French term (edited): contreventements

My partner is a carpenter, he's clarified for me that travées and pannes are not the same thing.

You can have several types of "panne": panne faitière, panne sablière, panne intermédiaire and so on.

They are the horizontal pieces of wood in a wooden structure, and they have different translations depending on which term is used.

A "travée", meanwhile, is the space between pieces of wood (the span).

The way this sentence is written is a bit odd as obviously you can't install a span.

What you CAN install is this "contreventement", which is a load-bearing element in roof structures.

For wooden structures, which I believe is the case here, that can be either a "strut" or "prop".

These links may prove useful to you:
https://build.com.au/roof-framing-elements
https://en.wikipedia.org/wiki/Timber_roof_truss

Dicobat is particularly helpful for definitions in French and often gives translations in English: https://www.dicobatonline.fr

Typical bracing over one span of a multi-girder bridge (slab not shown) - https://www.steelconstruction.info/Bracing_systems
Contreventement - bracing - https://context.reverso.net/traduction/francais-anglais/Cont...
travée - 1. (= rangée) row, [de pont] span - https://www.collinsdictionary.com/dictionary/french-english/...
A bracing system is a secondary but essential part of a bridge structure. A bracing system serves to stabilize the main girders during construction, to contribute to the distribution of load effects and to provide restraint to compression flanges or chords where they would otherwise be free to buckle laterally. - https://www.steelconstruction.info/Bracing_systems
Span is the distance between two intermediate supports for a structure, e.g. a beam or a bridge. A span can be closed by a solid beam or by a rope. The first kind is used for bridges, the second one for power lines, overhead telecommunication lines, some type of antennas or for aerial tramways. - https://en.wikipedia.org/wiki/Span_(engineering)#:~:text=Spa...
Span (architecture) Span is the distance between two intermediate supports for a structure, e.g. a beam or a bridge. A span can be closed by a solid beam or of a rope. The first kind is used for bridges, the second one for power lines, overhead telecommunication lines, some type of antennas or for aerial tramways. - https://en-academic.com/dic.nsf/enwiki/890176#:~:text=Span (...
Une travée (du latin trabs, « poutre »), dans le domaine de l'architecture, est une ouverture, un espace construit ou un élément de construction délimité par deux supports verticaux constituant les points d'appuis principaux ou les pièces maîtresses d'une construction (piliers, colonnes, arcs, fermes, poutres, etc.) - https://fr.wikipedia.org/wiki/Travée
stiffening - renforcement - https://dictionnaire.reverso.net/anglais-francais/stiffening
"Contreventement" - Élément de structure assurant la résistance d’une construction à des forces qui s’exercent horizontalement et sont dues notamment au vent, aux chocs et aux secousses sismiques.On utilise aussi le terme « contreventement » pour désigner l’action qui permet d’assurer cette résistance - https://langue-francaise.tv5monde.com/decouvrir/dictionnaire...

A 'travée' is a 'bay'. A bay is the part of a structural frame between two members. These members will be vertical for a wall bay or sloping or horizontal for a roof bay. Some or all the bays of a structure may be braced (single diagonal) or cross-braced (two diagonals forming an X, see diagram attached).

A 'panne' is a 'purlin'. It is a horizontal member of a roof structure between two framing elements (the two side members of a bay). As such it braces the structure (prevents the two framing elements falling in towards or out away from each other). This might be all they mean by 'panne de contreventement', in which case all the purlins would be 'bracing purlins'. But I suspect this particular 'bracing purlin' is distinct from other purlins. Possibly it is bracing of the frame members BEFORE purlins are placed.

Then there is this:
"Une panne de contreventement est une cornière (en forme de L) qui se raccorde sur l’alaise du bas des pannes voisines pour prévenir le roulement de la panne." But this seems very specific.

Without seeing the structure it's difficult to know exactly what they are referring to.

For 'travée de contreventement' you're safe with 'braced bay', but I'd refrain from calling the 'panne' a purlin, and refer to it rather as a 'bracing element'.

" Figure 5 [see below] depicts a typical multi-floor building elevation with CROSS-BRACED BAYS beginning at the foundation level. While Figure 5 shows only ONE BAY OF BRACING, the height and size of the specific structure may call for bracing multiple bays along a given column line. As with all braced-frame configurations, it's important to establish the location of these bays quite early in a project's development."
https://www.aisc.org/why-steel/architect/engineering-basics/...

Purlin or bracing span work too. This may be a more general term for the function.