determining beam size

huangyhg

determining beam size?
i am needing to determine what size beam to use;  i have a span of 36', simply supported at the ends, with a udl starting @ 10' of 840 lb/ft.  this udl continues for 16'.  if anybody could point me in the right direction it would be greatly appreciated.  what computer programs does anybody suggest to solve these types of problem?
thanks
if you are doing in reinforced concrete you can take the minimum size of beam as 30cm x 60 cms.you can design this by using staad software or any excel home made software
hi kc8
you can solve this problem by a couple of hand calculations,
firstly to get an idea of the maximum deflection of the beam
divide your span by 200, which equates to about 2.16" of deflection.
the deflection formula for a beam with a uniformly distributed load over part of its length is:-
x=[w*l^3/((e*i*384)*(1-2*a))]*(5-24*a^2+16*a^4)
where w=total load on the beam ie:-840lb/ft * 16ft
      e=modulus of elasticity of the beam material
      i=second moment of area
   
      l=span
      a= fraction length of beam at each end not loaded
         ie:- in your case 10ft/36ft
transpose this formula to find "i" which becomes if my maths is correct:-
            i=[w*l^3/((e*x*384)*(1-2*a))]*(5-24*a^2+16*a^4)
once you have the "i" value for the beam you can look up a standard beam and calculate stress from the following formula:-
       stress= my/i
where m=max bending moment (in your case
                             mmax=1/8*w*l*(1+2*a)
                             where w,l and a as the same as
                             described above
      y=distance from the neutral axis to the extreme fibre
        of the beam
hope this helps
desertfox
hi,
if you are familiar with mathcad, there is a great general beam solution file available at mathcad's civil engineering user forum. the file's name is "f_d_beam.mcd" and it can solve almost any type of beam with various boundary conditions. i strongly recommend looking at this file.
do you really need a computer program to do that?  beam formulas for simple beam - uniform load partially distributed can be found in any aisc book.  if you have the asd ninth edition in front of you, its on page 2-297.
if you want a steel section:
continue with the asd steel manual and once you have a moment, use the beam selection chart (p. 2-149 to 2-211) to choose a section size.  this will take ltb considerations into account.
for wood or concrete i could give you excel sheets that do the job.
peter von buelow
univ. of mich. - tcaup
beam analysis can be easily done by hand if the loading is simple (uniform load, point load at mid-span, third points, quarter points etc).  per pvb, steel asd can be used to determine internal forced such as shear and moment.  the asd beam diagrams and formulas aren't restricted to steel - it is general.  once the forces are known, design shall be based on appropriate material strengths and section properties.
it really bothers me to see so many engineers' reliance on computer software.  i sincerely hope that new generation of engineers realize that computer is merely a tool to achieve "our" (engineer's) design intent.  computer lingo from the early 80's: garbage in garbage out.
be careful, people.
the problem as stated should be done by hand; it's too easy for a computer.
that being said, a great (and as far as i'm concerned, the) program to check beam problems on is dr.beam and/or dr.frame, from drsoftware. i believe the url is
i appreciate all of the help.  at this point, i am just needing to get an idea of the beam size so i can include it in my cost of shoring up something.  thanks again.
the first paragraph in a white paper titled "analysis of simply supported beams" states:
'a solid grasp of simply-supported beams is one of the cornerstones of an engineer's education. this is even more important when the analysis is done by computer. to know and understand the relationship between loads, shears, bending moments, slopes and deflections serves as a check on the computer's output.'
the paper presents the relationship in very succinct form and concludes with a number of practical solutions including the question posed here.
view or download the paper here:
kc8,
getting back to your problem (however you want to calculate it) it looks to me like you've got a bit over 100 '-k to carry.  if you can look at the beam chart on 2-171 of the asd steel manual (that's the graphs not the sx table) you'll see that if you can brace it where the load starts, i.e. 10', then the graph shows either w16x40 or w18x40 as ok.  the bracing against lateral torsional buckling (ltb) is critical with the long span.  you can see on the chart the effect of unbraced lenght on the beam size.
peter von buelow
univ. of mich. - tcaup