Differential Temperature Analysis
Description
This form allows heating and cooling temperature profiles to be specified by either selecting a standard type or by explicit definition.
For type EN 1991-1-5 Linear, the effect of vertical temperature differences is considered by using an equivalent uniform temperature difference through the bridge deck from top to bottom. The temperature difference for different types of bridge deck is given in Table 6.1 of EN 1991-1-5, and adjustment factors for surfacing thickness are given in Table 6.2.
For type EN 1991-1-5 Fig 6.2 Non Linear, the temperature profile through a bridge deck for different types of bridge deck is given in Figure 6.2 of EN 1991-1-5 for a specified surfacing thickness.
Once the profiles are defined, a comprehensive series of data checks are carried out before the calculations are performed and the results displayed.
Outline Procedure
The temperature difference profile can be generated, when the Display radio button is set to Profile, by specifying only the deck type, surfacing details (for non linear type), and for deck type 2 (composite decks) the depth of the concrete slab. The program interpolates where explicit values are not available. The table shows the values generated, interpolating where explicit values are not available. The graphics show the heating and cooling temperature profiles drawn to scale. The program adjusts the top and bottom temperatures so that there is no axial effect in the section from the temperature profile. (This adjustment assumes that the effect of the reinforcement is ignored, and does not allow for the thermal coefficient having different values in different components of the section).
The table cannot be edited unless the profile type is switched to the User Defined type. With the User Defined type selected, the temperature may be specified at up to 20 heights through the section, including the top and bottom surfaces.
When the Display radio button is set to Stresses, the stress profiles are displayed graphically, and the principal values are displayed. More detailed results are available in the printout, which can be viewed by pressing the F5 key or clicking the Results button.
Note that the program calculations are based on horizontal strips. If the top or bottom surface of the section is not horizontal, the calculations will not be realistic, and a simplified model of the section may be more appropriate.
Heating differential temperature tends to produce a negative (hogging) moment and tension in a beam. Cooling differential temperature tends to produce a positive (sagging) moment and compression in a beam. The relaxing moments as reported in the results are those which need applying in the analysis to obtain the correct secondary effects.
Form Graphic
Field Help
Type
Select either a standard defined profile or the user defined profile option.
Type of Deck
Click on this field to display a drop-down list containing 6 options, which correspond to the 6 bridge deck types on Figure 6.2 of EN 1991-1-5. The groups in full are:
- Steel deck on steel box girders.
- Steel deck on steel truss or plate girders.
- Composite decks
- Concrete slab.
- Concrete beams or box girders.
- Concrete box girders.
Depth of concrete above steel
For most cases differential temperature profiles may be calculated from the type of deck and the overall depth of the section (which is calculated and stored by the program, and displayed on the form).
For the composite deck type however the depth of the concrete slab above the beam is required in place of the overall depth. This value must be specified in this field.
Surfacing
Click on this field to display a drop-down list containing 3 options, which correspond to the surfacing options in Table 6.2 of EN 1991-1-5.
If 'Surfaced' is selected, the field labelled 'Surfacing thickness' will become available. The surfacing factors for heating and cooling will be updated whenever the surfacing details are changed.
Surfacing thickness
Access to this field indicates that 'Surfaced' has been selected in the previous field. The default value corresponds to the surfacing thickness of Table 6.1 of EN 1991-1-5.
For surfacing thickness values not explicitly given in the tables of Annex B, the program will carry out a linear interpolation.
Surfacing factor - Heating
Surfacing factor - Cooling
The surfacing factors for heating and cooling are updated whenever the surfacing details are changed. The values are obtained from Table 6.2 of EN 1991-1-5, interpolating where necessary. The values may also be edited on the form.
Heating Temperature Diff. Height/Temperature
Cooling Temperature Diff. Height/Temperature
The table cannot be edited unless the profile type is switched to the User Defined type. With the User Defined type selected, these fields allow heating and cooling temperature profiles to be explicitly specified. The temperature may be specified at up to 20 heights through the section, including the top and bottom surfaces. Values entered under the 'height' column are distances from the TOP of the section.
Any previously standard defined profile will be supplied as default values.
Note that the program calculations are based on horizontal strips. If the top or bottom surface of the section is not horizontal, the calculations will not be realistic, and a simplified model of the section may be more appropriate.
Display
Toggle between Profile and Stresses to change the graphics accordingly.
Ignore reinforcement
The differential temperature primary stresses are calculated ignoring the presence of any reinforcement by default.
Click on this field to un-check the box so that reinforcement is included.
In all cases tendons are ignored.
If the reinforcement is included, it is allowed for only in the section properties used in the calculations, and not in the axial restraint.
Where both short term and long term values of elastic modulus are specified (i.e. for concrete properties) the short term value is used in the analysis.