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Steel Designers' Manual - 6th Edition (2003)
822 Foundations and holding-down systems
Ê z ˆ
q= 10. 7 ¥ NB 1+
Ë B¯
where N is the number of blows per metre and q, z and B are as before.
Cohesionless soils subject to flooding will suffer a reduction of capacity at water
table level:
capacity when flooded = K ¥ unflooded capacity
where K = (g - 9.8)/g and g is the soil bulk density given for convenience in kN/m3 .
27.2 Connection of the steelwork
27.2.1 Fixed and pinned bases
The function of a column baseplate is to distribute the column forces to the concrete
foundation. In general a plain or slab base is used for pinned conditions or
when there is very little tension between the plate and the concrete.A gusseted base
is used occasionally to spread very heavy loads but more generally for conditions
of large moment in relation to the vertical applied loads, the principal function of
the gusset being to allow the holding-down bolt lever arm to be increased to give
maximum efficiency while keeping the baseplate thickness to an acceptable
minimum. Gusseted or built-up bases give an ideal solution for compound or twin
crane stanchions in industrial shed buildings.
Fixed bases are used primarily in low-rise construction either in portal buildings
specifically designed as ‘fixed base’ or in industrial sheds in which the main columns
cantilever from the foundations. They are also used, though less frequently, in
multi-storey rigid-frame construction. In each of these cases it is assumed by definition
that no angular rotation takes place, and although this is unlikely to be
achieved it is generally accepted that sufficient rigidity can be obtained to justify
the assumption.
Pinned bases are those in which it is assumed that there is no restraint against
angular rotation. Although this is also difficult to achieve it is accepted that sufficient
flexibility can be introduced by minimizing the size of the foundation and
similarly reducing the anchorage system. Pinned bases are used in portal and in
multi-storey construction.
Typical pinned and fixed bases are shown in Fig. 27.6.
27.2.2 Baseplate design
27.2.2.1 Plain bases
The empirical method for determining the size of baseplates in BS 5950-1: 1990 was
not suitable for use with deep UBs or with bases that have very small outstand