DESIGN PROCESS
IMPACT OF LOAD ON THE FIXING: WHAT CAN BE ADDED TO THE SYSTEMS The design of the external façade will be dependent on how the brackets are fixed to the through-wall system. The manufacturer should provide the relevant guidance and calculations where the brackets are intended to be fixed onto the SFS studs (or through-wall). Helping hand brackets must always be directly connected into the studs of the SFS system or the structure. Although it is recommended that any insulation fixings/cleats should also be fixed into the studs of the SFS system or the structure, it sometimes may be necessary to fix these secondary fixings into the sheathing boards that do not coincide with the stud locations. Note: not all boards will be suitable for fixing into and this should be checked with the manufacturer. It is therefore essential that the pull-out strength of a fixing connected to the relevant boards is checked. Only approved fixings should be used for this application and checked with the system owner to ensure the board can provide the required level of support. PULL-OUT The pull-out strength of a fixing connected to a board is its ultimate capacity to resist separation and failure of the connection between the fixing and the board along the fixing’s long axis. Its capacity depends on the geometric properties of the screw and the strength and stiffness of the board material. The pull-out strength of approved fixings onto the relevant board should be provided by the manufacturer based on test results.
A fixing in shear is when there is a slip between the two components connected by the fixing – a stud and sheathing in a state of racking. A fixing is said to be in shear when it tends to tilt or bend and/or bear into the material because of the slip between the two materials connected by the fixing. The initial shear behaviour of a fixing into this state is used to determine its shear stiffness and as a result to calculate the panel shear stiffness necessary for diaphragm design. Further guidance on these topics is provided in the SCI publication P437 Design of stability systems for light steel framing.
INTERFACE OF INTERNAL COMPARTMENTS WITH THE EXTERNAL ENVELOPE
Where there are internal partitions interfacing with the external through-wall, both system suppliers should be consulted to ensure there are no compatibility issues, and that the performance requirements of both systems are maintained, and the system warranties are maintained. SERVICE PENETRATIONS Where services penetrate the through-wall, the designer should ensure that the services are firestopped to ensure that smoke and fire cannot enter the cavity. The services penetrations can take several forms: • Waste pipes • RWP • Inlets and outlets for mechanical ventilation systems. Regulation B3 of Approved Document B states: The building shall be designed and constructed so that the unseen spread of fire and smoke within concealed spaces in its structure and fabric is inhibited. Any service penetration penetrating the through- wall shall be designed to inhibit the spread of fire and smoke into the cavity of the through-wall.
RACKING With regard to systems designed to resist
diaphragm action (often referred to as racking), where various types of boards can be used to provide stability to the light steel-frame structure, this diaphragm performance must be derived from specific test results.
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