A. Introduction
I'd like to share my old experience with the Tendon Profile.
The Latest Engineering Trends and Issues
I'd like to share my old experience with the Tendon Profile.
To design structures, we must necessarily create load combinations. These combinations change depending on the state of the loads affecting the structure, and the coefficients considered for these loads vary according to standards. Therefore, while automatically generated load combinations are used for convenience in creating various load combinations, it is difficult to generate combinations that satisfy all conditions.
Designing a curved bridge was a challenge for me in every aspect. The tendon profile in MIDAS Civil, which we covered before, it’s a very well-known issue,
In India, the IS codes, or Indian Standards codes, play a crucial role in ensuring the quality, safety, and reliability of structures in India. It serves as an essential benchmarks to guide the design, construction, modification, and upkeep of structures. These codes are formulated by the Bureau of Indian Standards (BIS), a national body that develops and publishes standards to promote quality and consistency across various industries. These codes are reviewed from time to time and updated to reflect the latest developments in industries.
I would like to introduce the basic and advanced information about MIDAS API. I was surprised that many users wanted to use MIDAS API and asked when we could start to use it. Users have contacted us worldwide regardless of specific regions, company size, and areas.
In this section, we will discuss the damping method applied to Nonlinear Boundary Time History Analysis.
There are four types of damping methods.
Time History Load Cases - Damping Method
The four damping methods are categorized for analysis purposes as follows.
The method of applying damping varies depending on whether you want to apply the same damping to all elements of the structure or not.
The options you choose also depend on whether you use the Modal or Direct Integration methods. The two methods differ in how they account for damping, which can lead to much longer analysis times depending on the selected damping method.
Depending on the analysis method, the recommended damping method is as follows
The Mass & Stiffness Proportional method is Rayleigh Damping, which assumes that the damping matrix can be constructed as a linear sum of the mass and stiffness matrices, expressed by the equation below.
Here, a and b are the damping coefficients, which can be represented by the natural frequency (w) and damping ratio (h) of the two modes.
I n MIDAS CIVIL, enter the natural frequency (or period) and damping ratio (typically 0.05) for the two modes.
A common question we get is what values should be entered for Mode 1 and Mode 2. (Is it enough to enter the period values for Mode 1 and Mode 2, or what period values should be entered?)
Let's take a look at a quick overview of Rayleigh Damping to get a better understanding.
The graph above is for Rayleigh Damping (Mass - Stiffness Proportional Damping).
With two natural frequencies (or periods) and a damping ratio, the coefficients a and b can be calculated, and thus the damping ratio at any frequency can be calculated.
We typically apply a damping ratio of 0.05. However, the determination of two natural frequencies (W1 and W2) with a damping ratio of 0.05 requires engineering judgment.
So when the Direct Integration method is used, how should we define the initial load?
In this content, we will discuss the initial load of an analysis using the Nonlinear Direct Integration Method.
Defining the initial loading conditions is comparatively easier for nonlinear time history analysis using Direct Integration than for the Modal method.
Let's take a look at the same example from Part 1 and see how the initial load is defined for the time history analysis using direct integration.
The Analysis Method is selected as Direct Integration, and the End Time, Time Increment, and Step Number Increment for Output are the same as the Modal method in Part 1. (The "Order In Sequential loading" option can be considered for initial load consideration in Part 1, and selecting ST (static load case) is an inappropriate method for this option).
In this content, we will see how to consider the initial load using Initial Load (Global Control) in the Nonlinear - Direct Integration method.
Initial Load (Global Control)
Figure 2. Time History Load Cases - Nonlinear(Analysis Type), Static(Analysis Method)
Let's have a look at these options in a little more detail.
In the Nonlinear-Direct Integration method, the initial load using Global Control is defined as follows.
Figure 3. Load > Dynamic Loads > Global Control
Select "Perform Nonlinear Static Analysis for Initial Load",
Select the static load cases to be considered as initial loads.
