Exam Questions

Question 1

Give rough numbers for strength and stiffness properties of concrete (characteristic,
mean or design value)

Answer 1

e.g. C25/30 25 MPa char. compression strength,

Stiffness about 30 GPa (E mean)

Question 2

Give rough numbers for strength and stiffness properties of steel (characteristic, mean
or design value)

Answer 2

e.g. steel S235 characteristic strength 235 MPa (yield strength).

Stiffness about 210 GPa.

Question 3

Give rough numbers for strength and stiffness properties of timber (characteristic,
mean or design value)

Answer 3

C24 char. strength 21 MPa for compression parallel to the fibers. Bending parallel to the fibres 24 MPa.

Overall for timber, compression strength parallel to the fibers between 16-27 MPa.

Overall, stiffness between 4700 - 9400 MPa for normal construction timber.

Question 4

Compare the building materials steel and timber

Answer 4

Timber performs better in regard to fire

Steel has high carbon footprint, it is recyclable but it requires a lot of energy to produce. While timber generally has lower environmental impact.

Steel has higher strength than timber.

Steel is more ductile. With timber you really have to think more about the brittle failure modes.

Question 5

Name different approaches for pre-design of structures

Answer 5

• by using structural analysis software such as RFEM
• by experience and practical knowledge
• by analyzing similar structures
• simple checks for deformation/stresses
• by use of tables and formulae

Question 6

pre-design of truss by simple stress checks

Answer 6

approximate it to a simply supported beam.

calculate upper/lower chord of the truss:
F_Cc = - M/z ; F_T = M/z

Diagonals/verticals:

D_iC = - Vij / sin(a)

use the fact that the shear force is largest at the supports

estimate the stresses and choose cross section accordingly

Question 7

pre-design of an arch by simple stress checks

Answer 7

approximate it to a simply supported beam.

Arch normal force (center): F_C = - M/z

Horizontal support force: F_H = M/z

vertical support force: qL/2

F_c,max = sqrt( (F_v)^2 + (F_h)^2)

Question 8

simple stress check of an I-joist loaded by a bending moment and shear force

Answer 8

Divide into flanges and web.

F_C = -M/z

F_T = M/z

stress = F_C / A_flange
tau = 1.5 * V / A_web

Question 9

What criteria, aspects and boundary conditions need to be considered when deciding a structural system?

Answer 9

Load types

Span- and load bearing requirements

Lateral stability (stability against wind loads and seismic loads)

Deflection and vibration

Material affects strength and stiffness and therefore suitability for load-bearing capacity and structural efficiency. Also maintenance and durability.

Question 10

Compare frame structures with trusses
- sketch / describe different systems
- boundary conditions
- external loading
- internal forces

Answer 10

1. Sketch/Describe Different Systems
   Frame Structures: Consist of beams and columns connected rigidly, allowing bending moments to be transferred.
   Trusses: Consist of interconnected triangular elements, primarily carrying axial forces (tension or compression).
2. Boundary Conditions (Supports, Hinges)
   Frame structures often have rigid or semi-rigid joints, allowing moment transfer. Supports can be fixed, pinned, or roller.
   Trusses typically use pinned joints, meaning only axial forces are transferred, with supports being pinned or roller.
3. External Loading
   Frame structures handle vertical loads (gravity) and lateral loads (wind, seismic) by bending, shear, and axial forces.
   Trusses distribute loads through axial forces, making them more efficient for spanning large distances.
4. Internal Forces
   In frame structures, beams experience bending moments, shear, and axial forces, while columns primarily take axial loads and bending moments.
   In trusses, all members experience only axial tension or compression, reducing material usage but requiring precise joint design.

Question 11

Sketch / describe possible shapes of arch structures
- geometry of the arch -> influence on internal forces
- different loading situations
- hinge settings

Answer 11

Circular arches are good for equal load distribution.

Parabolic arches are ideal for uniform loads and reduce bending moments.

Pointed arches reduce lateral thrust.

The arch shape will determine how forces are transferred: a flatter arch develops more bending moments. A semi-circular or parabolic arch primarily experience compression which makes it very efficient.

Different loading situations:
* uniform loads: result in compression forces along the arch
* point loads cause localized bending moments and shear forces
* assymmetric loads will generate additional bending and thrust

Hinge settings:
* three hinged arches, they are statically determinate
* two hinged arches, which provide better stability than a three hinged
* fixed arch, fully restrained at the supports which makes them stiffer

Question 12

Name one global environmental issue and give a short explanation (including problem and indicators/examples).

Answer 12

Climate change. Caused by excessive emissions of greenhouse gases which leads to for example rising global temperatures and extreme weather events as we have seen and are currently seeing. Some indicators are for example co2 concentration and melting glaciers.

Question 13

Explain in your own words the concept of “Sustainable Development”

Answer 13

Sustainable development means meeting current needs without compromising the future generations’ ability to meet their needs. It balances economic growth, environmental protection, and social well-being.

Question 14

What are the three pillars of Sustainability? Give an example for each pillar

Answer 14

Environmental: Reducing carbon emissions in construction (e.g., using low-carbon materials).

Social: Ensuring safe and comfortable housing (e.g., accessible building design).

Economic: Cost-effective, long-lasting infrastructure (e.g., energy-efficient buildings).

Question 15

Why do you think that nowadays sustainability is link to the environmental pillar of the sustainability pillars?

