You can download a spreadsheet that will help you design a pre-stressed concrete beam.
The spreadsheet is not yet complete and has not been extensively checked, so use with caution

Download spreadsheet
 

Pre-stressed Balsa bridges

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Winner highest strength to weight  92.9g  Ultimate Load 600N  Strength/weight = 659
 
 
 
 

Winner lightest   90.7g  Ultimate Load = 75N