Bracing Systems Prezentare

5/28/2018 Bracing Systems Prezentare

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5. Bracing systems of the industrial buildings

5.1. The functions of the bracing system

The transversal frames are not able to take loads that act perpendicular to this plan. Thestiffness 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.

5.2. Classification of the 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!e"pansion) two running transversal frames

are bounded together in a stiffen block.

5.3. Braces for the truss

They are (fig.#):

!horizontal longitudinal bracing at the bottom flange of the truss their position being

along the structure in the e"ternal panels between the $oint in the supports of the trusses and the

ne"t $oint by longitudinal elements (rods). These braces take the horizontal reactions from the

top part of the intermediary columns that carry the longitudinal walls. If the truss is fi"ed to the

column then the bottom chord of the truss being in compression 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.


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%ig. #. &racing systems at the bottom chord of the truss in two situations: when the distance rods are

placed in the truss $oints; when the distance rods are placed intermediary between the truss $oints (smaller

loads applied to the bracing system)

%ig. ' esign 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)

!horizontal longitudinal bracing at the top chord of the truss are mostly used for the

case when the height of the columns and the lift capacities are small. They are disposed in the

same positions only they are part of the roof as lattice girders with the top chord flanges made of

the roof purlins.


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The longitudinal horizontal bracing of the truss reduces the deformations of the structure

at the roof level from the effect of the important horizontal transversal loads. They are designed

as continuous lattice girder.

%ig. . ifferent situations imposing the layout of various forms of bracing systems at the bottom chord

of the truss: fi"ed 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 a"is of the structure and in the end bays of the structure

in the bays ne"t to the tolerance distance between ad$acent blocks and at about *+ m along the

structure. ,ome 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:

ma"+'+ NP = (#)

whereNmax is the ma"imum a"ial stress in the top chord of the truss coming from the

vertical loads on the roof. The force - is distributed to the $oints of the bracing system as in a

lattice girder after that the final stresses being determined (fig.). It is a lattice girder with two

slopes but in the design and computation stage is assimilated with a plane system.


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%ig. . esign of the internal members of the horizontal transversal bracing

!the vertical bracing of the marginal and central struts of the truss(fig.) are put in the

bays with and intermediary at !* 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 $oints of the bottom chord of the truss.

Braces of the skylight

!horizontal transversal bracing at the top and bottom chord of the skylight that are

placed in the end bays of a block of skylight (from fire protection conditions the length of a

block does not have to go over +.../+ m);

!the vertical bracing of the marginal and central strutsare put in the bays with and

intermediary at ... bays along the skylight.


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%ig. *. 0ertical bracing system of the struts of the truss: marginal and central

Braces for the columns

!vertical bracing of the columns (fig./) are put in the a"is 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 fle"ible rod is put at the top part of the column. 1enerally the

bracings are put both at the top and at the bottom part of the column in a bay situated in the

middle of the longitudinal a"es or if the building is long at 23 2 being the longitudinal in

plane dimension;

!horizontal bracing of the columnsare used in the case when the bays are very big their

purpose being mainly that of reducing the effective length of the intermediary columns of the


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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 (fig.4):

%ig. /. esign of the vertical bracing of the columns in the central rigid frame

%ig. 4. &asic elements for the layout and design of the bracing system and individual elements

a

f

necz

a

f

necyzy

li

li

; .......................................................................(3)


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The computation of the bracing considering strength criteria is done only for the bracing

from the inferior part of the column part are placed in the frame in the middle of the industrial

building (fig. 5). The efforts in the braces will be determined from the condition that they can

carry the resultant from the wind forces and the longitudinal horizontal forces that appear at the

rail level due to the speeding up or slowing down of the crane (surge effect):

( ) ( ) +;+ =+= TipA LVhF! ()

then:

( ) ( ) tgFL

hFV p

T

i

p +=+= (*)

6lso:

( ) cos';+

"FF p#=+=

(/)Then:

cos'

pF"

+= (4)

The bracing elements are generally compound sections from two hot rolled profiles!

channels angles hollow sections etc. interconnected. The connection between the braces and

the structural elements are bolted on gussets these last elements being welded to the column

section.

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