foundation for 12 kips pull-out force

huangyhg

foundation for 12 kips pull-out force
hi all:
i am designing a small steel assembly for a mechine, which has four "legs".  the legs of this assembly will be bolted to an industrial floor on grade.  due to the overturning moment, the pullout force for legs is approximiately 20 kips per leg. the self weight of the assembly can provide about 2 kips (500 lb per leg).  the floor slab is a conventional 5" reinforced concrete slab which will not be able to resist this force. therefore, i need to make concrete blocks underneath these legs.
can anyone please tell me how to determine the size of the concrete block (20 kips will need a monster block by self weight alone)? how to take the friction between soil and concrete block into accout or not? do you have any other method besides concrete blocks?  please do not ask me to use "rock anchors" or "tie back" since it is supposed to be a small project. please let me know your thought and experience.  i will really appreciate.  thank you.
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shp6
if the legs are not too spread out, a mat foundation under the entire structure would work well.  the overturning check would then engage the whole mat and not just a single block under each leg.
alternatively, you could use a drilled pier of some sorts and drill deep enough to provide the necessary holddown weight.  i'm always reluctant to utilize soil friction for holddown/overturning conditions as soil parameters are sometimes hard to pin down without a large variability in safety factor.
you can also dig down and construct a large footing with a pedestal for the columns.  the footing, if wide enough, can engage the soil above and your calculations would then include the weight of the footing, the pedestal, and the soil on top of the footing.  some engineers use the soil directly over the footing while others (myself included) use a splayed area of soil (30 degrees from the vertical) that add to the downward counteracting force.
a spreadsheet can be quickly written to let you try different ranges of footing size, thickness, depth, etc. to get an optimum design.
can you use one foundation for all legs? in this case you will have weight of equipment plus weight of concrete foundation and moment.
if you can make eccentricity (e= m/p) less than half of foundation length (e<l/2) you may use equation for bearing pressure:
q= 2p/[3b(0.5l-e)]
this may be better than 4 monster blocks.
hi:  thank jay and whymrg.  the reason i posted this question is because we do not want to open a big hole inside our mechine shop.  the "legs" i have so far is about 1 foot by 1 foot and bolted down with two 1" diameter a490 bolt or threaded rods.  my idea is to build some kind of underground block to anchor these bolts.  we only want to open a hole say 2 feet by 2 feet the most to place each leg.  the depth may vary depends on the method we choose.  i heard that there is some kind of drilling deep anchors which may be able to help.  if anyone has other ideas, please let me know.  thanks.
hi shp6,
re the "deep drilling anchors" maybe you are referring to atlas or manta-ray, basically galv steel screw anchors. but this does seem to be getting close to a "soil anchor" which you said you wished to avoid.
but if you are indeed considering vertical anchors...you could consider a passive (or active) vertical soil nail/bolt - drill a 6" or 8" hole using your tei rig (this is the one that fits thru a standard door way and is electric powewered, 6 foot high mast), drill it about 10 feet deep (depending on your site conditions this will take less than 1 hour per hole is no casing is required), install 3.5" dia hdpe corrugated duct for corrosion protection, install 150 ksi threaded bar, grout inner and outer annulus, cure, proof test to 20 kips (you have all the stuff to do this), use the 150 ksi bar, plate and nut as anchor attachments for machine.
is the loading from the machine static or dynamic? for a "small machine" it sure has a good sized overturning moment/uplift.
i know i may be way off track, but since i know you and the "gear" that your shop, has you may wish to revisit this option.
hth
shp6,
if you want to hold each 20 kips uplift on each legs just by weight of concrete, you would need a block weighing 30 kips (safety factor = 1.5), that is only 7.5 cubic yard of concrete (6' by 6' by 6' block)
other solutions would be the mat footing under the entire equipment or the anchors described by jae, whymrg and ingenuity
aef
i'm just curious - what kind of machine that weighs about 3 kips produces a possible uplift on each leg of 20 kips?
