sizing for a residential steal beam

huangyhg

sizing for a residential steal beam.
as a mechanical engineer i have a tendency to question things but structural engineering is not my expertise.  i hired an architect for a full house redesign but i think his beam calculation is off.   the current beam is w8x15 with a single column which i'm looking to remove.
details:
single floor (bedrooms)
span length = 18ft
tributary width= 10ft (2x10's on 16 oc)
using the aisi residential steel beam span tables and a 40psf for the live load and a 20psf for the dead load i'm coming up that a w8 x10 beam will be sufficient.   the architect is saying i need a w8x48 beam which seems to be a little overkill.  
am i looking at this right or am i just dreaming about not having to change out the beam since i'm doing the work myself.
thanks for the help,
steve

looking just at strength and deflection, i'm coming up with heavier than an 8x10 (that is a very light beam), but lighter than an 8x48.  is it only supporting one floor?  it is likely helping to support the roof as well.
additionally, i don't do residential work so it's possible there is a more stringent deflection criteria.
finally, vibrations should be considered.  you would hate to get woken up from a good sleep because your kids are jumping in the next room, right?
would you like an architect to comment on your mechanical design?
i would suggest asking him, if he works for you, you should be free to ask and he should be able to justify the choice of beam size.
i did talked to him and he said that's what he always specs for this type of situation.  i also asked him if he had any calculations and or codes that show that is the required beam size to which he said no.
steve
how is the architect qualified to design steel beams?
sorry steve, i did not know that you already talked to him. his reply seems unacceptable to me, but i am not sure you can do a lot. after all it is him signing the drawings, is he not?
i wouldn't let an architect design any structure in my house!
you should make sure you are accounting for all of the possible loads.  are you considering any attic live load?  from asce 7:
attics with out storage = 10psf
attics with storage = 20psf
also, for a new beam in an existing house i am pretty strict with deflection.  the limits i use are referenced against total load deflection.... not live load.   the last thing i want is the plaster to crack or the doors to not work because there is a deflection problem.

first of all, architect have nothing to do with structural calculation. hire another structural engineer while telling the first designer that you failed to provide me the design as requested.
i'd probably use a w10x22 - ll delf is .28'' with total load defl at .43''.  while this meets code - it would not be good for a tile or marble floor.  also, your dining room cabinents loaded with glasses might tinkle a bit when you walk by.  
beam comes in at 39% - very conservative - but defl is the problem....
if you really want little deflection - try a w12x22 --- less than .33'' total deflection - but your clear basement ceiling height might suffer.  or use two or three sill plates to raise the level.
assuming sheetrock is attached below, the deflection limit is l/360 (ll) and l/240 (dl+ll); which commonly controls the design.  in the irc 2003, the deflection limits are shown in table r301.7 and in the ibc 2003, table 1604.3 (i just happen to have the 2003 on my desk, but they haven't changed).  
at a quick glance it looks like you're closer, but a little light.  although, to have him redesign it, he will likely defer to an engineer.  you would then need to pay the engineer to design the beam; which would likely not save much expense, unless there are numerous beams to save on bulk.  
for example, a w8x48 will run you about $688 at the plant and a w8x18 would cost about $258; which will save you $430.  a nice chunk of change, but an engineer will probably charge you a minimum of $300 dollars to design and seal it, so your down to $130 savings and your architect is not very happy with you.
although there appears to be alot of waste, i would probably tell you to leave it alone.  just my two pennies.
best of luck.