Challenge 3
Q1) The sectional properties we need upfront for designing a metro viaduct are (i)Area, (ii) Perimeter, (iii)Centroid (iv)Moment of Intertia along the major and minor axis as well as a torsional moment of inertia. The supporting software for identifying the sectional properties is AutoCAD which extracts the sectional properties from the cross-section diagram.
Q2)First the nodes are defined properly as per the design detail and then nodes are connected with the beam elements in the STAAD Pro software. After this support is defined and assigned in the software. Then under the tab of general isometric properties, we will input the sectional properties parameters extracted from the AutoCAD and then it will be assigned to the appropriate beam elements.
Q3)The SLS load combinations for the live loads are DL+1.2*SIDL+1.1*LL*I. Here I is the impact factor which is calculated as per the formulae given in the IS code. Moments for the section is calculated by observing the maximum moment experience under LL, SIDL, and LL at critical section like mid or support and then applying the formula of the SLS load combination mentioned above.
Q4)Prestressing is the introduction of compressive stress in advance at the bottom of the girder in order to counter the tensile stresses which can occur due to its service load. The advantages of prestressing are:
Efficient use of the compressive strength of the concrete which we know is almost ten times that of its tensile strength of it. Also, it helps in avoiding the stress reversal in the concrete during its service load and increases the load-bearing capacity of the girder.
Q5)Prestressing forces lost during the stressing, locking, and service of the girder are called prestressing loss. Broadly prestressing loss has been classified into two categories immediate and time-dependent losses which further have been classified into three each.
Immediate Losses are (i) Elastic shortening, (ii) Anchorage slip (iii) Friction and Time-dependent losses (i) Creep (ii) Shrinkage (iii) Relaxation
The prestress loss is significant when it comes to the long-term design for pre-stress which must be introduced in the tendons in order to remain operable even after all the losses in the tendons in the long run.
Q6) While fabricating the girder, an initial compressive force of prestress is applied on the girder using the cable which is stressed at both ends and then it is locked from both ends. After the locking, the cable tries to come back to its original position which naturally compresses the girder inducing compressive force. During this process, a Hydraulic Jack is used to induce the initial compressive stress.
Q7)Stresses in the girder are calculated by subtracting all the resultant prestressing losses from the compressive stress imparted on the girder through cable.
Q8)Transverse analysis of the open web structure of U girders in STAAD pro takes place in the following steps:
(i)Create geometry of the structure by defining the whole transverse section
center line dimensions are used to model the geometry
First of all, nodes are created and then plate elements are added in the Geometry section
(ii)Supports is assigned next.
(iii)After that thickness of the plates is added in the Properties section.
(iv)Then material type is defined for the entire section.
(v)After that Various types of loadings are assigned which are Dead Load(Selfweight), Super Imposed Dead Load(SIDL), and Live Load.
The whole load of the full track, both side plinths, rails, rail pads, and third rail are considered in SIDL.
For live load, we consider the configuration of Bogies and its wheel load on the plates of the girder which we calculate and apply
And then finally we do the analysis.
Q9) The longitudinal analysis of the open web structure of the U girder is performed using the line model in the STAAD Pro and follows the same steps as explained in the previous question. For the Live load, we apply the moving vehicle load as per the IS code and generate the Load combinations.
The final design bending moments and axial stresses are extracted in the following ways. At the first maximum bending moment or axial load from an individual load case is extracted from the STAAD pro output and then it is combined in appropriate load combinations along with the proper load factor multiplied.
In the STAAD Pro software, after analysis, we go to the post-processing and then enter into beam results where we can find the Bending moment and axial force magnitudes at different locations of the beam. From there we identify the maximum BM and axial force in magnitude for individual load cases and then as explained earlier, we combine it with appropriate load combinations as per the IS code along with the proper load factor multiplied with it.