w-shape with channel cap - s values

huangyhg

w-shape with channel cap - s values
i am beginning the structural steel design of a processing plant, i am looking to get approximate steel sizes to help me in determining my dead loads.  the main process building is 100' wide and has a bridge crane running the full width and length of the building.  my braced frames are at 20' o.c. and i am currently attempting to get an initial size on the beams that will stretch from bay to bay and directly support the crane rail.  the way i see this accomplished most often is with w-shape capped with a channel (then the rail on top of this built-up assembly).  
when i referenced the 13th edition steel manual (table 1-19) i found that there are two s values listed for x axis bending (s1 and s2).  is anyone familiar with the difference between these two values and how they are used?  i see the equation and graphic on the top of the page and am still a bit lost for the concept of what is happening.  
thank you.
s1 is used to calculate stress in the bottom flange and s2 for the top.  
any shape that is asymmetric will have two different s-values about the assymetric axis due to the fact that the extreme fibers are not equidistant about the neutral axis (because the neutral axis is not at mid-height.)
s1 is the bottom flange section modulus and s2 is the top flange section modulus.  they are used to determine the stresses in the top and bottom of the runway girder.
what capacity is the crane?   
that makes sense, thank you both.
ctw -
the crane capacity is assumed to be 25 metric tones, there may be another smaller 10 tone crane that can also be in the same bay at the same time, i am planning to design the system for maximum capacity of both cranes lifting at the same time.  i do not know what spacing distance we will limit the cranes travel near each other.  
any advice on selecting the rail itself?  does it depend on the manufacturer's chosen wheel style or is it driven by the expected loads?  at the moment i am assuming the astm a759 profile 135 for a starting place for dead load assumptions.
the cranes should be positioned side by side to obtain the multiple crane wheel spacing.  unless there is a physical stop between cranes, there is no way to guarantee that the cranes will be limited to specific areas, so the worst case scenario needs to be considered.  most crane manufacturers will provide anti-collision devices on the cranes if they know ahead of time that more than one crane will be on the runway.
the rail is normally sized by the crane manufacturer.  i don't have my references at hand, but a previous 30 ton crane project i dealt with used asce 135 lbs rail.  
based on your statements and questions, this appears to be the first crane structure you've designed.  this is not something to take lightly and you're doing the right thing by asking questions.  there are a lot of details that need to be considered in the design to avoid future problems.
search the forum to find other threads related to crane runway design if you haven't already.
aisc design guide 7: industrial buildings--roofs to anchor rods (second edition) is a good publication to own.

i agree with ctw - allow crane manufacturer to size and install the runway rail. there have been a lot changes in the crane industry where they are going with more and smaller wheels were smaller rail is used. this crane could use as small as 40# if it's a moderate duty 8 wheel crane.
suggest the crane builder install the rail - they will do a better job of insuring it is aligned accurately.
crane guys are often more economical in sizing beams too. it's what they do, day-in, day-out and they are well versed on the tricks of being more efficient with sizing.
i myself avoid capped beams if at all possible. the wider flange family of beams without cap is more econoimical 9 times out of 10. i.e. a w24x68 with c12 cap costs much more than a w24x104 or even a 24x117 ...... 20#/ft is the threshold depending on availability and a few other factors.
for 20' spans, i agree that a capped beam is likely not economical.
for longer spans, i would have thought that capped beams are an economical choice although i'm not sure where the cutoff point is between one piece and capped.
for preliminary sizing of a capped beam, i would assume the main beam carries all of the gravity load (including impact) with the compression flange continuously laterally braced.  then i'd assume the channel cap carries all of the lateral loads over the full span and is braced about its weak axis by the main beam.  
in the end, the built-up shape has to be checked using s1 and s2, but the above procedure gives you a starting point.
ba
don't forget to apply those horizontal loads up at the top of the rail. this will create torsion in your beam which you will need to resolve.
another good reference:  cmaa 70, crane manufacturer's association of america publication 70, "specifications for top running bridge & gantry type multiple girder electric overhead traveling cranes."
if you "heard" it on the internet, it's guilty until proven innocent. - dcs