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Bridge Vehicle Loading

ABSTRACT

The purpose of this document is to guide STRAP users on how to use the Bridge module to analyze and design a Vehicle Overpass Bridge. Note that the loading magnitudes and results output might not be close to the expected output. This document was prepared by Elisha. P. Tungamirai BEng Hons Civil (NUST).

The document will equip the user to understand the following concepts in STRAP:

Lane Definition
Generating Influence Lines
Application of Traffic Loading
Displaying Output Results
Transferring Output to Results module and Design combinations

DESCRIPTION OF BRIDGE

The semi-integral bridge consists of an overall length of 68.58m, with a 3-span configuration. The twin spine post-tensioned reinforced concrete deck has 17.14m jack spans and a 34.28m center span supported on reinforced concrete piers and closed cantilever-type abutments with return walls. The deck consists of solid deck sections over the piers and abutments mainly due to excessive shear stresses over the supports. The twin split oblong-shaped piers are cast integrally with the deck and supported on pot bearings over the abutments. Both the abutments and piers are founded on Augur piles and the skew of the bridge is 7°.

Figure 1 Full Bridge Render (Beam Model)

Figure 2 Typical Deck Cross Section

Figure 3 Jack Span Elevation

Figure 4 Centre Span Elevation

STRAP Bridge Module

Bridge design codes used by engineers around the globe require designing every point on the bridge (both longitudinal and transverse direction) for the load combination and arrangement that produce the most severe load effects on the bridge. The designer calculates influence lines at every point on the bridge for each result type. The influence lines guide the designer on where to apply the loading to achieve the most adverse moments, shear, deflections, etc. The process becomes complicated as the bridge size and geometry change. STRAP Bridge reduces the amount of work significantly by computing various result types for every point on the bridge. All the calculations are computed according to the designer’s input that includes the number of lanes, the width of lanes, length of segments, load intensities, etc. The program produces accurate graphical and tabulated results for any point on the bridge saving a great deal of time compared to the strenuous manual methods.

NA LOADING (Normal Loading)

Type NA loading, as specified by TMH7 (1981, revised 1988), consists of a distributed part and a concentrated part acting in conjunction with each other. The distributed part can either be in the form of two equal and parallel line loads spaced 1.9 m apart or in the form of a distributed load over the full width of the notional lane. The concentrated part of type NA loading can either be in the form of two equal point loads spaced 1.9 m apart, or in the form of a knife-edge load over the full width of the notional lane. (Malan & Rooyer, Jan. 2013) (Authorities, 1981). NA loading already contains provision for dynamic effects and no further adjustments should be made. NA loading closely resembles HA Loading in the BS5400 (2006) and BD37 (2001). See Figure 5 below for the NA vehicle load curve.

Figure 5 Loading Curve for Type NA Loading

The bridge deck overall width is 14.45m and 13.4m between the F-type parapets on both sides. The carriageway width is 8.7m and sidewalks of 2.35m on either side. This example will concentrate only on the STRAP Bridge module with the assumption that the user has already completed the bridge geometry modeling. Below is a list of other parameters associated with the analysis of the bridge:

Bridge spans (m) = 17.14m, 34.28m, 17.14m
Deck overall width (m) = 14.45m
Deck width between parapets (m) = 13.40m
Sidewalk width (m) = 2.35m
Carriageway width (m) = 8.70m
Number of notional lanes = 3
Width of notional lanes (m) = 2.90m

Lane Definition

To apply traffic loading on the bridge in STRAP, first complete geometry definition in the Geometry module then navigate to the Bridge module to commence lane definition.

Step 1: NA Notional Lanes

When the Bridge module is open, click on Define > Simple Line (Pick two nodes) to define the notional lanes. Click the end nodes to define segment/ notional lane length. A window like Figure 6 to define Lane segment properties will appear. For the lane width, according to TMH7 Part 2 Clause 2.6.2, the number of notional lanes is 3 for a carriageway width between 7.4m up to and including 11.1m. The width of each individual lane is 2.90m. For the lane offsets, since there are 3 notional lanes, the extreme left lane, central, and extreme right lanes will have -2.9m, 0, and 2.9m, respectively. The vertical tolerance is the distance that the program searches for nodes from the imaginary perpendicular plane drawn on the connected nodes. Since all nodes are falling in the same plane between the end nodes, set a nominal value of 0. 1.

