Local buckling of thin-plate elements - The Behaviour and Design of Steel Structures to EC3 - page 118

Tension member

Chord Web strut

(a) Equivalent truss

Flange Web stiffener Tension field

d

(b)Tension fields

a a a

Figure 4.19 Tension fields in a stiffened web.

Notonlymusttheintermediatetransversestiffenersbesufficientlystifftoensure
that the elastic buckling stress τ cr is reached, as explained in Section 4.3.1, but
they must also be strong enough to transmit the stiffener force

√

( a / d ) 2

N st = f y dt
2

3 τ cr
f y

a
d −

1 − (4.32)

1 + ( a / d ) 2

requiredbythetensionfieldaction.Intermediatetransversestiffenersareoftenso
stocky that their ultimate strengths can be approximated by their squash loads, in
which case the required area A st of a symmetrical pair of stiffeners is given by

A st = N st / f y . (4.33)

However, the stability of very slender stiffeners should be checked, in which
case the second moment of area I st required to resist the stiffener force can be
conservatively estimated from

I st = N st d 2
π 2 E

(4.34)

which is based on the elastic buckling of a pin-ended column of length d .

4.4 Plate elements in bending

4.4.1 Elastic buckling

The thin flat plate of length L , width d , and thickness t shown in Figure 4.20
is simply supported along all four edges. The plate is loaded by bending stress