Design Considerations in Heavy Construction
Heavy construction has more complex structural arrangement compared to common construction of moderate size. Due to limitation in size of construction raw materials, large amount of weldings and boltings are needed in fabrication and installation. This fact should be carefully considered in the design stage. The following is important factors in structural design and analysis of heavy construction:
1. Modelling Inaccuracy Factor (FEA), Fmi
Modelling inaccuracy in Finite Element Analysis is mainly due to mistake in meshing. Coarse mesh in hot-spot zone or zone of high stress fluctuation can give wrong analysis result. Thus, a good understanding of how structure responds to load is required. Engineer should treat FEA as an additional tool. Understanding of Mechanics of Material remains the most important thing in design.
To cover this computation error, we give modelling inaccuracy factor which ranges from 1.05 to 1.20. Selection of this factor depends on complexity of model.
2. Fabrication Defect Factor, Ffd
Welding, bad casting and misalignment of structural members are two main causes of fabrication defect.
a. Weld defects such as undercut and incomplete fusion cause local reduction of strength and lead to failure. In addition, stress flow which passes through transition zone between base material, HAZ (Heat-Affected Zone), and weld material, will fluctuate due to residual stress and change in mechanical properties.
b. Using low quality casting products can be dangerous to structures because usually it has porosity and rough surface in which crack will develope. In severe condition, the part just break away under high load. If possible, avoid using casting product for critical region of structure. If not possible, make sure it is free of defect.
c. Misalignment of structural members such as wavy steel plate and slightly bent beam will disturb the stress flow and initiate buckling failure in compressed member.
Depending on complexity of structures and quality of workmanship, fabrication defect factor ranges from 1.10 to 1.40. Refer to Weld Defects article for estimating value of fabrication defect factor in design and calculation.
3. Material Defect Factor, Fmd
All structural materials such as plates, beams, bolts, etc. have micro and macro defect.
a. Micro defect is distributed evenly in material. If a beam of length L has N amounts of micro defect, then beam of same material with length 2L has 2N amounts of micro defect. In heavy construction where size of structural members is big, probability of failure due to micro defect is higher because number of defect increases as material size increases.
b. Macro defect is material which doesn’t meet the specification. For example:
- Slightly thinner plate.
- Corroded beam due to bad preservation which leads to slight decrease in thickness.
- Notches due to lifting chain
Material defect factor ranges from 1.05 to 1.10.
4. Dynamic Factor, Fdy
Dyamic factor is to cover load fluctuation when structure is exposed to dynamic environment. For example:
- Ship rolling motion causes angular acceleration which produces transverse load.
- Acceleration and deceleration of drive motor causes torque fluctuation in the driven shaft.
- Vibration caused by rotating equipment.
Dynamic factor can be omitted when detail information about dynamic load is known and modelled in calculation. In the absence of load information, a dynamic factor ranges from 1.20 to 1.60 is sufficient to cover moderate load fluctuation.
5. Safety Factor, Fsf
Safety factor is to partly cover accidental and unknown loads which might happen due to human error or force majeure. Example of accidental loads:
- Wirerope failure during lifting weight causes the weight to hit structure in the vicinity.
- Brake failure causes moving structure to hit stopper and produces impact.
Safety factor also provides margin for future modification or equipment upgrade. For example:
- Mounting a new equipment on an existing structure.
- Increasing power and speed of a conveyor belt to meet increasing coal production.
Safety factor ranges from 1.40 to 2.00.
6. Design Factor, Fds
Combining all previously mentioned factors, we obtain design factor:
Fds = Fmi x Ffd x Fmd x Fdy x Fsf
For example, if design factor is 3, then allowable stress is:
σ,allow = σ,yield / 3
Based on the this information, design optimization can be done by:
- Perform detailed Finite Element Analysis to lower down modelling inaccuracy
- Perform detailed supervision of fabrication process and quality control to lower down fabrication defect factor
- Perform detailed check on quality of purchased material. Handle and preserve carefully to lower the material defect factor.
- Collect detailed and actual dynamic effect endured by structure. Include it in Finite Element Analysis (Point 1) to cut down the dynamic factor to zero.
- Select safety factor based on detailed risk assessment on each structural component and its consequence in case it fails.
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