Final Flashcards
What is obtained in preliminary structural design?
Location, spacing and scantlings of the principal members.
What are the six main steps of rationally-based preliminary structural design?
1) Construction of the finite element model.
2) Determination of the external loads.
3) Finite Element Analysis.
4) Limit State Analysis.
5) Formulation of constraints.
6) Optimization.
Name three load effects at the hull girder level.
1) V ertical Bending Stress
2) Horizontal Bending Stress
3) Shear Stress
Also: (4) Torsional Stress
What is the single most important load effect in the analysis and design of ship structures?
Maximum hull girder bending moment.
What is a “limit state” and what are the two main categories?
A “limit state” is any condition in which a structure or a structural member has become unfit for one of its intended roles because of one or more loads and/or load effects. The two main categories are unserviceability and collapse.
What are the three main types of structural failure?
Plastic deformation, instability (buckling) and fracture.
For a closed, rectangular box girder, in what regions would simple beam theory underpredict the longitudinal normal stress? Why?
Simple beam theory predictsthat the inner edge of a rectangular box girder doesn’t experience as much stress at the outer edge.
Meaning that the outer edge experiences more stretching, therefore more longitudinal strain and longitudinal stress.
The longitudinal normal stress on the outer edge can therefore be underpredicted.
What is the idea behind the definition of “effective breadth”
Effective breadth is a way of accounting for shear lag. Using this breadth in your moment of inertia calculations will give
you the maximum stress Vmax across the width of the flange, which will account for any shear lag.
The total longitudinal force of the flange should be equal actual and simplified cases.
Compare steels, aluminum alloys, and fiber-reinforced plastics in terms of cost, density/weight, stiffness, strength, corrosion resistance, and fire protection.
Steels
Cost:
steel < AA (aluminum alloys) < FRP ( fiber-reinforced plastics )
“ $500/ton for shipbuilding steel, $ 2900/ton aluminum, $ 7500/ton FRP, and $ 15 000/ton composites. “ (pg. 20-1)
Density/Weight:
FRP < AA < steel
“ very good flexural stiffness and strength for low weight “ ( On FRP, pg 20-12)
“The advantages of aluminum alloys over steel are low den- sity, high strength-to-weight ratio” (On AA, pg 20-9)
Stiffness:
FRP < AA < steel
“ readily formed into complex shapes” ( On FRP, pg 20-12)
Strength:
FRP < steel < aluminum alloys
“ A major difference between these steels and ordi- nary strength steels is that the higher strength steels have special additions such as aluminum, niobium, and vana- dium, which promote microstructural improvements and strengthening. “ (pg. 20-6)
“Disadvantages are (for FRP): …low through thickness strength.” (pg. 20-12)
Corrosion resistance:
steel < aluminum alloys < FRP
“seawater resistant with little or no corrosion; the material is virtually maintenance-free, compensating high initial costs by low maintenance costs” (on FRP, pg. 20-12)
“Aluminum alloys generally do not experience excessive corrosion under normal operating conditions.” (on AA, pg. 20-11)
Fire protection:
FRP < AA < steel
“Disadvantages are (for FRP): … need for adequate fire protection” (On FRP, pg. 20-12)
“Compared with steel, aluminum alloys have relatively low melting points and tend to lose strength rapidly upon exposure to elevated temperatures. Aluminum does not burn in an exothermic reaction in the presence of flame. How- ever, a fire in a compartment can heat the deck above, quickly weakening the deck sufficiently to let heavy objects fall through.” (On AA, pg. 20-11)
What is warping
The warping of a cross section is the net axial deformation pattern that occurs when a cross section deforms out of plane. It does not include the axial displacement caused by a pure rotation of the section (about the neutral axis, due to bending). Rather it is the net axial displacement pattern over the cross section after the rotation due to bending is subtracted out. Warping is caused by shear stress, arising either from transverse
shear force or from torsion.
For what type(s) of ships, the provision of adequate torsional strength is vital and difficult? Why?
A containership hull is essentially a beam of open cross section. Open sections have much
less torsional stiffness than closed sections; that is, they undergo more rotation for a given twisting moment. Also, they exhibit much more warping. Hence provision of adequate torsional strength is vital and difficult.
Warping function is found to be positive, does this point move backwards or forwards in the x direction
Given:
𝑀𝑥 is negative
𝜔𝑛 is positive 𝑢 = 𝜔𝑛 ∗ 𝜃′
And we know 𝑀𝑥 = 𝐺𝐽𝑑𝜃 . Hence 𝜃′ = 𝑑𝜃 will be negative.
𝑑𝑥 𝑑𝑥
𝑠𝑖𝑔𝑛(𝑢) = 𝑠𝑖𝑔𝑛(𝜔𝑛) ∗ 𝑠𝑖𝑔𝑛(𝜃′) = +𝑣𝑒 × −𝑣𝑒 = −𝑣𝑒
Thus
Which means the displacement is negative and hence the point will move backward.
When can the simplified version of the warping equations be used?
The simplified form of equations can be used if:
▪ The section has an axis of symmetry which causes the center of twist to lie on
that axis.
Since ships are usually symmetric about their vertical center-plane, we shall describe the calculations for the case when the z-axis is an axis of symmetry.
▪ If the ship has a flat bottom, then the 𝜁 and 𝜂 axes can be such that the origin is placed at the center of the keel
* h𝐷, the distance to the tangent line of any point, is zero for all points in the bottom.
* s would be measured from the center line and greatly simplifies calculations.
In frame analysis, if a beam element is subjected to distributed load, the load is divided into two parts: an internal load and an external load. What are the internal and external loads?
Internal loads are inside the beam and are responsible for internodal bending.
One example of internal loads is the internal bending moment of the beam Mz(x) in beam bending problems. Another is the components of stress for elasticity problems. You can also express internal loads in terms of the nodal displacements.
External loads act on the entire beam or strucrure, and are located at the boundaries of a beam, for example at the nodes.
What is the idealized “elastic, perfectly elastic” stress-strain curve? Why is it an “idealized” model?
The idealized “elastic, perfectly elastic” stress-strain curve is a model to describe the behavior of a material. You can use a “elastic, perfectly elastic” stress-strain curve for beams to show how the yeilding spreads throughout a cross-section. “Elastic, perfectly elastic” stress-strain curves can show when the bending moment goes farther than the intial yeild value and when the beam reaches the plastic moment, where the “elastic, perfectly elastic” stress-strain curve will start to hinge.
This is an idealized model because the real stress-strain curves tend to increase in stress towards the end of the cross section and having the idealized model is more predictable.
What is a plastic hinge (in the context of beams and plate strips)?
A plastic hinge is where the stresses are equal in the upper and lower sections of the cross section.
A plastic hinge is formed when all the fibers of the beam in that section have reached their maximum load and can’t take anymore bending moment. At that point, the linear line starts to curve horizontal in stress, creating what is know as a plastic hinge.
What are the main contributing factors to fatigue in ship structures
- Structural dicontinuities from the configuration of the ship.
- Configuration of the weld details.
- Weld material defects (porosity, lack of fusion, solidifaction of cracks). 4. Insufficient quality of workmanship in the welding.
- Use of higher tensile steels.
- Cyclic loads or wave-induced loads.
- Corrosive environments.
What are the three common fatigue design strategies? What are the differences?
- Safe-life design: designed to have a high survival rate with little to no regular inspections. (Can be the most expensive and inefficient).
- Fail-safe design: designed around moderate survivial probability with regular inspections held. Expect to make some repairs when they happen.
- Damage tolerant design: provides moderate survival probability with cracks detected non-destructively and calculations are made to determine remaining lifetime constantly.
