Crane and supporting structural design
Crane and supporting structural design
During the weekend, I have conducted some studies on the Crane and Supporting Structural design and like to share the study findings with you. It should be advised that we are talking about Crane and Supporting Structures not only the Crane itself.
1. Design Approach
There are two types of design approaches,
• Static Design approach - Load Factor Method
• General Design Approach - Allowable Stress Method
1.1 Static Approach,
A safety factor should be applied for Crane and Bearing Structural Design when considering static loading. According to ASME specifications ASME30.20 and ASME BTH-1.
• Use a Safety Factor 2.0 for Category A lifters (Cranes), and – Category A, Lifting loading is defined;
• Apply a Safety Factor 3.0 for Category B lifters (Cranes), -Category B, Lifting loading is undefined or lifting loading is severe.
No dynamic or impact factor is to be applied when applying static approach,
1.2 General Design Approach
We are using this method in our crane tower structural analysis and design using structural analysis program SACS.
If using general design approach, AISC allowable stress (generally 0.6Fy) should be applied for Crane and Bearing Structural Design. In addition, an impact or dynamic factor has to be applied for the lifting load.
Different specifications specified different dynamic impact factor as shown in the following.
• Impact factor 1.2 to 1.5 (not more than 1.5) per specifications ASME30.20 and ASME BTH-1.
• Impact Factor 1.25 per ANSI MH27.1 & MH27.2,
• Dynamic Factor 1.33 per API 2C for fixed pedestal/tower cranes, to be discussed in section 3.
Practically, onshore crane design shall apply impactor factor of 1.25 based on ANSI MH27.1 & MH27.2 specifications, see references attached [1, 2]. Actually, the combination safety factor is almost the same as in load factor design method. Considering that the allowable stress of 0.6Fy, it implies that a safety factor is 1.67, when apply dynamic impact factor of 1.25 for lifting load, the overall safety factor for lifting load is,
It is relatively higher than 2.0 specified in Load Factor Design Method for structures of Category A lifters. That means both load factor method and allowable stress method are based on almost the same strength evaluation criteria for design of crane and bearing structures.
2 Theoretical Impact
Theoretically, as a moving loading imposed on a structural system, additional dynamic loading impact shall be deployed to the structure due to the following reasons:
Regardless of effects from wind, shocking (due to stopping or starting), moving with loading would induce dynamic impact which has to be considered in structural analysis and design.
2.1 Theoretical Impact
If considering the vertical lifting speed only, the dynamic impact is to be counted following the equation,
Where, K with unit of lb/ft as Vertical Spring Rate or Structural System Stiffness;
SWL with unit of lb as static working load or crane rated load;
g gravity, 32.2ft/sec2;
relative velocity of lifting, ft/sec.
2.2 Calculation per our Structure
Based on the crane tower structural analysis, the calculated structural displacement is 3.333” or 0.2777’ at the 40’ tip under static load of 40 kips. Actually the term of
is equivalent to the term of
Thus we can calculate the dynamic impact factor in accordance with the lifting speed as shown in the shown in Table-1.
Table-1, Example of Dynamic Impact Calculation per Current Tower Structure
It is seen from the table that the dynamic factor is about 1.25, 1.33, 1.5 and 2.00 as lifting speed reaches 0.75 ft/sec, 1.0 ft/sec, 1.5 ft/sec and 3.0 ft/sec respectively for our tower structure.
Above example is based on structural deflection of our crane tower with the boom at 135 degree in plane. In fact, the structural deflection is varying due to loading intensity, boom rotation and loading application distance from tower center in plane. Therefore, the theoretical dynamic impact is changing in one structural system in accordance with the actual loading intensity, lifting speed and boom position. Practically, to simplify analysis, a constant impact factor is always used for structural analysis and design as specified by related design specifications.
In general, the higher the lifting speed, the severe the dynamic impact and the stiffer the structure the higher the dynamic impact. When designing a crane supporting structure, a structural layout is developed and the structural stiffness is defined, therefore, the lifting speed is a critical factor concerning dynamic impact for lifting load.
As lifting speed is one of the main factors in dynamic impact evaluation, the specification requires that experienced operator should be employed to control the lifting speed. However, control of speed could not be guaranteed, specification further requires that a “Qualified Professional” should determine the dynamic factor to simplify the calculations.
3 Onshore VS. Offshore
From Section 1 and 2, we can conclude that there is no significant difference from different specifications. The impact factor is in a range of 1.2 to 1.5 depending on the structural stiffness and lifting speed.
Regardless if the specification is for offshore or onshore structure, the dynamic impact factor follows the same theory of structural dynamics. The difference between offshore and onshore structures is the movement of the crane supporting structural base. Actually, the dynamic impact calculation for offshore structure is based on the same equation as that used for onshore structure. The only difference is the speed calculation counting crane base movement as shown in Equation 3-1.
For onshore structures, the speed Vh in Equation 2-1 represents the lifting speed for cranes supported by a fixed structure. For offshore structures, on the other hand, the speed is summation of three directional speeds due to movement of supporting structural base as shown below.
Assuming Vd and Vc to be zero, equation 3-1 returns to equation 2-1, the same as dynamic factor calculation for onshore structures. Therefore, applying API-2C factor 1.333 for fixed structure to onshore structures is reasonable. However, the factor of 1.333 is relatively conservative with 6% higher than the factor 1.25 specified in ANSI specification.
I believe either applying dynamic impact factor 1.25 per ANSI or using factor 1.333 per API is reasonable. However, impact factor 1.25 specified in ANSI specification for onshore structures is strongly recommended for our crane tower structural analysis and design.
I am expecting technical discussions to determine the impact factor for moving load.