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Steel Designers' Manual - 6th Edition (2003)
limit state. If plastic design is appropriate, the effective breadth is modified to
(span/3). 5
In the design of continuous beams, the effective breadth of the slab may be based
conservatively on the effective span of the beam subject to positive or negative
moment. For positive moment, the effective breadth is 0.25 times 0.7 span, and for
negative moment, the effective breadth is 0.25 times 0.25 times the sum of the adjacent
spans. These effective breadths reduce to 0.175 span and 0.125 span respectively
for positive and negative moment regions of equal-span beams.
21.7.2 Modular ratio
The modular ratio is the ratio of the elastic modulus of steel to the creep-modified
modulus of concrete, which depends on the duration of the load. The short- and
long-term modular ratios given in Table 21.3 may be used for all grades of concrete.
The effective modular ratio used in design should be related to the proportions of
the loading that are considered to be of short- and long-term duration. Typical
values used for office buildings are 10 for normal weight and 15 for lightweight concrete.
21.7.3 Shear connection
Table 21.3 Modular ratios (a e) of steel of concrete
The shear resistance of shear-connectors is established by the push-out test, a standard
test using a solid slab. A typical load–slip curve for a welded stud is shown in
Fig. 21.9. The loading portion can be assumed to follow an empirical curve. 9 The
strength plateau is reached at a slip of 2–3 mm.
The shear resistance of shear-connectors is a function of the concrete strength,
connector type and the weld, related to the diameter of the connector. The purpose
of the head of the stud is to prevent uplift. The common diameter of stud which can
be welded easily on site is 19 mm, supplied in 75 mm, 100 mm or 125 mm heights.The
material properties, before forming, are typically:
Ultimate tensile strength 450 N/mm 2
Elongation at failure 15%
Duration of loading
Type of concrete Short-term Long-term ‘Office’ loading
Normal weight 6 18 10
Lightweight
(density > 1750 kg/m
10 25 15
3 )
Basic design 613