Figure 6 Lane Segment Properties

Set a value of 68 for Divide segment into to have the segment equally divided into successive 1m lengths. For the number of subs strips, set a value of 3 to divide the notional width into 3 strips. The load train will move 3 positions in the transverse direction per notional lane.

The more the number of strips and sub strips, the more the number of generated load cases. To this example, the load train moves in 1m intervals in the longitudinal direction and 3 positions in the transverse direction per notional lane. Since there are 3 notional lanes, 3 segments will be defined for the bridge. Click on Another segment to define the second lane, change the lane offset to 0 for the middle notional lane and an offset of 2.9m for the last lane then click End definition. After successfully defining all the 9 lanes, a layout like Figure 7 below will be displayed.

Figure 7 Bridge Notional Lanes

Step 2: Displaying Influence Lines

To generate influence lines, click on the toolbar Options > Load Distribution > Beams > All Beams to distribute the bridge loads over all beams since the deck is modeled using beam elements. If the deck was modeled using plate elements, click on distribute bridge loads over elements. Click on Options > Load Direction > Global X3 Direction for traffic load direction. On the toolbar, click File > Solve to instruct the program to generate the Influence lines. To display influence lines, click on Results > Display Influence Lines for Selected Result > Highlight the member> Result Type M2. The program will draw influence lines for the selected member. Refer to Figure 8 for the influence line diagram of the central member for jack span.

Figure 8 Jack Span Centre Influence Line

From the influence line shown in Figure 8 above, a conclusion can be drawn that for a maximum longitudinal moment on the jack span, fully load the first span, skip the adjacent span, and load the jack span on the other side. The same goes for the influence line for the central span. The central span must be fully loaded with no traffic loading on the adjacent spans. The program will automatically apply the loading according to the generated influence lines for any point on the bridge to obtain the most severe result intensity and arrangement.

Step 3: Application of NA Loading

NA Loading is not included in STRAP’s load train library by default. Only NB24, NB36, and NC load trains are included. To apply NA loading, click on Lane Load located on the sidebar to the far right of the main window. Proceed to click on Define > Lane Load. A window like the one in Figure 9 will be displayed. For notional lane 1 and the other remaining 2 lanes, input a uniform load of 36KN/m per notional lane under uniform load as stated in the TMH7 Part 2 Clause 2.6.3.1. The code states that for loaded lengths in excess of 36m, the load is derived from the formula Q_a=(180/√L)+6 and 36KN/m per notional lane for loaded lengths up to 36m. However, STRAP calculates the load reduction automatically from the maximum loaded length even when the load length exceeds 36m.

For notional lane 1, max length is 68.58m, Factor table select South African and input 144KN for Knife-edge load for both shear and moment envelope. The Knife-edge load is derived from Clause 2.6.3.2.2 of TMH7 Part 2 from the equation 144/√n where n is the number of the notional lane. As the number of lanes increases, change the max length and Knife-edge load. The uniform load should not be changed because STRAP automatically calculates the load reduction factors with an increase in max length on different notional lanes. It should be maintained at 36KN/m per notional lane. See Figure 9 for the Lane Load Input.

Figure 9 NA Loading Input

Below is Table 1 for NA Load input. The 3 lane loads are defined as follows:

Lane Loaded	Lane Load Name	Uniform Load	Max Length	Factor Table	Knife Edge Load
1	NA Load Lane 1	36	68.58	South African	144
2	NA Load Lane 2	36	137.16	South African	102
3	NA Load Lane 3	36	205.74	South African	83

Table 1 NA Loading Input Table

To display the uniform load factors according to TMH7 Part 2, click on Output > Display Load Factor Tables. To create the Load cases, click on Load Case on the sidebar below Lane Load. Click on Define > Load Case Definition and assign the load to the lanes as shown in Figure 10. After assigning the loads to lanes, click on Create load cases by permutation > From lane > 1 to 3.