With this setting, a nonlinear static analysis of the selected loads is performed. The results are used as initial conditions for the time history analysis.
After selecting Initial Load in Time History Global Control, select "Initial Load (Global Control)" in Time History Load Case as follows.
Figure 4. Nonlinear - Direct Integration with Initial Load(Global Control)
The initial load applied in Global Control is considered as the constantly acting initial load. Therefore, "Keep Final Step Loads Constant" is always "Checked On".
"Cumulate D/V/A Results" is an option to select whether to combine the results of the time history analysis with the results of the initial load analysis.
A detailed description of both options is explained in Part 1.
In MIDAS CIVIL, elements with nonlinear properties such as seismic isolation, vibration control bearings, and dampers can be represented in the analysis model with the General Link option.
Numerous impressive structures were created before the formulation of standardized design codes, but challenges existed. The shift to modern design codes introduced a systematic and scientific approach to bridge engineering, enhancing safety, consistency, and reliability in design and construction. Design codes also play an important role in protecting bridge engineers by providing a framework for legal compliance, standardization, risk mitigation, and professional accountability.
The above example is difficult to consider in terms of practical use. Therefore, to make a calculation that can be applied to an arbitrary cross-section, we will go over one-by-one through the formulas and calculations that are needed.
The example cross-section is a PSC box shape as shown below, and the input of the cross-section is in the form of consecutive coordinates. When using the calculation program, the input should be in a general coordinate system, but for the convenience of the calculation in the example, the following coordinate system is used where the upper right corner is the origin (0,0) and the lower left direction is positive.
Figure 1. Example of a PSC box cross-section
Sectional properties are calculated using Green's theorem from the input coordinate data. The required section properties for the calculation are the area, second moment of area, and distance from the section's top edge to its centroid.
Figure 2. Cross-Sectional Properties
AASHTO LRFD Heating case - Zone 3 is considered.
Figure 3. Differential Temperature load
To ensure the accuracy of the calculation, the change point of the temperature gradient load must be included. Therefore, the change point of the temperature gradient load was added to the cross-sectional coordinates, and the temperature gradient load was applied to each node.
Figure 4. Temperature gradient load at Each node
The restraint force can be calculated using the equation derived in section 3, but since the temperature and width vary linearly on the z-axis and y-axis, respectively, we can write linear equations in terms of z for temperature (t) and y for width (b) and substitute them into the equation. Therefore, the equation can be expressed as follows:
Figure 5. Equation of a straight line based on changes in width and temperature
Figure 6. The formula for calculating restraint force
Now, if we apply the formula for calculating restraint force that has been determined to each straight line and calculate it, we can obtain the following restraint force.
Figure 7. Restraint force
Using the calculated acceleration and temperature gradient load, the residual stress at each node is determined as follows.
Figure 8. The equation for Residual Stress
Figure 9. Residual Stress
This is an Excel spreadsheet designed using VBA based on the formulas introduced above. It allows users to input the loads examined in Part 1/Part 2, calculates the residual stress accordingly, and generates a graph.
Figure 10. Sample Calculation
Now, let's verify the created spreadsheet. First, we will use the same cross-section as in the example, and the loads are defined as follows, and the results are shown in the spreadsheet accordingly.
Figure 11. Calculation example for verification 1
Figure 12. Calculation example for verification 2
Figure 13. Calculation example for verification 3
The verification was performed using MIDAS CIVIL. The four simple spans with the same cross-section are created as shown in below the example and analysis is performed by applying the loads according to each design standard.
Figure 14. MIDAS CIVIL model for verification
The results are as follows.
Figure 15. Top Stress - MIDAS CIVIL
Figure 16. Bottom Stress - MIDAS CIVIL
As expected, the results show a 99% match with the values obtained from the spreadsheet.
Figure 17. Values obtained from the spreadsheet
We have examined the effect of temperature gradient loads on beams according to each design standard. Hopefully, this has provided a basic understanding of temperature gradient loads.
