I am about to remove a light-weight fiberglass tub and install an ultra-heavy cast iron on the second floor. I have open access underneath which is great and my beams run perpindicular to the tub. ASSUMING that my new tub exceeds the psf limitations of the exsisting floor, in detail, how do I go about stretgthing or "suring" up this area? I am an novice DIY'er at best but I follow instructions really well and would appreciate some help. Pictures are a plus.

Thanks,
Desperate for a bath...tub!

Do you have access to the full length of the floor joists? If so, I would sister in some identical 2x10s by screwing and glueing them in place. Try to reach the end of the current joists and use a joist hanger.