kokosnow;
What I would do is to hire a local metallurgical lab/field services firm that can perform PMI (Positive Material Identification) and portable hardness testing to attempt to type the material. This is relatively inexpensive and can be done in bulk quantities (girders) with minor surface preparation.
Once you type the material chemical composition, you can evaluate the grade of carbon steel ultimate tensile strength from the portable hardness test results, and find a published yield strength.