Challenge 3

Q1) What are the sectional properties we need upfront for designing a metro viaduct? What is the supporting software for identifying the sectional properties and designing them?

--> Following are the sectional properties we need to upfront for designing a metro viaduct:

i) Area of section: 2.1735015659 m^2

ii) Perimeter of section: 15.875865 m

iii)Bounding Box: X: , Y:

iv) Centroid of section: X: 0.000, Y: 0.000

v) Moment of Inertia: X: 7.403E+ 1 1 , Y: 6.7578E+ 11

All these values are from AutoCAD and STAAD PRO will identify this values and it helps us to design the structure.

Q2) Explain the detailed steps involved how elements are created in STAAD Pro and how are the inputs for sectional properties assigned?

--> Steps to create Elements in STAAD PRO are :

Step 1: To set the UNITS--> Type Units-->Metric and Force in Kn.

Step 2: Now create Nodes--> Click Nodes--> Span of structure= 27m but Actual length begins from 25mm.

Step 3: To create element--> Click on Structure--> Beam--> Connect the nodes to create beam

Step 4: For supports--> Click Supports --> Create Support --> Pinned--> Add -->Use Cursor to Assign--> Click Assign--> Place it on Node 2-3

Step 5: Using Sectional Properties --> Click Properties--> Define-->General--> Put the values in the box

Q3) What are SLS combinations used for live loads, with respect to impact factor and how the moments for the section are derived?

--> A moving vehicle load is to be applied on structure

Loads--> Create Live Load--> Definitions--> Vehicle Definitions--> Add

The SLS Combination for Live Load= Dead load+ 1.2*SIDL+1.1*Live Load*I

where,

Impact (I)= 1+(0.15+8/6+L)

= 1+(0.15+8/6*25.6)*(90/116)

= 1.313

Moment (M) = (4183.55+1.2*1797.228+1.1*2969.161*1.313)

= 10628.58 kNm

Q4) What is prestressing in girders? What are advantages of Prestressed girders in a metro project.

--> Prestressing in girders is a concept in which we introduce compressive stresses at bottom of girder and those compressive stresses counter balance the tensile stresses that come on the girder. Prestressing tendons are provided in the girders stressed and locked in it and after locking they compress the girders and helps to take care of girder in tensile stresses service.

Advantages of Prestressed Girders in a metro project:

i) The use of the prestressed technique in concrete gives it greater strength, by providing an internal compression force which counteracts the traction force produced by the loads of the structural elements.

ii)One of the great benefits of the prestressed girders is the elimination of cracks from all of the load stages. This absence of cracking supposes the structure’s greater capacity to withstand loads, impacts, vibration and knocks, while minimising the possibility of corrosion in the steel, thus increasing the durability of the construction.

iii) The prestressed girders saves the cost of construction and gives lighter weight material.

Q5) Explain about prestressing losses? What are different type of prestressing losses and mention the significance of the prestressing losses?

--> Losses in prestress can be divided into two categories: immediate and time-dependent. Anchorage set, friction, and elastic shortening all-cause immediate losses. Creep, shrinkage, and relaxation losses are time-dependent.

For Immediate Losses:

i) Elastic Shortening: Elastic shortening of concrete (EC) occurs when the prestress in tendon is transferred to the concrete beam, which causes the beam to shorten and the tendon to shorten with it, resulting in a prestress loss in the tendon.

It is governed by equation: Δfy= mfc

where,

Δfy = Drop in prestress

m = Modular ratio

fc= Stress in concrete

ii) Friction Loss: During stretching a curved tendon in a post-tensioned part, friction is formed at the interface of concrete and steel. There is a reduction in prestress along with the member from the stretched end. The stretching must also overcome the tendon's wobbling in addition to friction.

It is governed by equation: Px= Poe^μ α-k

where,

Po= Prestress force at jacking end

μ= Coefficient of friction between cable and duct.

α= Cumulative angle in rod through which the target to cable profile has turned between any two points

k= Friction coefficient

iii) Anchorage slip: Anchorage is a component that is used to anchor the tendons into the concrete while terminating the tendons. The main function of anchorage is to transfer the stressing force to the concrete once the stressing process is completed. In the case of pre-tensioning, the tendons are monolithically embedded into the concrete. So, no external anchorage is provided and hence there is no loss due to anchorage slip. In case of the post-tensioning system as the tendons are provided inside the ducts of a precast concrete member.

