building.
The structure's resistance to a global bending moment is provided by the connections between the portal trusses and the columns.The portal structure has loads applied to it by purlins and side rails.
The two column bases are pinned in the second case because the connection between the truss and a column does not resist the global bending moment.The top level of a simple structure requires bracing in both directions because of the wind on the side walls and the gable walls.Longitudinal stability is provided by a wind girder in the roof.
There are tension and compression elements.The bracing resists the shear forces and the compression and tension resistance in the top and bottom of the truss help with bending under gravity loads.There is a wide range of forms that can be created.The choice of the individual elements and overall geometry can vary.Below are some of the commonly used types.
Long span buildings can range from 20 to 100 m in span.Diagonal members are in tension for gravity loads.This type of truss is used in areas with a lot of gravity loads.The diagonal members are in tension for the load.In open buildings, this type of truss may be used where the majority of the load is from the ground.
It is possible to provide either a single or double slope to the upper part of a roof supporting truss.An example of a double is shown.
Diagonal members are either in tension or compression.The Warren truss has the same compression and tension web members as the Pratt one.A modified Warren truss can be adopted where additional members are introduced.
Industrial workshop-type buildings use the North light trusses for short spans.They allow maximum benefit to be gained from natural lighting by the use of glazing on the pitch which faces north or north east to reduce solar gain.Large column-free spaces can be found on the steeply sloping portion of the North Light truss.
Dynamic thermal modelling should be used to explore the impact of north lights on carbon emissions.Although north lights reduce the requirement for artificial lighting, they can also increase the demand for space heating by increasing the volume of the building.The Target Zero Warehouse buildings design guide gives further guidance.
The saw-tooth truss is a variation of the North light truss.The vertical face of the saw-tooth truss is similar to the North light one.
The economy of steel weight for short-span high-pitched roofs is offered by the Fink truss.There are many ways of grouping the internal members.
For the same steel weight, it is possible to get better performance with a truss than with an I beam.The difference is bigger for long spans and heavy loads.If the height of the truss is not limited by criteria other than structural efficiency, the full use of this advantage can be achieved.There is a limit on the total height of the building.Modern fabrication equipment is very efficient, but it is still more time consuming to fabricate a truss than an I beam.
The equipment of the fabrication factory, the local cost of manufacturing, and the steel unit cost all affect the balance between minimum weight and minimum cost.There is an economic solution for spans over 20 m.
ducts and pipes that are required for operation of the buildings services can be installed through the truss web.service integration
The range for the ratio of span to truss depth is 10 to 15.The external geometry of the building is determined by the architectural design.The intended use of the internal space can lead to the choice of a horizontal bottom chord.To allow maximum space to be provided, the conveyor must be hung under the chord.
There are many solutions available.The magnitude of the internal forces, the ease of connections between members, and the need to connect pre-made sections on site are all factors that affect the choice of members.The out-of-plane resistance and the resistance under reversed loading will be important when selecting members.
For smaller spans, tee sections are used for internal members.The internal members can be bolted or welded.The back-to-back angles or channels can be used for longer spans or heavier loads.
Rolling sections are typically found in UKC sections for large trusses and heavy loads.They are usually welded.Bolts are used to complete any necessary connections.
hollow sections are chosen for their structural efficiency and aesthetic reasons.The parts will be welded in the workshop.It is important to verify the resistance of the joints as the joint design may dominate member selection and final truss geometry.The members should be selected carefully so that they don't have to strengthen the trusses.
It is possible to design bolted or welded connections for all member sections.In steelwork construction bolted site splices are preferred for their economy and speed of erection.It is necessary to evaluate the consequences of slack in bolted connections.Pre-loaded assembly to produce non-slip joints are recommended in order to reduce these consequences.
Open sections are usually connected by bolting or welding with the use of gusset plates.Guidance on the design of welded joints for hollow sections is available from Tata Steel.
The small trusses can be welded whole from the fabrication factory to the site.In the case of large roof trusses which cannot be transported whole, welded sub-assemblies are delivered to site and either bolted or welded together on site.
In light roof trusses welded connections are more preferred than bolted connections due to their higher fabrication costs.
The members have to be designed against out-of-plane buckled.The upper and bottom parts of the trusses are in compression for gravity loading.The portal trusses have compression and tension in them.
The purlins and the wind girder are used to restrain the upper chord.
Additional bracing may be necessary for the restraint of the bottom chord.The diagonal members transfer the restraint forces to the level of the top chord, where the general roof bracing is provided.
The longitudinal element which closes the bracing in the lower region is called the Vertical roof bracing.
A horizontal wind girder with longitudinal elements can be created at the level of the bottom chords.
The longitudinal members need to act in tension, so it is convenient to arrange a wind girder at each end of the building.
In buildings where the roof trusses are not portalized, it is necessary to provide a longitudinal wind girder between the braced gable ends.
It is encouraged by design Standards to assume the joints are pinned and to verify the members for the load only.
With all internal members pinned, trusses are analysed with continuous chords if loads are applied.Both bolted and welded connections have the same assumptions about pinned joint behavior.
The extra moments produced by the eccentricity are usually allowed for in the design of the chord members.
It's convenient to work on restricted models.It is usually justified to work with 2D models instead of a global 3D model for a standard building.When it is articulated to the columns, a truss can be modeled.
The clearance introduced into the bolts in shear and bearing connections can have a significant effect on the displacement of the nodes.
The bolts are located in larger holes in order to facilitate erection.A clearance hole is a 2mm bigger than the bolt and is used for standard bolt sizes.
The bolt must come into contact with one of the connected parts in order for the connection to work.This slip can be considered an extension of the member in tension.The slip is considered a reduction in length that is added to the elastic shortening of the compressed member.
A significant increase in displacements can be caused by the total slip in the many different connections of a truss structure.
To control the effect of connection slack on the displacements is essential to the structure.It is necessary in order to do this.
If the chords have been modelled as continuous, they may be subjected to bending moments.
The different modes of instability are taken into account when evaluating the resistance of a member to compression.
The only things that need to be evaluated are the compressed members in the plane of the structure and out of it.
The cross-section's resistance is reduced by applying a reduction to the buckling resistance.The elastic critical force is what determines the reduction factor.
The elastic critical force for the diagonals and the vertical is determined from the length of the member.
It is possible to take the length of the plane in which the members are in uniform compression and divide it by the system length.
The length of the buckled out of plane must be taken between the support points.
In this example, the roof is supported by the truss and the purlins are at the level of the upper chord.
There are only vertical members between the top and bottom of the trusses.The chords can be near or parallel.