Calculating Soil Bearing Pressure under Mobile Crane Outriggers by Travis C. Smith, P.E. Mesa Associates, Inc. Knoxville, TN June 27, 2014 Revised July 8, 2014 The process of determining the soil bearing pressure under mobile crane outriggers is relatively simple process of taking the maximum outrigger load and dividing it by the effective area of the bearing pad/mat under the outrigger. The following formula may be used to calculate bearing pressure under a mobile crane outrigger.
=
×
where, Q = bearing pressure Pout = maximum outrigger load Le = effective length of bearing pad/mat W e = effective width of bearing pad/mat While determining the soil bearing pressure is relatively simple, determining the maximum outrigger load is not so simple. Calculating the maximum outrigger load is complicated and over the skills of the riggers, supervisor etc. Most crane manufactures have charts that can be used to determine the maximum outrigger load for a given crane configuration. However, these charts are not typically available as most crane manufactures do not like to disseminate this information. Determining the outrigger loads without manufactures charts is a complicated process of crane data collection and mathematical calculations. The following crane data is required to determine outrigger loading of which not all is readily available from the manufactures published technical data on the crane. Data is required to determine outrigger loading: W c = Carrier weight dc = Carrier center of gravity horizontal distance from axis of rotation W u = Upperworks weight, including counterweight du = Upperworks center of gravity horizontal distance from axis of rotation W b = Boom weigh and its center of gravity from boom king pin dl = Distance between front and rear outriggers dt = Transverse distance between outriggers Xo = Outrigger center of gravity from crane axis of rotation t = Distance from boom king pin to crane axis of rotation
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Calculating Soil Bearing Pressure under Mobile Crane Outriggers R = Crane radius (i.e. distance from axis of rotation to load) Lb =Boom and jib center of gravity horizontal distance from boom king pin θ = Boom angle L = Lifted Load Equations for determining outrigger loading:
= + cos = + − = + + = − − = = +
in addition to moment outriggers must support the vertical load, which is supported equally by each outrigger
Lift over rear, outrigger load,
=
± 4 2"
moment portion is added for rear outrigger and subtracted for front outrigger
Lift over side, outrigger load,
# =
+ + + 4 2" 2"
Boom side front outrigger
# =
+ − + 2" 4 2"
Counterweight side front outrigger
=
+ + − 4 2" 2"
Boom side rear outrigger
=
+ − − 4 2" 2"
Counterweight rear front outrigger
where, Mb = Boom moment Mu = Moment of upperworks Vu = Vertical loads of upperworks
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Calculating Soil Bearing Pressure under Mobile Crane Outriggers Mnr = Moment of all loads about outrigger center of gravity lifting over rear Mns = Moment of all loads about outrigger center of gravity lifting over side V = Total vertical load Moment effect values remain constant for a particular load, operating radius, and boom position relative to longitudinal axis—only the sign changes. The same condition exists when the boom is positioned over the corner at an angle from the longitudinal axis of crane, operating over the rear. In this case, the value of Mnr and Mns will be calculated:
= cos ∝ − − = sin ∝ α is the slew angle of boom from rear of the crane. Mnr is portion of moment due to effect load lift
over the rear, and Mns is portion of moment over the side. The individual outrigger pressure is in combination of vertical load and moment. Lift over the corner,
# =
1 + ) − * 4 2 " "
Boom side front outrigger
# =
1 − ) + * 4 2 " "
Counterweight side front outrigger
=
1 + ) + * 4 2 " "
Boom side rear outrigger
=
1 − ) − * 4 2 " "
Counterweight rear front outrigger
It is not unusual for the calculated value for one of the outriggers to be negative. This means that the crane lifts free of an outrigger beam or even lifts a float. In this case, it is considered that the reaction at that outrigger is 0. When two outrigger floats lift or two reactions have negative calculated values, the crane is in the process of tipping. The sum of all outrigger reactions must be equal to total weight of crane + load + boom. In addition, the sum of all moments about crane longitudinal centerline and about transverse line must be 0. A reasonable approximation of the maximum outrigger load can be made using the information in the Australian Mobile Crane Code of Practice (2006). Section 10.2.6 Calculating pressure applied by outriggers states the following: If a crane is designed in accordance with AS 1418.5: Cranes, hoists and winches – Mobile cranes, the crane will overturn within the stability part of the load chart when the maximum safe working load (SWL) is multiplied by a factor of 1.33. In reality, a crane will not approach this
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Calculating Soil Bearing Pressure under Mobile Crane Outriggers condition, provided the operator does not overload the crane. However, a reasonable approximation for maximum ground pressure applied by the outriggers is detailed below. 2
Pressure (tonnes per m ) applied by outrigger feet: =
0.65 × total crane mass + lifted load individual outrigger area =
0.65 × C; + L area
Therefore a reasonable approximation of the maximum outrigger load is as follow. = 0.65 × = + >
where, Pout = maximum outrigger load W C = Total Weight of Crane (including counterweight) W L – Total Weight of Lifted Load (including block, lines, rigging, load, etc) A good source for further information on soil bearing pressure under crane outriggers and crane pad/mat design is a paper titled “Effective Bearing Length of Crane Mats” by David Duerr, P.E. that was presented at the 2010 Crane & Rigging Conference in Houston, Texas.
References: 1. Effective Bearing Length of Crane Mats” by David Duerr, P.E., presented at the 2010 Crane & Rigging Conference in Houston, Texas 2. Bechtel Rigging Handbook, 2nd Edition, Bechtel Equipment Operations, Inc. 3. Australian Mobile Crane Code of Practice 2006, Queensland Government, Department of Justice and Attorney-General.
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