Figure 10 NA Load Case Input

By clicking Create load cases by permutations, an instruction is passed to the program to create load cases by interchanging the lane loads. The program will interchange the loading amongst the 3 lanes as mentioned above. The most adverse effects are not necessarily obtained when the bridge deck is fully loaded. To achieve all the various load case scenarios, the designer can opt to load the first two notional lanes and skip the third. To do this, select the option Create load cases by permutation > By lane groups. Put lanes 1 and 2 in the same group and lane 3 in a different group. Please refer to Figure 24 Straddling of Lanes in TMH7 Part 2 for guidelines on Bridge deck transverse loading. Several load cases can be defined depending on the number of load arrangements the designer desires. See Figure 11 below for the definition of permutation groups.

Figure 11 Definition of permutations groups

Step 4: Display Output Results

To display the results within the STRAP module, click on Results > Draw Applied Loads for Selected Result > Click on any beam > Absolute Value Maximum > Beam Results > M2 > At 5/10 of Length. An output like Figure 12 will appear. The program will indicate the load train position and in the case of NA loading, the position of the knife-edge load and the Maximum M2 moment value will be shown. The maximum moment for the mid-span sag moment is 8164.969KNm. The result type ranges from Axial, V2, V3, Torsion, M2, M3, and deflections. The next step is to transfer the output to the Results module. The output can also be displayed in a table for a detailed output of every member and nodes. The same applies to influence lines output.

Figure 12 Max M2 Moment Output

Step 5: Transferring Output to Results

To transfer the output to the Results module, click on the toolbar Results > Update STRAP Results File. See Figure 13 for the Update STRAP results file table. Give the Load a name NA Max abs envelope and click ok to transfer the output.

Figure 13 Update STRAP results file

Click on the Results module > Draw result > Beam Result Diagram > M2. See Figure 14 below,

Figure 14 NA M2 Bending Moment Envelope

Once the results are exported to STRAP Results module, they can be included in design combinations together with other load cases that include self-weight and other defined load cases on the bridge. Under the Loads module, navigate to existing loads. The output from the Bridge module will form part of the loads as an existing solution. Bending moments, shear forces, immediate deflections, etc for the individual load case can be displayed similar to that of other loads like dead loads, super-imposed dead loads, etc. See Figure 15 below,

Figure 15 List Of Load Cases

The NA – Bridge: Max abs envelope load case is combined with other relevant load cases to generate several load combinations as shown in Figure 16 below The NA load case is combined with several load cases as defined in the design code the designer is using.

Figure 16 Definition of Load Combinations

N.B The advantage of using the STRAP Bridge Module for traffic loading is that the program automatically calculates the torsion introduced in the deck due to eccentric applied loads because of the load train moving from left to right in the transverse direction. The bridge module incorporates guidelines stated in Appendix 2.A Clause 2.A.2.4 Figure 24 (a and b) for Extreme transverse positions for NA Loading on 2, 3 Lanes in TMH7 Part 2.

NB LOADING (Abnormal Loading)

TMH7 Part 2 Clause 2.6.4.2 states that the NB vehicle shall be taken to occupy any transverse position within the length of the carriageway it is in and to be within 0.6m from the face of a kerb, except that when the distance between the kerb and the balustrade exceeds 0.6m it shall be placed up to within 0.15m from the face of the kerb. For this example, the NB vehicle will be placed 0.15m from the edge of kerb on the sidewalk because the distance between the yellow line and kerb exceeds 0.6m. NB loading is a unit loading representing a single abnormally heavy

vehicle. NB loading closely resembles HB Loading in the BS5400 (2006) and BD37 (2001). Only one NB vehicle is permitted on a bridge at any given time and does not coexist with any other vehicle load. NB loading is typically applied in two magnitudes namely NB24 and NB36, with the number referring to the number of units applied. NB24 has an axle load of 240 kN and NB36 an axle load of 360 KN. The magnitude of NB loading is determined by the class of road and the relevant authority. There is no allowance for dynamic effects on the NB vehicle load train. See NB load train in Figure 17 below,

Figure 17 NB Load Train from TMH7 Part 2