We can take this one step further by using these results to calculate axial strain and bending moment, which can then be converted into equivalent linear temperature loads. By doing so, we can predict the impact of temperature gradient loads in indeterminate structures.
In design, temperature loads are often included in most load combinations, and if the design is done within the range that does not allow tensile stress, the impact of temperature loads can be significant and cannot be ignored. I hope that the following article will be helpful in design.
#Temperature Gradient #Non-linear Temperature #Temperature Gradient #Temperature difference # Design Calculation #BS EN # AASHTO LRFD #BS 5400 #NCHRP #DMRB #CS 454
GOODNO, Barry J.; GERE, James M. Mechanics of materials. Cengage learning, 2020.HAMBLY, Edmund C. Bridge deck behaviour. CRC Press, 1991.
Would you like to use the Excel Spreadsheet in the content?
Submit the form below right away, and receive the file for calculating temperature gradient loads.
(Note! This spreadsheet requires access to the MIDAS CIVIL API for utilization.
If you have any inquiries regarding the CIVIL API, please feel free to leave a comment.)
For curved and skew bridges, when the overall coordinate system (Global Coordinate System) and the diagonals/orthogonals of the piers are not parallel in the analysis, users need to consider the seismic inertial forces in the horizontal direction acting on the entire pier in the "most unfavorable direction.”
Through Part 1 & 2, we looked at how the temperature gradient load of a bridge is calculated based on the design criteria. Now, let's examine how the calculated load affects the bridge deck.
Wouldn't it be convenient if the sections of the drawings you have could be seamlessly integrated with the analysis program without any additional work? Integrating analysis models with irregular section structures, without the need for section extraction or calculation! In this content, we will show you how to easily solve the calculation of irregular sections when creating structural analysis models by integrating CIVIL NX and CAD.
In BS EN, the temperature gradient load for bridges is described in section 6.1.4.2, "Vertical temperature components with non-linear effects (Approach 2)," of BS EN 1991-1-5. The load is specified differently depending on the type of bridge deck, which can be steel, composite, or concrete.
In addition to these rules, the magnitude of the temperature gradient load also varies depending on the thickness of the pavement and the height of the structure. This information is provided in Appendix B of BS EN.
BS EN 1991-1-5 Annex B
It should be noted that the temperature gradient load provided in BS EN is inherently more complex than that of AASHTO LRFD, and there are several errors and incomplete parts, making it difficult to calculate the load.
Therefore, let's look at BS 5400-2:2006 together, and determine the correction and load calculation method for it.
For reference, the latest information on this load can be found in CS 454 - Assessment of highway bridges and Structures provided by DMRB (Design Manual Road Bridge).
The steel deck provides 4 types of load depending on the girder shape and temperature. The temperature based on thickness is divided into three categories: unsurfaced, 20mm, and 40mm.
In the case of 1b, the temperature according to the surfacing thickness is not provided.
In BS 5400-2, 1b is separated into group 2, and a table showing temperature changes according to surfacing thickness is provided, so it should be applied accordingly.
BS EN 1991-1-5 Figure 6.2a & Annex B Table B.1
BS 5400-2 Figure 9 & Annex C Table C.1a & C.1b
The composite deck has a total of four different temperature gradient load categories which are divided based on Normal/Simplified procedures and temperature effects. Additionally, ten sets of temperature gradient loads are provided taking into account the variation of surfacing thickness according to the height of the slab.
In "Heating", it is expressed as "h2" and is applied across the entire cross-section.
In Cooling, the lengths for h1 and h2 are missing.
Regarding T2, although the diagram shows 4℃/-8℃ for a 100mm surfacing, no table is provided for other conditions.
There is no table provided for slab depth and pavement thickness in the Simplified Procedure.
Heating insets are replaced with those of BS 5400-2.
The length is applied in the same way as heating.
T2 uses 4℃/-8℃ as a fixed value.
It is not used in the case of the simplified procedure.
