Civil Engineering Reference
In-Depth Information
anchoring
plate
prestressing
tendons for retrofit
tightened bolt
3000
(a)
(b)
prestressing
tendons for retrofit
prestressing
tendons for retrofit
(c)
2
φ
32
−
(d)
Fig. 3.44 Arrangement of prestressing tendons.
b. Computation of internal force: the process of force computation can be divided into
two steps. Firstly, original structural force at service is to be calculated according to the
method mentioned in previous section. Dimensions and axial forces of the members of a
trapeziform roof truss under external loads are shown in Fig. 3.45(a). Secondly, the internal
force under prestressing effect, which is taken as the external force, is to be calculated.
Summing up the values obtained in both steps will come to the final result.
Prestressing tendons are placed along 2-10, 10-9, and 9-6 members, and vertical tensions
are exerted at points 11 and 8; thus some changes occur in the axial force of this concrete
roof truss system after prestressing force has amounted to 100 kN. See Fig. 3.45(b). It
can be concluded that each member with prestressing bar decreases by 100 kN in value of
internal force, while other ones experience no change. Similarly, if the 11-10, 11-9 and 9-8
members are equipped with prestressing tendons and tensions are put at 11 and 8 joint,
internal force reduction will be limited in those members above.
Prestressing force has a wider range of effect when prestressing bars are not arranged
along the axis. The value of axial force in the members changes after prestressing tendons
are connected to joint 1, 10, 9 and 7, which is in turn loaded 100 kN force. See Fig. 3.45(c).
It is evident that axial forces in member 11-2, 11-10, 10-4 and 10-9 are reduced, while those
in member 11-1, 1-2, 2-3, 3-4 and 2-10 rise, especially for the 2-10 chord (which should be
given more attention).
c. Check calculation of the bearing capacity: this step can be carried out after modification
in the resulting internal force of each member with regard to practical load condition. The
