Civil Engineering Reference
In-Depth Information
between steel bars and concrete will be reduced, but also main reinforcements will erode,
and concrete coverage will spall;
d. When longitudinal cracks appear at the abutments of both ends, it is concluded that
anchorage at bars is unreliable, or the reinforcement and anchoring steel plate are not well
jointed. In addition, ineffective anchorage of one tensile web member will cut down its
internal force, and simultaneously raise that of the other neighboring ones;
e. In case stiffness of the roof truss system is inadequate, deflection of the structures of
this type will often surpass the prescribed limits of deformation demand in accordance with
Criteria of Reliability Assessment for Industrial Buildings
(
>L/
400) ahead of full-loading
state;
f. If the roof truss structure without any protection is surrounded with super-high envi-
ronmental temperature, or extra high level of humidity, or corrosive media, and the width
of cracks is over the prescribed value of 0.2 mm;
g. When steel bars become erosive due to carbonization of concrete or some other causes,
and covering concrete is exploded;
h. The lack of integrity of roof bracing system causes vibrations and swings of the roof
truss when cranes or other machines are at work.
i. Erection sag of the roof trusses cannot meet the criterion requirement and no supporting
measures have been adopted.
3.4.2
Retrofit Method for Concrete Roof Trusses
As the interaction of each member in the roof truss system seems so dramatic it is of
great importance to choose an appropriate retrofit method and detailing design as well.
Hence analysis for internal force in the structure both before and after retrofit can guide
method selection and also provide useful information for design. In the following section,
key points in internal force analysis will be covered at first, and then some common methods
and examples will be discussed.
1. Key points in load calculation and internal force analysis for reinforced concrete roof
trusses
(1) Loads and load combinations
The accurate determination of the loads to which a roof truss will be subjected requires
consideration for the practical situation, and prediction of the following two load combina-
tions: one is superposition of dead load and entire-span distributed live load exerted, and
the other is dead load together with live load distributed in the range of half span. And
then the most dangerous combination situation is selected.
(2) Diagram for calculation and computation of internal force
Strictly speaking, entirely cast reinforced concrete roof truss systems are termed as multi-
ple statically indeterminate trusses with restrained connection, and therefore rather compli-
cated calculation procedures are required. But in general they can be simplified as pin-
connected trusses. The diagrams for layout and calculation of one mansard roof truss
structure are shown respectively in Fig. 3.43(a) and Fig. 3.43(b), where
P
a
,
P
b
,
P
c
,...,
P
n
correspond to concentrated loads from roof plate,
g
is defined as the weight of upper
chords, and
G
1
,
G
2
and
G
3
respectively represents the gravity load of web chords, lower
chords and bracings (which are transformed into the joint loads). The load action on the
upper chords comprises forces on the joints and between the joints and the latter component
brings in flexible deformation and bending moment as well along the upper chords. Although
cast together with the upper chords, the web members have little constraint on the bend-
ing deformation of the upper chords, considering the greatly decreased stiffness of the web
chords, thus, such roof trusses can be modeled as trusses with continuously pin-connected