BS EN 1991-1-5 Figure 6.2b & Annex B Table B.2
BS 5400-2 Figure 9 & Annex C Table C.2
Loads are provided in two types according to temperature, but 36 sets of temperature gradient loads are provided according to the section height and pavement thickness, so several linear interpolations are required to apply them.
In Heating, when h is more than 0.8, it is indicated as 13.0℃, but in Annex Table B.3, it is indicated as 13.5℃.
In Cooling, the range notation of h3 is incorrect.
In cooling, h2/h3 is set to be larger than 0.20m.
In B.3 Table, although Cooling T1 is indicated as 4.3 for a slab depth of 1.0m and surfacing thickness of 200mm, it should be interpolated to the intermediate value for the depth of 0.8/1.5.
It is applied in accordance with BS 5400-2, as 13.5°C.
Range notation follows BS5400-2.
In Cooling, h2/h3 is set to be less than 0.20m.
It is revised to 4.8 instead of 4.3.
BS EN 1991-1-5 Figure 6.2c & Annex B Table B.3
BS 5400-2 Figure 9 & Annex C Table C.3
There are no guidelines other than the specified slab height and surface thickness. However, based on experience, linear interpolation within the range is acceptable. linear interpolation is performed within the range and the closest value is taken for the value exceeding or less than this.
BS EN 1991-1-5 Annex B. Table B1 to B3
Conceptually, the temperature load on the top surface of a slab decreases as the thickness of the surface increases.
In the case of Type 2 & 3, when the thickness is unsurfaced, i.e., zero, the value is calculated to be smaller than when there is thickness. Then, in the case of types 2 & 3, should the unsurfaced and 50mm be interpolated for the surface thickness of less than 50mm? A question may arise.
This can be seen by referring to BS 5400-2, which specifies that the surfaced thickness includes waterproofing thickness. This means that Types 2 & 3 can be divided into two types of surfaces: one with waterproofing and another without any surfacing.
Therefore, for sections with a surfacing thickness of 50mm or less, it is necessary to interpolate the value between waterproofing thickness and 50mm, and CS454 provides accurate information on this.
BS 5400-2:2006 Annex C
CS 454 Appendix D2.3
In the load combination of BS EN, the uniform temperature and temperature difference are not separately dealt with, but expressed as one “Thermal action”.
BS EN 1991-1-5 6.1.5
Based on the above, the calculation sheet for determining the temperature gradient load can be prepared as follows:
Calculation Example by Deck Type
BS EN 1991-1-5 covers a wide variety of applications for temperature gradient loads based on the shape and variation of temperature. However, it has inherent errors that can be confusing for engineers encountering the standard for the first time. Therefore, it is necessary to compare it with BS 5400-2:2006 to understand it better.
If possible, It is recommended to apply the latest information contained in DMRB CS454 as much as possible.
Imbsen, Roy A., et al. Thermal effects in concrete bridge superstructures. National Cooperative Highway Research Program, 1985.Shushkewich, Kenneth W. "Design of segmental bridges for thermal gradient." PCI journal 43.4 (1998): 120-137.AASHTO, LRFD Bridge Design Specification, Ninth Edition, American Association of State Highway and Transportation Officials, Washington, D.C., 2020.AASHTO, LRFD Bridge Design Specification, SI Units, Fourth Edition, American Association of State Highway and Transportation Officials, Washington, D.C., 2007.BSI, BS EN 1991-1-5, Eurocode 1 : Actions on structures - Part 1-5: General actions - Thermal actions, British Standard Insititution, London, 2003.BSI, BS 5400-2, Steel, concrete and composite bridges - Part 2: Specification for loads, British Standard Insititution, London, 1978.BSI, BS 5400-2, Steel, concrete and composite bridges - Part 2: Specification for loads, British Standard Insititution, London, 2006.England, Highways, CS 454 Assessment of highway bridges and structures, The National Archives, Kew, London, 2022.Emerson, Mary. Temperature differences in bridges: basis of design requirements. No. TRRL Lab Report 765. 1977.
