When is a truss most often used

For large trusses and heavy loads, typically found in transfer trusses in buildings, members may be rolled sections; typically UKC sections. Nodes are usually welded. Any necessary connections are completed with bolted splices within the length between nodes.

For many exposed trusses, hollow sections are chosen for their structural efficiency and for aesthetic reasons. Nodes will generally be welded in the workshop. As part of the truss design, it is essential to verify the resistance of the joints in accordance with BS EN [1] as the joint design may dominate member selection and final truss geometry. Members should be selected carefully to avoid expensive strengthening of trusses fabricated from hollow sections.

For all the types of member sections, it is possible to design either bolted or welded connections. Generally in steelwork construction, bolted site splices are preferred to welded splices for economy and speed of erection.

Where bolted connections are used, it is necessary to evaluate the consequences of 'slack' in connections. In order to reduce these consequences typically, the increase of the deflections , pre-loaded assemblies to produce non-slip joints are recommended.

Hollow sections are typically connected by welding whilst open sections are connected by bolting or welding , which will usually involve the use of gusset plates. Small trusses which can be transported whole from the fabrication factory to the site, can be entirely welded. In the case of large roof trusses which cannot be transported whole, welded sub-assemblies are delivered to site and are either bolted or welded together on site.

In light roof trusses, entirely bolted connections are less favoured than welded connections due to the requirement for gusset plates and their increased fabrication costs. It is necessary to design members in compression against out-of-plane buckling. For simply supported trusses, the upper chord is in compression for gravity loading, and the bottom chord is in compression for uplift loading. For portal trusses, each chord is partly in compression and partly in tension. Lateral restraint of the upper chord is generally given by the purlins and the transverse roof wind girder.

For the restraint of the bottom chord, additional bracing may be necessary, as shown below. Such bracing allows the buckling length of the bottom chord to be limited out of the plane of the truss to the distance between points laterally restrained; the diagonal members transfer the restraint forces to the level of the top chord, where the general roof bracing is provided. Blue - The purlin which completes the bracing in the upper region Green - The longitudinal element which closes the bracing in the lower region Red - Vertical roof bracing.

It is possible to create a horizontal wind girder at the level of the bottom chords, with longitudinal elements to stabilize all the trusses.

It is convenient to arrange a transverse wind girder at each end of the building so that the longitudinal members need act only in tension. It is necessary to provide a longitudinal wind girder between braced gable ends in buildings where the roof trusses are not 'portalized'. The general arrangement is similar to that described for a transverse wind girder :.

Although joints in trusses are often hardly pinned in reality, it is generally satisfactory and encouraged by design Standards to assume the joints are pinned and to verify the members for axial load only. If loads are applied between nodes, trusses are often analysed with continuous chords, but with all internal members pinned. These assumptions about pinned joint behaviour apply to both bolted and welded connections. Where member centre lines do not intersect at a node the joint geometry may have been adjusted to increase the strength of the joint , the additional moments produced by the eccentricity are usually allowed for in the design of the chord members.

Several questions arise in respect of the modelling of a truss. It is usually convenient to work on restricted models. For example, for a standard building, it is common and usually justified to work with 2D models portal, wind girder, vertical bracing rather than a global 3D model.

A truss can be modelled without its supporting columns when it is articulated to the columns. When the connections between elements which make up a truss are bolted, with bolts in shear and bearing category A in BS EN [1] , the clearance introduced into these connections which allows slip can have a significant effect on displacement of the nodes. In order to facilitate erection , the bolts are located in holes which are larger than the bolts themselves.

For standard bolt sizes, holes which are 2 mm bigger than the bolt are usually made usually referred to as a clearance hole. Select basic ads. Create a personalised ads profile. Select personalised ads. Apply market research to generate audience insights. Measure content performance. Develop and improve products. List of Partners vendors. Construction Design. By Juan Rodriguez.

Juan Rodriguez is a former writer with The Balance who covered large-scale construction. He is an engineer with experience managing and overseeing large civil works construction. Learn about our editorial policies. Updated on October 07, Figure Ideally, the system lines centre lines of truss members should precisely meet at the panel points. In practice, this can hardly be achieved. Trusses almost invariably have eccentric connections, which exert additional stresses on members and connectors.

Adequate margins to stress limits need to be allowed for. Rule of thumb: If system lines of three truss members cannot meet in one point e. The philosophy is to maintain the basic design of the truss as per the samples given in chapter 4 and 6 and then adjust the span of the truss and the distance between individual trusses to suit the design of the building and the load of the roof.

Proceed as follows: 1. Determine the shape of your roof, the span of the truss and the loads S' using the recommendations in chapter 2. Select the desired truss from the samples given in chapters 4 and 6 considering the span that comes closest to your design.

Compare your design span with the span of the chosen sample. Compare the load of your roof with the load of the sample. A combination of steps 3 and 4 is possible! However, the stability of the truss against lateral movement is not yet ensured. Compression chords tend to buckle in a direction perpendicular to the plane of the truss. In addition, wind loads on gables are also exerting forces perpendicular to the plane of the truss.

A lateral restraining or bracing system is therefore required. Purlins and battens may form part of this lateral bracing system. It is important that at least each panel point is braced against out-of-plane movement.

X-bracing in the plane of the roof is added to form a structurally sound system. Figure The eaves must take up the wind forces on gables and must lead these safely into the foundations either through solid walling or braces.

Lateral restraining systems including wind braces consist of minimum two braces in the plane of the upper chord. Alternatively, special bracing trusses in the upper chord plane may be used. Attic trusses allow for storage space and sometimes even added living space.

Common uses: Residential home construction projects that require an attic or additional loft living space. One downside to many trusses is that the webbing can often limit attic space. This can be a dealbreaker for homeowners looking for additional storage or living space. Luckily, there are attic trusses, which are built to allow for attic space. Attic trusses look similar to queen post trusses, but the two vertical posts are spaced further apart to allow for attic space. The wider the building structure is, the bigger the attic space will be.

The steeper the roof pitch is, the taller the attic ceiling will be. Common uses: Residential home construction projects that require vaulted ceilings. Scissor trusses make it possible to create vaulted ceilings.

The bottom chords of scissor trusses are sloped, creating the dramatic ceiling in the room below. Scissor trusses combine the speed and convenience of using pre-engineered wood trusses, while still getting the aesthetic benefit of high ceilings. Gable trusses are usually used in combination with other types of trusses. They have two top chords, one bottom chord, and multiple vertical posts. Gable trusses are built on each end of the roof framework to support roof sheathing.

From metal roofs, to asphalt shingles, to vaulted ceilings and flat roofs, there is likely a truss that can help you get the job done. Roof trusses are versatile, cost-effective, and convenient.