Generation of Load Cases Due to Prestress (Prestressed Concrete)

Robot generates loads resulting from prestress in concrete elements (prestressed concrete). Losses in prestressed elements are calculated according to the following codes:

• American code ACI 318
• EuroCode 2 (ENV 1992-1 : 1999)
• Polish code PN-B-03264:1999
• French code BAEL 91.

After selecting one of the codes listed (Tools menu > Job Preferences > Codes), highlighting a concrete structure element, and selecting Analysis > Analysis of Pre-stressed Elements from the menu, an application from Spreadsheet Calculator calculates and generates a prestress load. Data from the highlighted structure element is passed to the Spreadsheet Calculator application (element length as well as section dimensions) using the COM interface. After the calculations are performed, the structure is modified (load cases are added to the structure).

Note: To use the Analysis > Analysis of Pre-stressed Elements option, both Robot and Spreadsheet Calculator programs must be installed on your computer.

Data between Robot and Spreadsheet Calculator is exchanged using COM interface. Applications (sheets) are closely linked with Robot. That is why you must have the Robot protection that lets you use COM interface.

Currently, there are 3 applications for prestressed (post-tensioned) structures that calculate and generate prestress loads considering immediate losses caused by:

• Friction between a tendon and its duct
• Anchorage slip
• Elastic deformation of concrete.

These applications work independently of each other. Each application cooperates directly with a structure model defined in Robot. Individual applications calculate one of the immediate losses:

• Prestressed structures - Friction loss

  The application lets you calculate the impact of prestress forces on an element as well as losses induced by friction between a tendon and its duct. As a result, the calculations give a value of friction loss in (n) cross sections and 2 load cases are generated.

  These cases include loads caused by prestress (without considering losses) and with friction loss considered. Adding up the effects caused by these cases yields the final load from prestress with friction loss considered. If cases from losses and prestress are distinguished, you can generate your own calculation cases using the combination generator in Robot that can also consider different values for safety factors. (Note to Author: Please verify. Also, tell the reader how to access the combination generator. If the combination generator is hidden from the user, then remove information about it from the sentence.)

• Prestressed structures - Loss due to anchorage slip

  The application lets you calculate the value of the loss induced by a slip of a tendon in an anchorage. As a result, the calculations provide a value of the slip loss in (n) cross sections and a load case is generated in the Robot.

• Prestressed structures - Loss from elastic deformation of concrete

  The application lets you calculate the value of the loss caused by elastic deformation of concrete. This loss results from elastic shortening of a concrete element from loading it with an external force (prestress). As a result, the calculations provide a value of the loss due to elastic deformation of the concrete in (n) cross sections and a load case generated in Robot.

Details concerning the operation of the previous applications can be found in the Help provided in the Spreadsheet Calculator program.

Note:

• Loads from prestress can be generated only on bar elements.
• Tendon profile can vary in one plane only.
• A load can be generated on only 1 selected bar.
• Design is based on the resultant cable.

Rules underlying the calculations

1. Theoretical basis of the calculations

   If the code provides relevant formulas for calculating problems for designing prestressed structures, they are used directly in Spreadsheet Calculator. These formulas are assigned an appropriate number referring to the code formula. If an appropriate formula is not given in the code, then rules based on literature or derived from the assumptions described in the code are used. If so, the issue is thoroughly described in the Spreadsheet Calculator help.

2. Elements with several tendons in the cross section - rules

   When designing elements that are prestressed by more than one tendon, which is frequently the case, 2 methods can be employed. The first method is based on the resultant cable and the second method is based on multiple using the application.

   • Resultant cable - This method assumes that calculations are performed by reducing the profiles of the tendons to a resultant profile. It is often used in designing prestressed structures. Errors resulting from this assumption are not large for typical cases and can be used at least in the initial stage of design.

     This method calculates the resultant properties and profile based on the data about the individual tendons; or it assumes these properties and profile in the initial design stage and later defining the profiles of the individual tendons, so their resultant profile and properties are consistent with those assumed in the beginning.

   • Multiple calculations - You can perform successive calculations and generation of forces in Robot, because you can change the name of the load cases and tendon profile. If you change the names for each generation of load cases, then you get the load cases that correspond to the individual tendons. That way, you can apply the standard Robot relations to create a group responsible for total prestress with the relevant losses considered. This makes it easier to analyze accidental cases, such as a rupture of a single tendon.

     If you do not change the names, you can get 1 case that includes many load records corresponding to individual tendons. You can create summary tables of losses by combining several applications (sheets) into one workbook comprising the analysis of the entire element, within which the applications (sheets) focus on a single tendon.