Would you like to use the mentioned Excel Spreadsheet in the content?
Submit the form below right away, and receive the file for calculating temperature gradient loads.
(Note! This spreadsheet requires access to the MIDAS CIVIL API for utilization.
If you have any inquiries regarding the CIVIL API, please feel free to leave a comment.)
Temperature gradient load is one of the loads that is generally used in the design of the superstructure of a bridge. For designing a bridge superstructure, you will likely need to consider the effects of temperature gradient load.
Many structural analysis programs, including MIDAS Civil, allow for the input of temperature gradient loads and can relatively accurately calculate their effects. However, as one becomes more familiar with using structural analysis programs, it is easy to overlook the impact of these loads on the structure and why certain results are obtained.
AASHTO LRFD (American Association of State Highway and Transportation Officials Load and Resistance Factor Design) and BS EN (British Standards European Norms) are two major design standards used for bridge design. Let's take a closer look at how temperature gradient loads are calculated in these design standards.
AASHTO LRFD's temperature gradient load is described in section 3.12.3 "Temperature Gradient," and it has remained unchanged from the 1998 2nd edition to the most recent 2020 9th edition. The same calculation method has been used consistently in all editions, without any significant changes.
AASHTO LRFD - Temperature Gradient
The temperature gradient load in AASHTO LRFD is relatively simple in its calculation method, making it easy to apply and consider for the design.
Now let's take a brief look at how AASHTO LRFD calculates its temperature gradient load.
According to the commentary of AASHTO LRFD, the loads applied to concrete bridges are based on NCHRP Report 276.
NCHRP report 276 - Figure A-3 Positive vertical temperature gradient within superstructure concrete
NCHRP report 276 - Figure A-5 Negative vertical temperature gradient within superstructure concrete
Loads are classified according to pavement conditions. Plain/Unpaved, 2 in. Blacktop, or 4 in. Blacktop.
In the case of positive loads, the current AASHTO LRFD load size and load distribution are similar, but for negative loads, there is a significant difference.
The changes in these differences can be seen in "Design of segmental bridges for thermal gradient, KW Shushkewich, PCI journal, 1998". The figure below compares the temperature gradient load for each standard in the case of a plain concrete surface with a height of 8ft, Zone 3.
Design of segmental bridge for thermal gradient - fig 2. Comparison of thermal gradients
As seen in the figure, AASHTO 89 directly cites NCHRP Report 276.
AASHTO 94 has since changed to a form that is similar to the current one, but for Negative values, it still uses -0.5 times the value for Positive values.
AASHTO 98 is currently the most similar to the current form, using Negative values as -0.3 times Positive values. According to this article, the results have been validated for the AASHTO 98 standards.
For steel bridges, the pattern of the Australian bridge specifications was used, and AS 5100.2 states that the temperature after passing through the slab is applied directly to the entire steel girder. AASHTO LRFD also uses the same concept.
AS 5100.2 Figure 18.3 Design effective vertical temperature gradients
The Excel Spreadsheet is created for calculating AASHTO LRFD temperature gradient loads as follows.
Calculation Example - AASHTO LRFD
Would you like to use the mentioned Excel Spreadsheet in the content?
Submit the form below right away, and receive the file
for calculating AASHTO LRFD temperature gradient loads.
(Note! This spreadsheet requires access to the MIDAS CIVIL API for utilization.
If you have any inquiries regarding the CIVIL API, please feel free to leave a comment.)
We have discussed the process of calculating cross-sectional properties using "Green's theorem."
Hello civil engineers . This is Connor and Ben!! 👬
In railway or highway design projects, structure engineers always receive decided alignment from the project manager. Highway or Railway alignment usually changes slightly because of many external factors while working on a project. Sometimes, slightly changed alignment can have potential critical issues to construction or other project steps, serviceability, and even a legal issue. For example, a transition curve is related to drivability and driving safety. If so, structural engineers must recognize and consider that minor change. Let's look into how structure engineers change alignment during their projects.