It is governed by equation: P/A = EsΔ/L

where,

Δ= Slip of anchor

L=Length of cable

A=Cross sectional area of cable

Es= Modulus of Elasticity of steel

P= Prestressing force in cable

Anchorage System	Anchorage slip (Δ)
Freyssinet System 12T 15Cables	4mm
19T 15cables	6mm
Magnet System	8mm
Dywidag System	1mm

iv) Creep and Shrinkage Loss:

Shrinkage of concrete:The volumetric changes of concrete structures due to the loss of moisture by evaporation is known as shrinkage of concrete. Due to shrinkage of concrete, the tendons are loosened and loss in prestress occurs.

It is governed by equation: ΔFp = Ep* ∈sh

Creep Loss: Deformation due to rearrangement of molecules over a period of time under the application of a constant load is called creep. The deformation of concrete along the direction of tendons, makes them loosened thereby causing loss in prestress.

It is governed by equation: ΔFp = Ep* ∈cr

where,

ΔFp= Loss in prestress

Ep= Modulus of prestressing steel

∈cr= Ultimate Creep strain

∈sh = Ultimate Shrinkage strain

v) Relaxation of steel: Relaxation of steel is defined as the decrease in stress with time under constant strain. Due to the relaxation of steel, the prestress in the tendon is reduced with time. Just like creep, relaxation also depends on time. It depends on the type of steel, initial amount of prestress, and the temperature.

Q6) How a girder is being prestressed while fabrication? Explain the components/materials used during the prestressing?

--> While fabrication the highly tensile steel or wires are inserted in the beam from one end to future beam. These prestressed tendons are inserted for prestressing of the girder. The steel strands are stretched to create an extremely high state of tension or stress and anchored in bulkhead at the end of the beam.The long prestressing and fabricating bed allows more than one beam to be made at the same time for economy and efficiency the steel strands are pulled and tensioned through two beams at once.After all the steel is in place the metal forms are positioned on both sides of the steel cages for the concrete mix. The metal forms will create the traditional I-beam shape of the finished prestressed concrete beam and concrete is poured into the tops of the forms.When the forms are removed the finished concrete beam emerges displaying its iconic I-beam shape. It is now “prestressed pretensioned” because the concrete has formed and hardened around the tensioned steel strands.Hence the girder is ready to take to the site and used in construction.

Q7) How are stresses in the girder calculated with the resultant prestressing losses?

--> As per IS:14268 of Prestressing Steel used in design:

Characteristics:

Nominal Area of strand (As) = 140mm^2

Nominal Ultimate Stress (fpu) =1860 MPa

Maximum Jacking stress: 0.765 fpu= 1422.9 MPa

Modulus of elasticity (Ep) =195000 MPa

We take No.of strands (n)= 72

Prestressing Force Applied (P)

=(0.765fpu x As x n x (1-Loss)/1000 kn

=(1422.9x140x72x17.5%)/1000

=11832.8 kN

For Section Modulus (Z)= I/y

For checking at bottom: Ybot= 0.594m

Z= 0.74035/0.594

= 1.2464 me3

For Stress at Bottom= P/A + P*e/Z - M/Z

= ( 11832.8/2.1735) + ( 11832.8*0.494/1.2464) - ( 10628.58/1.2464)

= 1.607 N/mm^2

Centre of Gravity of Section to COG of tendons (e) = 0.594-0.140 = 0.454 m

Q8) Explain the step wise procedure of transverse analysis of open web structure of U girders in STAAD pro along with how the vehicle parameters and loadings are fed in the STAAD software.

--> For Transverse Analysis: Finite Modelling is done in transverse analysis

Step 1: Set the Units--> Metric, Force in kN

Step 2: Create Nodes--> Select Node--> Enter the values in the box

Step 3: Click Geometry--> Translational Repeat

Span =27m - 0.7-0.7= 25.6m

For plates at particular interval= 25.6/16 = 1.6m

Step 4: Go to View--> View from Y direction--> Select Nodes--> Translational Repeat--> 1.6m--> Link Steps

Step 5: For adding plates--> Click on Geometry--> Select Add Plate--> Click points to create

Now Select plates with Plate cursor--> Click on Geometry--> Translational Repeat--> Enter the values for start and end plate

Step 6: Now Create beams on ends--> Go to Translational Repeat-->

Step 7: For Breaking Beams--> Select Beam--> Break Beam--> Select Delete Duplicate Beams

Step 8: Now Add Supports--> Assign Supports--> Create Pinned Support--> Enter Beam no.

