Design a flat roof meeting certain mechanical performance criteria. The basic structure of the roof is a series of rectangular-section 'joists', made from softwood, that span the space to be covered by the roof.
• Length of joist between supports: L = 3 m
• Width of joist: a = 50 mm
• Modulus of joist material: E = 14 GN m-2
• Load due to roof covering: mC = 600 N m-2
• Snow loading specification for southern Britain: mS = 750 N m-2
• Acceleration due to gravity: g = 9.8 m s-2
(a) By modelling the joists as independent simply supported beams, determine the minimum depth of a joist needed to support a 100 kg person without deflecting more than 20 mm at any point.
(b) Calculate the maximum permissible distance (centre-to-centre distance) between joists of this depth to deflect no more than 10 mm at any point when subject to the dead load of the roof covering and a live load from snow.
The construction standards applied to structures of this sort insist that a margin of error is incorporated into the design calculations. Explain, in your own words, the principles behind limit state design.