balsa wood through or under???

lord_jebus

hi,

i have to design and build a balsa wood bridge for a class at university,

i've been reading up and it seems for this type of material its best to use a truss bridge as its only good in tension or compression

the load will be at the bottom i think

wondering whether a through truss would be better than an under truss and why? cant find any info on why an under truss isnt used much apart from clearance/space issues.

also which is better for a load directly applied in the centre, the pratt or the howe? im not certain

any advice would be greatly appreciated

thanks

andy