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Steel Designers' Manual - 6th Edition (2003)
352 Plane frame analysis
span/height to eaves = 25/7.6 = 329 .
rise/span = 3.75/25 = 0.15
wL ( total load on frame) = 9. 48 ¥ 25 = 237 kN
2 wL
2 = 9. 48 ¥ 25 = 5925 kN m
Horizontal thrust at feet of frame ( Fig. 11.5)
= 0. 21 ¥ 237
Moment capacity of rafter ( Fig. 11.6)
= 49. 8 kN m
= 0. 0305 ¥ 5925
Moment capacity of leg ( Fig. 11.7)
= 181 kN m
= 0. 059 ¥ 5925 = 350 kN m
2
Assuming a design strengh of 275 N/mm ( S275 steel)
:
S x required for rafter 3
= 181 ¥ 1000/275 = 658 cm
S required for leg 3
= 350 ¥ 1000/275 = 1270 cm
x
Trial sections (NB these are first trials and may not be adequate):
Rafter 406 ¥ 140 ¥ 39 kg UB ( S275 steel)
;
3 4
Sx = 721 cm , Ix
= 12 500 cm
Leg 457 ¥ 152 ¥ 60 kg UB ( S275 steel)
;
S 3 = 1280 cm , I
4
= 25 500 cm
x x
It is suggested that the next stage is to check the overall stability of the frame. The
main reason for this is that the only way to correct insufficient stability is to change
the main member sizes. If any other checks are not satisfied, additional bracing can
frequently be used to rectify the situation, without altering the member sizes.
References to Chapter 11
1. Kleinlogel A. (1931) Mehrstielige Rahmen. Ungar. New York.
2. British Standards Institution (2000) Structural use of steelwork in building. Part
1: Code of practice for design in simple and continuous construction: hot rolled
sections. BS 5950, BSI, London.
Further reading for Chapter 11
Baker J.F. (1954) The Steel Skeleton, Vol. 1, 1st edn. Cambridge University Press.