huangyhg

wheel load on slab-on-grade
a crane truck operats on a slab-on-grade (6" thick, with mesh raber at mid-height), other than the punching shear, my supervisor asked me to check "bending" of the slab.
have you ever done this? is there any significant bending developed under the wheel in the slab supported by compacted sturcutral subgrade (100%)?
thanks for advice.
the mesh in the slab is not going to help you in bending especially being located at slab mid depth. the maximum flexural stress occurs at the very top and bottom of the slab! the secret to a good slab on grade design is the base and subbase conditions and compactions. i think 6-inch slab is reasonable. place your joints no more than 15 feet apart in both directions. have them saw cut as soon as possible. there are many joints for construction type.
this site will be of tremendous help for you since they have all kinds of notes, tables and details and other details plus software.
there is a bending in the slab due to structure of soil.
aci 360r "design of slabs on grade" has chapter 4.2 "vehicle loads" and appendix a has information and example with charts for different soil conditions, wheel loads and spacing. this publication is very helpful.
another publication is crsi engineering data report number 33 "the structurally reinforced slab-on-grade" has information on slabs with rebars. it will give you amount of reinforcement if slab is not thick enough.
try to find those and good luck. (i used them for floors with forklift and truck loads.)
save your money, do not put mesh because it hardly ever gets in the proper position. it is like pulling teeth with contractors top get them to do the right thing.
i agree absolutely! use rebar, not mesh.
a crane truck operats on a slab-on-grade (6" thick, with mesh raber at mid-height), other than the punching shear, my supervisor asked me to check "bending" of the slab.
is the crane a temporary condition, or will it be used for the life of the pavement? if the crane loads represent the planned use of the pavement, then you need to pay careful attention to the subgrade as well as the pavement thickness and reinforcement.
you will also need to carefully plan the location of the expansion joints so that you can add reinforcement in those areas, assuming the pavment will be wider than 20 feet and joint intersections may be subject to repeated crane wheel loadings. these pavement corners tend to break off under repeated heavy wheel loads - so additional bars are needed within about three or four feet of the joint if you care about the pavement life and appearance. they should run 45° to the main bar pattern (both ways.) size and spacing of the extra bars? i'd use no. 5's at 12 inch spacings - in addition to your normal reinforcement. that's 4 layers of bars; you will have to thicken the pavements in those areas, too.
these details are not needed for most rc pavements because of the difference in loads and the usual absence of joint intersections where heavy wheel loadings are likely to occur -
j1d, you said that a crane truck "operates" on a 6" slab on grade. does that mean you are analyzing an existing slab or designing a new slab? the previously posted comments all seem to assume a new slab is to be designed. i would think their comments could be different if you have an existing slab.
the problem with wwf (mesh) is that when the concrete is being placed the workers are stepping on the wwf and their weight pushes it to the bottom. with the weight of the concrete and its density the wwf does not"rebound" to its originally placed position. this same problem exists for rebar if and when there is no adequate chairs or spacing of chairs to hold it up as the workers step on it. if the rebar spacing is at least 12" on center each way they may be able to miss pushing it down, but as they are trying to place the concrete and moving about with shovels, screeds or even pump hoses they are not concerned with the final placement of the rebar unless the inspector or the engineer is onsite to review with them the importance of not relocating the reinforcement during placement.
when i specify reinforcement i state that the placement of chairs, or what i prefer is concrete blocks having the same f'c as the concrete to be placed, used as chairs, be located such that the maximum deflection in the bars (or wwf) is no greater than 1.5" when supporting the weight of the workers. this ensures that a) the workers can walk on the reinforcement without pushing it all the way down, and b) provides for adequate stiffness so that as they the workers pass over, the reinforcement will spring back to the original location.
in the documents referred to by others you will find that there is both positive and negative bending in the slab. therefore reinforcement placed at the mid-depth accomodates both. the positive moment is directly below the wheel load and the negative moment occurs between wheels on the same axle. similar analyses are used for high storage rack post spacing as well as fork lift traffic and their related loads.
one last observation. i reviewed a construction failure where the contractor had pushed the wwf down to the bottom of the slab. it was a reinforced slope pavement under a bridge. besides walking on the wwf and pushing it down, the contractor did not notify my firm that they were going to make the pour so we were not present on site to instruct them to pull the wwf back into position ( as we had on a previous pour). well they also did not follow our plans that instructed them to turn the side walls down and embed a minimum of two feet into insitu soil. this bridge was over a creek that had been flooding regulary - so much so the original rip rap slope protection had been washed away a few months earlier due to a 100 year storm event. this contractor refused to watch the weather reports and did not adequately protect his work. in a few days after placing the reinforced concrete slope pavement it rained, the creek waters rose and eroded the side of the slope pavement, water then piped underneath the pavement and washed all the soil from under the pavement. gaps as large as two feet deep by 12 to 20 feet wide were observed. we made them tear out the slope pavement. now the reason for this story: i was there to observe the demolition and thought that it would go quickly since the reinforcement was not located in concrete as specified and it was a nominal amount. they used a medium size bull dozer to attempt to break up the slab. the reinforcement had been completely pushed down to the bottom of the slab. this could be seen after the waters receded; re
thanks for your input!
this is a new slab-on-grade, the operation of crane is kind of temporary, for equipment installation and maintenance (once a couple of years). thanks pmkpe for the advice and story, i'll use reber and check the stiffness of it w.r.t. chair location.
p.s. is there a simple way to calculate the bending?
regards,
j1d:
my specifications instruct the contractor to make the check in the field. that way i don't dictate to them where to place the supports. its a performance requirement to limit deflection to 1 1/2". how they choose to do it is up to them.
for the design of concrete slabs-on-grade with and without reinforcement, for any and all types of loadings, for expansive and "normal" soils - refer to designing floor slabs on grade: step-by-step procedures, sample solutions, and commentary by ringo and anderson from aberdeen press and amazon.
get a copy of designing slabs-on-grade by ringo & anderson. it is invaluable when designing slabs for vehicle loads, racks, etc.
lateral shear stress imposed from the subsidence determines your mesh strength parameters according to their anchored lock by the granular mass utilized, which is related to the punch shear force as it would cause a drawdown, but of course relieve the problem as you crane sinks into the mire at one point load, instead of spread load across the mesh.
driving fast is no answer, where most of these mesh systems come from, aircraft landing strips on the bering straights aleutians and italy and france, but then things were lighter in those days and they often sank when at rest.
the note as indicated by another reply is your bedding materials must have a good granular effective stress, but perhaps of importance is the lack of shear stress imposed by the wheel forms on the formed surface and water retention causing pond hollows to form, as mesh is designed to flex the only way to prevent this is to either design a sliding slab that pulls back, or fix a rigid system (re-bar as indicated by others) and "float" it.
agreed. but the forestry commission has experience of this with access load bearing sites on bog as do the institute of hydrology from plynlimon, wales wallingford england
mikehydrophys
mdshydroplane

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