BRACING SYSTEMS OF
THE INDUSTRIAL BUILDINGS
►FUNCTIONS OF BRACING SYSTEMS
►CLASSIFICATION OF BRACING SYSTEMS
FUNCTIONS OF BRACING SYSTEMS
The transversal frames are not able to take loads that act
perpendicular to this plan.
The stiffness of the whole structure is insured with longitudinal ties
between the plane structural elements.
• The bracing system insures the spatial character and a greater
stiffness. Their roles are:
a spatial collaboration between the transversal frames;
small deformations of the structure under horizontal transversal
actions;
stability during the assembling stage;
diminish the effective lengths of the structural elements in
compression;
take over the horizontal loads due to wind actions and the surge
effects due to cranes.
CLASSIFICATION OF BRACING SYSTEMS
• The following structural elements are usually braced:
• roof trusses
;
• skylight
(if there are);
• columns
;
• crane girders
(particular situations).
• In the longitudinal direction of the structure, generally
in the bays situated in the middle of the blocks for
temperature tolerance (contraction-expansion) two
running transversal frames are bounded together in a
stiffen block.
BRACES FOR THE TRUSS
• -horizontal longitudinal bracing at
the bottom flange of the truss
Along the structure in the external panels, between the joint in the supports of the trusses and the next joint by
longitudinal elements (rods);
Take the horizontal reactions from the top part of the intermediary columns that carry the longitudinal walls; if the truss is fixed to the column, the bottom
chord of the truss is in compression and has to be provided with bracing in the second panel.
The positions of the bracing in the case of multi-spans structures when both the heights of the columns and the lift capacities of the cranes have small or medium values are presented and also for big heights and heavy lift
capacities. Bracing systems at the bottom chord of the truss in two
situations: when the distance rods are placed in the truss joints; when the distance rods are placed intermediary
between the truss joints (smaller loads applied to the bracing system)
Design elements of the horizontal bracing system at the bottom chord of the truss (the sheeting is light and it cannot take
wind loads in its plane)
Different situations imposing the layout of various forms of bracing systems at the bottom
chord of the truss: fixed connection between the truss and the column, light cranes and
small heights and heavy cranes and big heights (also, cranes placed at different levels)
horizontal transversal bracing from the bottom and the
top chord of the truss
• Are placed perpendicular to the longitudinal axis of the structure and in the end bays of the structure, in the bays next to the tolerance distance
between adjacent blocks and at about 50 m along the structure. Some of the purlins are part of the system of horizontal bracings (under the skylight the purlins are missing so rods have to be placed).
• The bracing from the bottom chord of the truss are necessary in order to avoid the local buckling of the elements in the bottom chord of the truss. • The bracing from the top chord take the horizontal reactions coming from
the top of the intermediate columns of the walls in the gable (reactions coming from the wind action normal to the wall) or alternatively, to a horizontal conventional force:
• P=0.02Nmax (1)
where Nmax is the maximum axial stress in the top chord of the truss coming from the vertical loads on the roof. The force P is distributed to the joints of the bracing system as in a lattice girder after that the final stresses being determined (fig.4). It is a lattice girder with two slopes but in the design and computation stage is assimilated with a plane system.
• the vertical bracing of the
marginal and central struts
of the truss
• Are placed in the end bays of the structure and intermediary at 3-5 bays along the structure (in order to insure the stability at the
assembling stage).
• They may also be put under the supports of the skylight and in the case of the overhead travelling cranes at the joints of the bottom chord of the truss.
Design of the internal
members of the horizontal transversal bracing
Vertical bracing system of the struts of the truss: marginal and central
Braces for the columns
vertical bracing of the columns
• Are placed in the axis of the columns in the longitudinal direction along the building;
• In the end bays, because of the tolerances that are to be taken into account, only a flexible rod is put at the top part of the column;
• Generally, the bracings are placed both at the top and at the bottom part of the column in a bay situated in the middle of the longitudinal axes, or if the building is long, at L/3, L being the longitudinal in plane dimension;
horizontal bracing of the columns
They are used in the case when the bays are very big to of reduce the effective length of the intermediary columns of the longitudinal walls and to help them in taking the horizontal forces transmitted by the wind to the walls. • The design of the cross sections of the braces is based on the slenderness
assumption, because of their small dimensions:
Design of the vertical bracing of the columns in the central rigid frame
(∑M)A =0;(W +Fp)⋅hi −V⋅LT =0 ( ) (W F ) tgα L h F W V p T i p ⋅ = + ⋅ + = ( F) 0;W Fp 2Dcosα H = + = ∑ α cos 2 p F W D= + (7)