NCCI: Design of Roof Trusses - Upper Chord Design

Oct 29, 2026

NCCI documentation explains how to analyze and design roof trusses when common assumptions do not hold, including eccentric joints, non-nodal (distributed) loads, and reversed wind load effects. It focuses on the behavior of the upper chord, detailing when only axial forces are relevant and when bending moments arise due to continuous chords or roof sheeting/purlin arrangements. Design considerations include buckling in and out of plane, lateral restraint requirements, and relevant Eurocode references for stability and member checks.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages14

Reading Time35 min

AccessFree to download

General

Design of upper chord

General

Schematic example of non-nodal loads on upper chord

Design of lower chord

Design of web members

Eccentricity

Examples of truss to column connection and truss splice

References

NCCI: Design of roof trusses This NCCIdeals with some special cases that can occur when designing a roof truss. Such cases are for example how to treat eccentricity in the joints, non-nodal loads and reversed loadings Contents 1. General 2 2. Design of upper chord 2 3. Design of lower chord 5 4. Design of web members 9 5. Eccentricity 10 6. Examples oftruss to column connection and truss splice 11 7. References 12 1. General The traditional analysis of a truss assumes that all loads are applied in the joints and that all joints in the truss are pinned. Even though this is generally not the case, since the upper and lower chords are normally continuous and the web members are often welded to the chords, it is still a common and acceptable procedure to determine the axial forces in the members. In a situation when the dimensions of the upper chord is very large and the overall depth of the truss is small the moments due to continuous upper chord has to be considered. However, this is rarely the case for roof trusses in industrial buildings that is treated here. Bending moments have to be considered in other cases, which will be explained here. In this NCCI the considered roof trusses are assumed to be simply supported as in Figure 2.1, i.e. the truss to column connections are not fixed. The analysis of a truss is rather simple using the assumptions in the previous paragraph andwill not be discussed here but there are some special cases that complicate things: 􀂉 When for example the roof sheeting is attached directly to the truss or if purlins are used and they are not placed only in the truss joints, bending moment will be introduced into the upper chord. 􀂉 Due to eccentricity in the joints between chord and web members, moments might be introduced that need to be taken into account in the design. 􀂉 For low pitch roofs, reversed loadings due to the wind load can cause compression in the lower chord and it has to be designed for lateral buckling. 2. Design of upper chord 2.1 General If all loads are introduced in the joints, only axial forces should be considered. When the upper chord is compressed, both buckling in and out of plane have to be considered if not constrained in the lateral direction. According to Annex BB of EN 1993-1-1 [1], the buckling length is equal to the distance between the truss joints for buckling in the plane of the truss. The buckling length out of plane is the distance between the purlins. If the upper chord is a built-up member it should be designed according to §6.4 of EN1993-1-1 for buckling in the lateral direction. The above statement that the buckling length out of plane is the distance between the purlins requires that the purlins act as lateral support which means that they cannot move in their axial direction. This means that the purlins should be restrained by bracings in the roof structure and by vertical bracings in the walls. It is also possible to use the roof sheeting, if it is stiff enough to work as a diaphragm and fulfils the requirements for structural class I or II according to EN 1993-1-3 [2], instead of bracings. In SS050 examples of stabilising trusses and bracings are explained and shown. If the roof sheeting is a deck placed directly on the roof truss, i.e. without purlins, the buckling out of plane of the upper chord is restrained. This requires that the roof sheeting is stiff enough to work as a diaphragm and is in structural class I or II according to EN 1993-1-3 [2] . In the ideal case the roof will be attached to purlins that will be placed in the joints ofthe truss and only the axial forces need to be checked. This is not always the case, sometimes the roof sheeting is directly attached to the roof truss. In such cases the upper chords will be subjected not only to axial force but also to bending moment. In Figure 2.1 an example is shown with non-nodal loads applied to the roof truss. Figure 2.1 Example of a roof truss with

FAQs

What situations complicate truss design beyond the standard joint-load and pinned-joint assumptions?

Special cases include roof sheeting attached directly to the truss, purlins not placed only at truss joints, eccentricities at chord–web joints, and reversed loadings from wind that affect compression in the lower chord.

When do bending moments need to be considered in the upper chord?

Bending moments arise when loads are introduced in a non-nodal way, such as when roof sheeting is attached directly to the upper chord or when purlins introduce forces away from truss joints.

How is buckling length determined for the upper chord when compressed?

Buckling length in the plane of the truss equals the distance between truss joints, while buckling length out of plane equals the distance between purlins (assuming purlins provide lateral support).

Why must purlins be restrained if they provide lateral support out of plane?

Out-of-plane buckling length assumptions require purlins to act as lateral support without moving axially, which means they should be restrained by bracings in the roof structure and vertical bracings in the walls.

Oct 29, 2026

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