hi:  
    thanks to jay, whymrg, ingenuity, dlew, and pylko.  just for you curiosity for this mechine, let me try to describe this setup for you.  as ingenuity knew, i am working in a company has a lot of stressing rams.  these rams can take as much as 1700 tons of force.  these rams includes two part: outer shell and inner piston.  these two parts are screwed together internally.  from time to time, we need to disamble them to clean inside and calibrate the ram.  believe or not, to unscrew these rams it takes about 60,000 ft-lb of torque based on our previous experience.  the largest ram has a size of 2 feet in diameter and 3 feet height.  it weights 6000 lbs (sorry, not 3000 lbs as i mentioned previously).  in order to produce this 60,000 ft-lb at a distance of 2 feet, i need to use a hydrolic piston to produce 30 kips force.  in order to make this entire setup smaller, i have two floor beams length 5 feet each and the reaction force from the legs is approximately 20 kips in tension or compression. i hope this will give you some sense about this setup.
    ingenuity, i think i may go for the method you mentioned.  i need to get more info about this method.  thank you for your help.  i hope you are doing everything well there.
try
hi shp6,
you remind me of fond but frustrating memories!
the first time i did 0.5" x 31 strand stressing we blew the gland seal on the jack!! it was in a remote area of the world and although a 31 strand jack weighs only 900 lb, returning the jack back to origin for repair was not an option.
to remove and replace the gland nut took an almighty human effort. we had very limited tools available to us, so we chain blocked the jack base to three 40' containers (120 degree angles), bolted long steel sections to the top face of the gland nut, and with 4x250lb men (i did not fit this category, so i guess i supplied about 1/10th the required force) proceeded to tighten/loosen the nut. only done it once by hand, hopefully never again!
for a 6000 lb jack that can stress 91 strands if i am not mistaken - that is an absolute monsters!
i take it that the 30 kip hyd piston is in a horizontal position. so take care of the horizontal force at the foundation too!
i know you want a compact, self-contained base, but maybe it is worth splitting up the jack base from the piston base. seperate them by say 10 feet horizontally and use the mechanical advantage - less hyd force required, hence less foundation reactions. you can attach a steel lever section to the top of the gland nut using the tapped holes on the nut circumference. on the infrequent times that you have to do this setup i am thinking that the bases could be demountable so reduce congestion in your shop - just flush recesses for bolting the removable base with temporary covers for forklifts. just a thought.
also, soil anchors can also work in compression - mini/micro/pin piles - and have tremendious capacity in the right soil conditions.
i have designed and constructed several soil anchors/mini piles in the past, but local geotech knowledge is worth a lot (everything!), so speak with your geostructural guys for specific details.
good luck. you get some interesting projects!!!
hth
hi: ingenuity, thanks for your advise.  after talking to our shop manager, we may want to do a mat foundation instead of mini pile as you mentioned.  the reason for that is we do not have the right equipment to drill a 10 feet deep hole here and the soil condition is unknown, either.  based on my quick calculation about the mat foundation, we need a 7'x7'x2' concrete base with steel to form this foundation.  we figure that will be faster and less questions.  we do not make the decision yet.  i am actually doing both calculations to see which one is faster and cheaper.
the story you mentioned is very interested.  i did not know these pistons are so difficult to seperate.  these 1700 ton rams are like monsters.  i am actually designing a crane to be integrated into this system and a pipe column will be used.  this crane needs to take at least 8000 lb force and the arm is about 7 feet long.  this crane is another monster for us but i think i am getting closer to get this monster.  in addition, as you mentioned, i designed this system to be able to be disassembled and moved.  as you know, we are looking for an new office and prepare to move soon.  i have to make sure this system can be easily moved to our new home.  i designed everything bolted instead of welded.  you know bolting sometimes causes some problems but we will make it to work.
it is an interested project.  i wish you were here to see us building this system.  i will send you a picture to you when we completed it.  you can probably build the same system in your shop.
thanks again and good luck.