Basic methods of reinforcing steel structures - Assessment and Refurbishment of Steel Structures - page 95

Balanced co-operation between prestressed bolts in a frictional joint with other bolts
or rivets in one joint should not be assumed. Such co-operation can be expected only if
the joint is statically loaded. In dynamically loaded joints, co-operation of prestressed
bolts in friction joints only can be counted on with abutting joints. In such a case, the
capacity of the combined frictional and welded joint F b,w,Rd, is given by the relation:

(4.1)

where
F S,Rd is the designed capacity of the frictional joint
F w,Rd is the designed capacity of the welded joint
k b

is a coefficient depending on operating conditions; for statically loaded joints k b =1.0 and for
dynamically loaded joints k b =0.8.

4.4 Reinforcement by changes to the static system

Reinforcement by changes to the static system enables a more favourable division of
internal forces (as well as stresses) in the individual elements of a load-bearing structure.
It is effective method of reinforcement that offers a wide range of possibilities. At the
same time it is a difficult method with regard to monitoring changes to internal stresses
and their effect on the stresses and stability of structural elements.
Changes to the static scheme of various types of structures can be divided into those:

• Without significant change to the static scheme; e.g. by increasing the column strength
in transverse section to obtain a division of internal forces at the cross bonding of the
material
• With partial change to the static scheme, e.g. by adding a draw bar, a girder is changed
into a statically indefinite strut frame.

When the static scheme is changed, loading effects at points of bedding or support are
also changed. Such adaptations include changing single girders into continuous girders,
introducing grid systems, connecting independent elements and parts into the spatial
operation of the load-bearing system, adaptation to obtain the action of a frame (e.g.
changing a three-hinged frame into two-hinged one). The bedding of a structure can be
changed by altering fixed kinds of bedding or changing sliding bedding to fixed kinds.
Sometimes, on the other hand, the change is aimed at decreasing redundancy. Such cases
include those where the supports of continuous girders have collapsed or foundations that
were thought to be built in have slipped. The correct arrangement of stiffeners for
securing the spatial strength is very important, particularly for plane structures. It needs
to be remembered that the importance of stiffeners in older structures was often
underestimated; nowadays the need for corresponding spatial reinforcement is recognized
and in most cases is respected in designing new projects, as well as in completed
structures.