## Blog

The Latest Engineering Trends and Issues

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.

## 1. Why do we need design codes?

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.

## 4. Nonlinear Temperature Effects on PSC Box Section

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.

## (1) Section Information

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

## (2) Section Property

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.

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.

## (5) Restraint force

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

## (6) Residual Stress

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

## (7) Calculation and Verification

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

## (8) Conclusion

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.

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## 1. The usual method for calculating inertial force

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.”

## Nonlinear Temperature Effects on Beams

### (1) Basic Concept

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.

## 1. How to link a CAD cross-section with an analysis model at once?

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.

## (1) Vertical temperature components with non-linear effects

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).

### Type 1: Steel deck

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.

### Problem

• In the case of 1b, the temperature according to the surfacing thickness is not provided.

### Modification

• 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

### Type 2: Composite deck

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.

### Problem

• 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.

### Modification

• 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

### Type 3: Concrete deck

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.

### Problem

• 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.

### Modification

• 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

## (3) Conclusion

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.

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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.

## 1. AASHTO LRFD - Bridge Design Specifications (2020)

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.

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.

• AASHTO Segemtnal Guide Specification (AASHTO 89)
• AASHTO LRFD Bridge Design Specifications (AASHTO 94)
• AASHTO proposed segmental Guide Specifications (AASHTO 98)

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

### (2) Calculation of Temperature Gradient - AASHTO LRFD

Calculation Example - AASHTO LRFD

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Have you had a chance to explore the content "Green's Theorem Part 2: Calculating Normal Stress for Asymmetric Cross-Sections"?

## 2. Normal stress for Asymmetric section

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.

Have you checked the previous post, "Green's Theorem Part 1: A Step-by-Step Guide to Calculating Section Properties in Structural Design?

## 👉🏻 Check out previous post

Hello civil engineers . This is Connor and Ben!! 👬

Happy New years 🌞 civil engineers . This is CoBen!! 👬

It's time for a review, as usual. Do you remember Offset function in CIVIL NX?

Let me reiterate. You cannot apply a Beam End Offset at the same time when a Section Offset is in place. It is not possible to apply Offsets simultaneously.

For more details, you can click on to check it out!

## 0. Introduction

Explore how MIDAS CIM aligns with the latest BIM guidelines, focusing on Work Breakdown Structure (WBS) and Object Breakdown Structure (OBS). Discover the revolutionary Field Data feature in V190, allowing seamless input and management of object information.

## 0. Introduction

Embark on a journey into the realm of structural analysis with the spotlight on the transformative capabilities of Analysis Lines. In this blog post, we delve into the profound applications and insights gained through the utilization of Analysis Lines in 1D Element Analysis. Join us as we uncover the pivotal role these lines play in enhancing structural understanding and design precision.