Although I do not consider myself an expert, I have 12 years experience as a structural engineer and 6 years experience designing and building timber framed barns. Based on my experience, I feel a reasonable number to use for typical timber framed hay loft designs is around 60psf. Here is why. Our typical barn is a 24x36 bldg (864 sqft of loft) with a 4ft-knee wall. This building can accommodate approximately 500 bales of hay, which equals 0.58bales per sqft. Or, at 50# per bale, around 30psf. I like to design lofts for around 60psf, but I'm not uncomfortable with 50 or even 40 psf. A building with more overhead or large haymow would require a similar evaluation. In order for a 24x36 building to approach 300psf of loading, it would have to hold 5,184 bales of hay. That's a lot of hay. 300 psf seems excessive for all put the largest commercial hay storage facility. Any thoughts?