What are some good things about pratt-truss bridges?

I really need to know this for a science project ASAP! if you need to see an exact example here's a link:http://www.google.com/imgres?imgurl=http://www.past-inc.org/historic-bridges/pratttruss.jpg&imgrefurl=http://www.past-inc.org/historic-bridges/image-pratt.html&usg=__xOKooTnflAaaK6sqr-NTl3CxEVA=&h=227&w=600&sz=19&hl=en&start=0&zoom=1&tbnid=7pK-i7Qi8lkXYM:&tbnh=91&tbnw=241&ei=wIXQTZX0OYrl0QGz6rCIDg&prev=/search%3Fq%3Dpratt%2Btruss%2Bbridge%26um%3D1%26hl%3Den%26rlz%3D1G1GGLQ_ENCA397%26biw%3D1024%26bih%3D452%26tbm%3Disch&um=1&itbs=1&iact=hc&vpx=489&vpy=147&dur=1214&hovh=138&hovw=365&tx=247&ty=63&page=1&ndsp=8&ved=1t:429,r:2,s:0