Design Considerations in Heavy Construction

August 5, 2006 at 3:07 am 7 comments

Introduction

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:

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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.

** End of Article **

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Entry filed under: Construction, Design & Analysis.

The Weld Defects #1

7 Comments Add your own

  • 1. mariani  |  August 5, 2006 at 11:09 pm

    Good idea with the blog, Honey.
    A little bit more information for each point may help laymen understand it better.

    GOOD LUCK with your blog! XOXOX

    Reply
  • 2. isadikin  |  August 5, 2006 at 11:15 pm

    Thanks honey. 🙂 It is in continuous improvement edit. Will give as many information as possible.

    Reply
  • 3. Goio  |  August 15, 2006 at 7:18 am

    Arrrggghhhh.. he’s expanding to wordpresssss… oh noooo!!!! … along with her… double arrggghhh … welcome to wordpress, my dear friends :D…

    Gimme! Gimme! Gimme!

    Reply
  • 4. Widi  |  August 28, 2006 at 11:08 am

    I gave a quick look through this page and have one question for you. How did you determine the specified values of safety factor to each factor? Were they based on a theoretical concept or your practical experience?

    Reply
  • 5. isadikin  |  August 28, 2006 at 11:58 am

    Widi: Value for each factor is based on theoretical concept. Actually, almost everything is THEORETICAL here. For example, check the posting WELD DEFECT to see the reason for choosing weld defect factor. It is based on what we find out in practice: that weld contains defect of various degree of severity. Remember, there is no perfect material. The so called STAINLESS STEEL is not STAINLESS at all. it is only corroding at lower rate. Once you accept this REALITY in PRACTICE, the rest is THEORETICAL. For example, I accept that some weld undercut of average size N mm exists. Then theoretically it would reduce the strength of the structure. The amount of reduction is determined by theoretical reduction in area and theoretical stress concentration factor.

    Choosing the factor is THEORETICAL. Understanding welding process is also THEORETICAL. But to explain the undercut or incomplete fusion we must be PRACTICAL. Observing how a welder works, how the welding current setting affects the weld quality, and what the shape of an undercut are all PRACTICAL. Thus, an Engineer must have well-balanced theoretical and practical knowledge. We still have a long list of item for theoretical stuff: fracture mechanics of the undercut, plane stress or plane strain, phase alteration of the steel around the Heat-Affected Zone, and so on. But if we consider too much on these theoretical things, the thing will never be built. If we are not going to build a rocket-science equipment, then we just go practical and cover all these sophisticated theoretical stuff with a safety factor which we know is adequate from experience.

    Reply
  • 6. Adi  |  December 22, 2006 at 1:06 pm

    I agree with you ndra. I had experience to design linkage for rotary log grapple. I have calculated with assumption the system of grapple is stuck, and force came from cylinder. If i calculated by normal operation, i could chose 16 mm plate for link. But if stuck condition i must chose 36 mm plate. In addition, we need to assume worst condition, and the result must be compared with the other application in actual.

    Reply
  • 7. Deepak pooranachandran  |  August 6, 2011 at 10:52 am

    How these will be applicable in ship building please give me a brief description……..

    Reply

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This blog is intended to accommodate sharing of thoughts, ideas, and experience in heavy equipment design and construction. You are free to copy, print, and distribute material in this blog provided that you refer back to its source and you do not use it for commercial purpose. Feel free to drop comment. Have a nice day, mate. //

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