I would like to change the tuning of my concert ukulele from GCEA to GDAE. Which kind of string do I need for that (diameter) ? Is it dangerous for the bridge ? (Ref. my ukulele is a Lanikai LU-21 CE)
It would not be dangerous for the bridge if the four strings you select to produce that tuning have about the same tension (measured in lbs or kg) as the strings used for standard tuning.
Here is their explanation of the calculations involved:
Understanding what determines string tension.
In order to determine the tension at which a string will vibrate, you need three pieces of information: the Unit Weight, the Scale Length, and the Frequency of the string. You can use the formulas below to calculate the exact tension for any string using the scale length of your particular instrument. All of the charts illustrate string tensions for each string at a variety of pitches, in case you use alternative tunings.
UW- Unit Weight.
In all the charts and formulas in the brochure, unit weight is expressed in pounds per linear inch (lb/in).
L- Scale Length. This is the vibrating length of the string. This is determined by measuring the distance from the nut to the bridge of the instrument in inches (in).
F- Frequency or pitch. This is the pitch at which you will be tuning the string expressed in cycles per second (Hertz).
To calculate the tension of a string in pounds use the formula below, inserting the three variables described above:
T(Tension)= (UW x (2 x L x F)^2)/386.4
To convert the result into Newtons, simply multiply by 4.45.
If you know what tension you want the string to have, you can calculate the string unit weight. You can then use the charts in this guide to locate a string with approximately the same desired unit weight.
UW(unit weight)= (T x 386.4)/(2 x L x F)^2
Read the actual PDF for more information.