Step 9: For Thickness of Plate--> Go to Thickness--> Add Thickness for Slab Part, Top Web, Bottom Left Web and Bottom Right Web

After adding the values Go to View--> X direction--> Select Plate--> Assign to Selected Plates-->

For Slab Part R1:

For Top Web--> Select--> Assign--> R2

For Bottom Left Web-->Select--> Assign--> R3

For Bottom Right Web--> Select--> Assign--> R4

Step 10: After adding thickness to all the parts let us cross-check the U-girder: Go to 3D Rendering-->

Step 11: Now for Beams: Go to Define--> Rectangle--> Dummy --> Select Beams--> Assign

Step 12: For Loading--> Go to Load Case--> Add--> i) DeadLoad ii) SIDL iii) Live Load

iv) SLS--> Add--> Define Combination--> SLS LiveLoad--> DL-1, SIDL-1.2 and L.L- 1.1*1.313 (I) =1.443

For Load Casing Details:i) DeadLoad-->Add--> Selfweight--> Assign to View

ii) For SIDL--> Add--> Plate Loads--> Partial Plate Pressure--> Add the values in the box

Step 13: Select Plates in Top View--> SIDL--> Assign to Selected Plates--> Click on View Loading Diagram

For 1st: For LiveLoad--> 16/2= 8*9.81= 78.48kN, Single Plate = 1.6m, Gap between two Bogey=4.4/2=2.2m

LiveLoad--> Add--> Plate Load--> Concentrated Load--> 78.48 Force

For 2nd: Force= -78.48,Y= 0.245 , X= -0.2

For 3rd: For right--> Y= -0.245, X= 0.2, For left: X= -0.2 , Y= 0.245

Select Plate Cursor--> i) Click on Plate #101, #140, #102 and #141--> Load--> Assign to Selected Plate

Step 14: Now all the loads has been applied--> Select Run Analysis--> Postprocessing--> Plate Results--> Select Results Tab--> Select Load Case-->

Q9) How is the longitudinal analysis of open web structure of U girder performed, final design bending moments and axial stresses are extracted? Explain the software procedure for extracting the same.

--> For longitudinal analysis:

Step 1: Type Units--> Select Metric , Force in Kn

Step 2: Create Nodes--> Select Nodes--> Enter the values

Step 3: For Element--> Select Structure-Beam--> Connect to created nodes

Step 4: For Pinned Supports--> Click Supports--> Create Support--> Pinned--> Add--> Use Cursor to Assign--> Assign--> Place it on Node 2-3

Step 5: Using Sectional Properties

Click Properties--> Define-->General--> A dialogue box will appear enter the given values--> Assign to View--> Assign

Step 6: Now for Vehicle Live Load--> Click on Loads--> Create Live Load--> Definitions--> Vehicle Definitions--> Add

We will take 16 tonns of load = 16*9.81=156.96kN

Step 7: For Load Case Details: Add Load--> Dead Load--> Loading Type: Dead

Same goes for SIDL: Add Load--> SIDL--> Loading type: Dead

For adding Load in that cases: Click on Load--> Add--> Selfweight Load--> Direction Y--> -1 Factor

Click on Selfweight -1--> Assign to View--> Assign

Click on SIDL--> Add--> Member Load--> Uniform Force-->

Step 8: For Applying Load of moving vehicle (L.L): Load Generator-->Add No. of Loads (262) --> None--> Add

Now Add whole distance of metro train= 2.3+12.4+2.3*2+4.4=38.4m (Length of train) + 27= 65.4m

Now for load moving ahead 2.5m steps= 65.4/0.25= 262

Click Generate Load--> Enter the values in dialogue box

Step 9: Now Go to Analysis & Design--> Run Analysis--> Postprocessing --> Apply

Step 10: For Results--> Select Results Extraction--> Beam Results--> The results will appear

For seeing the values--> Click Annotate --> Beam Results--> Maximum Bending Moment & Maxm. Shearforce

For Maxm. BM when Live Load runs--> View Loading Diagram--> Generation--> The results will appear