Girders is one of those things that can make or break the look of building model, especially an industrial or military unit. If you are making buildings for 28mm or similar you are in luck as there are various types of plastic girder available for most model shops or online. But if you are building at a smaller scale, such as 15mm or even 10mm, finding scale girders can be difficult.
One good alternative is to use railway track. You can buy a section of flexi track for either ‘OO’ or ‘N’ gauge for just a few pounds. Simply remove the metal rails and cut to size with some metal snips and you have perfect I-beam style girders just right for small scale models.