the USA, and factors of safety for use in assessment were increased (Illustration 196) (Swailes, 1997). It seems very likely that the ripples of alarm crossing the Atlantic Ocean led to the introduction of conservative assessment methods by the London County Council (General Powers) Act, 1909. The LCC Building Act Committee at the time certainly made a study of the American codes (GLC, 1976). Structural framing details of the Darlington Apartments were deficient in that projecting bearings for steel beams resulted in large eccentricities of load on the cast iron columns (Friedman, 1995). Temporary lateral stability bracing was also inadequate. Reference has been made in Section 6.4.1 to 'historical damage' suffered by cast iron columns. In more recent times, numerous examples of roof collapse due to fracture of a supporting cast iron column by a fork-lift truck have been recorded (Lovejoy, 1988). Small diameter columns in low-rise buildings such as mill weaving sheds are particularly vulnerable. Guidance on minimising the risks has been issued by the Health and Safety Executive (HSE, 1999). The solutions are to remove the powered vehicle hazard, to protect the columns by barriers or suitable encasement, or to provide steel or ductile (SG) cast iron replacements with adequate impact resistance in vulnerable locations.
6.5
8
0
50
1W
Flat Ended Bars
150
200
250
Slenderness ratio LJr
Ill~tstrationI91 Experimental and theoretical strut buckling curves (Source: Swailes, 2004)
Cast iron beams
6.5.1 Details, manufacture and defects Early in the nineteenth century beams might be cast directly from the blast furnace. Later practice was to re-melt the pig iron in a secondary furnace, or cupola, giving more control over the operation and a better quality end product (Swailes, 1996). The choice of raw materials was sometimes made by an engineer, but for building works was often left to the experience of the ironfounder.
CROSS SECTlONAl
ARE4 (n)
x1Oamrn'
Illustration 192 Typical section property variations for a test column from Stanley Mills (Source: Swailes, 2004)
A re-usable wooden replica or pattern of the beam was made, very slightly oversize to allow for shrinkage of the iron on cooling. I-section beams were most often cast with their webs horizontal (Illustrations 197 to 203). The lower part of the mould for large beams would be formed directly in a deep layer of sand in the foundry floor. The upper part of the mould would be made in one, two or three mould boxes, and would normally be air-dried and then fitted in place over the lower part of the mould prior to casting.
A clayey sand mixture was used for moulding, compacted in layers, but not so densely as to prevent the escape of gases generated due to the damp nature of the 'green sand' mould. The 'ventilation' of the mould was an important element of the moulders craft as
0
20
40 60 80 Lateral deflection (mm)
100
Illustration 193 Test results for Column I (Source: Swailes, 2004)