base plate design- simple design

huangyhg

base plate design- simple design
hey,
i have to design a base plate for a post for a handrail and i am anchoring it down to concrete with epoxy bolts. the tube is a 2.5" sq tube. it is 3'10" tall and has a concentrated force of 200#.
my moment is 9200 #-in and the uplift at the base of the tube is ~3.6kips. i am going to weld around the tube with 1/4" weld, which is sufficient to keep the tube connected to the base plate.
i'm having a mental lapse on what is the uplift load on the epoxy bolts, and the forces in the plate. originally, i was just going to treat the plate as a beam from the point of compression on the one side to the bolt, with the tension force as a concentrated load in the center of the plate. but the uplift isn't distributed in the same way, it would seem to be more like a cantilever. now i know i've really wrapped myself around such a simple problem, and i know my 6x6 plate works, because i've had my boss design it with me  when i first started (i'm 2 years in, and i'm just choking right now on this).
any help?
rc
all that is necessary for the triumph of evil is that good men do nothing.
    edmund burke

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what uplift are you referring to with the 3.6 kips?  that seems like you are using 9200/2.5 =~ 3.6.  if you actually designed the baseplate then you had to get the bearing pressure distribution and uplift in the anchor bolts to size the plate thickness.  use those values.  the uplift in the anchor bolts will be less than the 3.6k because you have a bigger moment arm (most likely).
seit,
the 3.6 kips was just to show the progress of my design. that is the 9200/2.5. using t=c=m/d,etc. to obtain that. i'm looking at design guide 2 for "small moment" base plates but they use the modified asd, where the code in this jurisdiction is still ibc2000 so i'm using '89 asd. i'm asking how do i size the plate and get the tension all in one.
thanks for the quick response.   
rc
all that is necessary for the triumph of evil is that good men do nothing.
    edmund burke

i thought you said you already designed the plate?  
i don't believe this qualifies as a small moment situation.  a small moment situation is when the entire baseplate is in compression such that there is no uplift on the anchor bolts, that is not your situation.  you have uplift on the anchor bolts, hence you are in a large moment situation.
look at the aisc base plate design guide or use a program.  you know the uplift should be less than 3.6 k.
firstly use design guide 1 to calculate the lever arm and the tension in the bolts. design the bolts for this tension and their proportion of the shear.
use the bolt tension to calculate the bending in the plate and design the plate.
then design the welds around the post to take the required moment and shear.

csd72,
i did follow design guide 1 (i think i mentioned 2 above). and as mentioned by seit, i do have a large moment baseplate considering (e > e critical).
the load is only 800# if i follow the design guide, but that just doesn't seem to make sense. that seems too low.
i was thinking maybe half or so. i'm reading more about it now to see if i missed something.
rc
all that is necessary for the triumph of evil is that good men do nothing.
    edmund burke

as soon as i clicked submit, i realized since my e is so great that my moment arm is greater, and it is proportional to that. right?
i'm sure that is my answer right there.
rc
all that is necessary for the triumph of evil is that good men do nothing.
    edmund burke

a quick a calc to check this simple plate is the following:
beacuse i dont have the exact geometry of your plate i would make several assumptions. assume you have an 8x8 plate 2.5" sq attachment and 4 bolts with 1" edge distance and 6" bolt spacing. horizontal force @ top of the post = 200 lbs
1. moment = 46x200 = 9200 in-lbs
2. determine tension on the bolts. the moment arm would be
((6")/2 + (2.5"/2)) = 4.25" (you may add 2x the plate thickness but i would not recommend it unles you really nee it). therefore the tension t = 1.2x(9200/2x4.25) = 1298 lbs/bolt. (the 1.2 factor is to account for the plate flexibility - unstiffened plates)
3. the shear on the bolts will be 200/2 = 100 lbs
4. determine bolt allowables from mnfr. make sure to take reductions for edge distance and spacing violations if any (guardrails are usually very close to the edge of conc).
5. check if = t/tall + v/vall < 1.0
6. the plate bending moment will be 2t x moment arm. the moment arm for the assumed plate is (6"-2.5")/2 = 1.75". therefore, the monent m=2x1298x1.75 = 4543 in-lbs
check if plate thickness is adequate. fb=m/s if you solve for t you get: t(required) = sqrt((6xm)/wxfb)) = 0.355"
m = 4543 in-lbs
w = 8 in
fb = weak axis bending allowable = 0.75 sy (assuming a36)
      
i hope this helps
  
here is how we typically calculate the distribution to the anchors.  you bascially itterate until the results converge.  hope this helps, you may also want to try the manufactures anchor software.  they will typically do the load distribution for you.