As promised in the previous post, a spreadsheet for designing reinforced or prestressed concrete composite beams has now been uploaded to my web site.  It can be downloaded from: Composite Design Functions-GS.zip .  As usual, the spreadsheet includes full open-source code.

In this first version of the spreadsheet the Excel Goal Seek function is used to find the neutral axis depth and curvature of the section under the second stage (composite) loading.  A later version will use the QuadBrent Function to perform this task, which will provide much better performance, and will allow the entire procedure to be encapsulated in a User Defined Function (UDF).

The procedure for using the current version is:

• Enter load details for both stages of loading, and section details
• The spreadsheet will calculate the Neutral Axis depth and curvature for the first stage (non-composite), but estimated values must be entered by hand for these values in the composite section.
• Click the “Find NA and Curvature” button: the spreadsheet will use Goal Seek to find the values that provide equilibrium for the specified loading and cross section.
• If Goal Seek fails to find a solution try different starting values for the Neautral Axis depth and section curvature.
• Concrete and reinforcement strains, stresses, forces, and bending moments are then reported on the next sheet, for both Stage 1 and Stage 2 loading, using two UDFs.

Screen-shots showing the analysis of a precast pretensioned bridge girder with in-situ top slab are shown below.  More details of the usage of the program, and the VBA coding will be provided in later posts.

Note that this is a beta release and the spreadsheet has had only limited testing.  As always, if results are to be used for other than evaluation purposes it is essential that they are verified using an independent analysis.

Input Concrete and Reinforcement details from top to bottom (Stage 2 section in lighter grey)

Click the "Find NA and Curvature" button to find the Neutral Axis and Curvature that will provide equilibrium under composite loading.

The Stage 1 and Composite sections may be plotted, showing the Neutral Axis positions

Typical output showing Stage 2 (composite) results

Strain diagrams for Stage 1, increment under Stage 2, and combined Stage 1 + 2 strains