--- url: https://qsteelcad.com/en/blog/floor-and-roof-diaphragms language: en alternate: https://qsteelcad.com/blog/doseme-cati-diyaframi updated: 2026-11-06 publisher: RENMAE Teknoloji ve Yazılım LTD. ŞTİ. --- # Floor and roof diaphragms: who delivers the load to the braced wall? A strong braced wall is useless if nothing delivers the load to it. The diaphragm is the horizontal plate that does this job, working together with its sheathing, joists, tracks and screws. ## Short answer A diaphragm is the plate behavior of a floor or roof that collects horizontal force in its own plane and passes it to the lateral load-resisting walls. Sheathing, joists, tracks, collectors and screws work together. A strong braced wall cannot stabilise a building alone if no diaphragm and connections bring the load to it. ## Key points - A diaphragm carries horizontal force in its own plane and distributes it to the walls. - Floors are sized for earthquake action with the overstrength factor D (10.5a). - The transfer of force through connections is shown by calculation (10.5b). - Openings up to 3% of the floor area need no separate calculation; above that, the 2027 light steel regulation requires a shell model. - Floor joist bearing is at least 40 mm (10.5e). A diaphragm is a horizontal surface behaving like a deep, flat beam. When wind pushes on the facade or an earthquake drags the mass, the floor takes that force in its own plane and delivers it to the lateral load-resisting panels at the two ends. The sheathing acts as the web of the beam and the tracks along the edges act as its flanges. ![Plan view: horizontal load arrows entering the floor, with sheathing, tracks and a lateral load-resisting panel on each side](https://qsteelcad.com/assets/img/blog/diyafram-en.webp) ## Why is the diaphragm as important as the braced wall? A braced wall carries only the force that reaches it. If the force does not arrive at the wall, the bracing sits idle and the facade panels bend out of their own plane. This is why clause 10.5 of the Turkish Building Earthquake Code 2018 (TBDY 2018) treats floors under a separate heading: floors are calculated so that they transfer the earthquake forces arising in their own plane safely to the lateral load-resisting panels. ## What are floors made of in light steel? The regulation states that the floor system may be built from joists together with profiled steel sheet (trapezoidal), timber or cement-based elements. What matters is not the name of the material but that it is stiff enough to transfer the force and that its connections are defined. The regulation requires the necessary measures to be taken for this. ## What does the regulation require of a diaphragm? | Clause | Requirement | In practice | | --- | --- | --- | | 10.5(a) | The D factor in Table 4.1 is applied when sizing floors for earthquake action | Diaphragm forces are calculated in amplified form | | 10.5(b) | The transfer of forces to panels, joists and tracks through connections such as screws and bolts is shown by calculation | Connection count and spacing are a calculation output | | 10.5(c) | Where needed, collectors are used so that chord forces can be carried | The edge and collector line is treated as continuous | | 10.5(d) | If the opening area does not exceed 3% of the floor area, no separate calculation is needed | Stair and shaft openings enter the calculation from the start | | 10.5(e) | The minimum bearing for floor joist support is 40 mm | Checked in the detail over the panel | Section 7 of the 2027 light steel regulation keeps the same five conditions and adds one sentence: when the 3% limit is exceeded, the floor element is modelled as a shell element and the load transfer mechanism is shown. In other words, a floor with a large stair opening can no longer be assumed to act as a diaphragm. ## What are chord force and a collector? Staying with the beam analogy: under horizontal load, one edge of the floor works in tension and the opposite edge in compression. To carry these forces the edge line must be continuous. If the line is not resolved with a single member over its full length, the splices are connected so that they transfer this force. A collector, in turn, is the line that gathers force into a short braced panel. If only part of a wall is a lateral load-resisting panel, the force along the rest of the length is brought to that part through this line. When panel positions change in the drawing, this line changes too. ## How do openings affect the diaphragm? A stair, shaft or gallery opening is like a hole cut in the web of the beam. The force changes direction at the edges of the opening and the sheathing becomes discontinuous. For small openings the regulation does not ask for a separate calculation. Once the limit is exceeded, you show how the force flows around the opening. ## How does the roof plane work? The roof works on the same logic: it gathers the force arriving at the top of the upper-storey walls and passes it to the walls. The sloped plane, truss bottom chord, ceiling sheathing and blocking are considered together. The diagonals in the truss web are not the roof's lateral load system; they are the truss's way of carrying vertical load. How the roof passes its horizontal force to the wall top track is a separate detail. Read next: [Bracing, bridging, diaphragm, temporary prop: four look-alike duties](https://qsteelcad.com/en/blog/bracing-bridging-diaphragm-temporary-props) ## A checklist for diaphragms - Is the sheathing or sheet type and thickness defined? - Is the screw spacing of the sheathing stated in the project? - Is the connection from the floor to the panel track written down? - Is the edge line continuous and are the splices resolved? - Has the opening ratio been checked against the 3% limit? - Is the joist bearing at least 40 mm? - Are the braced panels where the diaphragm can deliver force to them? ## How are floors and roofs modelled in QSteelCAD? Automatic roof builds trusses, ceiling and roof panels. Because panels, trusses and walls sit in the same three-dimensional model, it is easy to see which panel a joist bears on and where the storey levels line up. The program does not perform the diaphragm calculation; the adequacy of sheathing, screws and collectors rests with the project engineer. ## Frequently asked ### Does an OSB-sheathed floor count as a diaphragm? It can, if the sheathing and its connections are designed to transfer the force. The name of the material alone is not enough; the regulation requires the transfer to be shown by calculation. ### Does a trapezoidal sheet floor act as a diaphragm? Whether the sheet works as a diaphragm depends on its section, fastener spacing and edge details. The regulation lists trapezoidal sheet among the floor system materials; the behavior is verified in the project. ### What happens if the floor opening exceeds 3%? TBDY 2018 gives 3% as the limit below which no separate calculation is needed. The 2027 light steel regulation requires the floor to be modelled as a shell element and the load transfer mechanism to be shown when this limit is exceeded. ## Related solutions - [Automatic roof](https://qsteelcad.com/en/solutions/automatic-roof)