Answer 15

Sustainability is often linked to the environment because climate change, resource depletion, and pollution are all very urgent global threats

Question 16

Why should civil engineers be involved in sustainability issues?

Answer 16

Civil engineers shape the built environment, influencing material use, energy consumption, and waste generation. By making sustainable designs we can reduce environmental impact.

Question 17

What is the built environment? What is the role of buildings regarding the sustainable development?

Answer 17

The built environment includes all human-made structures. Buildings consume energy, water, and materials, impacting the environment. Sustainable buildings minimize resource use and emissions.

Question 18

What are the main stages of in the life cycle of a building?

Answer 18

1. Raw material extraction
2. Manufacturing
3. Construction
4. use and maintenance
5. End-of-life (demolition/re-use/recycling)

Question 19

Name two environmental design principles (not DfD) you have learned in the course, give an example/approach how
do deal with it, and link them to the possible stages in the life cycle of a building and explain the link shortly.

Answer 19

Energy efficiency: for example passive house design reduces operational energy in the use phase of the buildings life. More energy efficient buildings are better for the environment since they typically have much lower carbon footprint.

Material efficiency: Use an optimized amount of material to reduce waste and co2 emissions. For example also using recycled materials where possible reduces resource depletion and lowers carbon footprint.

Question 20

What are the tools commonly used to quantify the sustainability of buildings? Why do we need measure
sustainability?

Answer 20

LCA, and with building certification systems like LEED, BREEAM, Miljöbyggnad.

Measuring the sustainability helps track the environmental impact and improve the buildings performance

Question 21

Explain the dominating linear approach in the building sector? And guess why it is like that?

Answer 21

the linear approach follows like a take-make-dispose model which consumes resources without reusing them. It probably persists because it is convenient economically and there is a lack of regulations regarding this. The building sector likes to follow traditional practice.

Question 22

Explain how a circular approach in the building sector could work and link it to sustainability.

Answer 22

By thinking more like a circle; by re-using and recycling and designing for adaptability. By changing to a more circular way of building we would reduce waste and lower carbon emissions.

Question 23

What is required for stability of a structure?

Answer 23

The structure must resist vertical and horizontal loads without excessive deformation or collapse.

Question 24

What kind of structural systems are used for the lateral stability of structures?

Answer 24

• Bracings
• Rigid moment frames
• Shear walls and /or plates
• Core systems

Question 25

Advantages/disadvantages of different stability systems?

Answer 25

Bracings: efficient but obstruct internal space Moment frames: flexible layout but higher material demand since the connections are rigid Shear walls: great lateral resistance but they are heavy and limit openings Core systems: strong and space efficient but require careful integration

Question 26

What are imperfections and how should they be used in the design of a structure?

Answer 26

In reality, structures are not perfectly aligned like in the calculations. This means excentricities which we need to consider. There are also material imperfections.

Question 27

What is the difference between first order and second order analysis?

Answer 27

First order theory considers only the original geometry without any imperfections. In second order theory we account for deformations affecting equilibrium which is very critical for slender structures.

Question 28

Which factors could influence your choice of a structural design model?

Answer 28

* what level of detail you need * what sort of analysis are you doing and for what purpose * stage of the project * structural type

Question 29

Which structural models could you use to analyze a concrete or timber slab?

Answer 29

* 3D slab with solid elements * 2D slab with shell elements * 1 dimensional beam

Question 30

What is the influence of rigid connections in trusses?

Answer 30

Members will experience bending moments and shear forces in addition to axial forces. The stiffness is increased, reduces deflections

Question 31

What is the influence of loading in nodes or along the members (in trusses)?

Answer 31

In nodes: - ideal truss assumption - all members experience only axial forces - prevents bending moments in members - common in roof trusses and bridges where loads are transferred through purlins or stringers to the nodes Along the members: - when loads act directly on the members - members experience not just axial forces but also bending and shear, making them behave more like beams rather than pure truss elements - when you have heavy trusses where secondary loads act diretly on the chords

Question 32

Explain the semi-probabilistic design concept in the Eurocodes

Answer 32

Loads and material properties naturally vary, so they are modelled with probability distributions. For example the distributions are modelled with historical data and measurements, and so instead of just using the maximum windload that has ever been recorded in human history you would choose a value such that the probability of failure of the structure is sufficiently low.

Question 33

How can you reduce the imposed load with regard to loaded area or the number of loaded floors or both?

Answer 33

The imposed load can be reduced with regard to both loaded area and number of floors, when certain conditions are met. For example, more than two floors of the same type and for loaded area you have to take into consideration slab sizes. When you reduce the imposed load in this way, in the load combinations you should not reduce the load further with phi-values when it is an accompanying load.

Question 34

Explain the difference between ULS and SLS

Answer 34

The limit states that concern the safety of people or the safety of the structure shall be classified as ultimate limit states. The limit states that concern the functioning of the structure or structural members under normal use shall be classified as servicability limit states. here is also included comfort of people using the building and the appearance of the building.

Question 35

Did you have any issues with robustness in your structure? Can the collapse of a vital member eg a column, beam or truss be overbridged? if not, how could you achieve sufficient robustness?

Answer 35

The system has good robustness as most of the load bearing elements are either statically indeterminant or there are many next to each other. for example, the ceilings are multispan structures and as such they will not collapse if one support fails. The outer walls are constructed as two span beams with the foundation, the ceiling of the ground floor and the roof as supports. Only the foundation support provides a vertical support. However if one column in the outer wall fails, since there are many columns close to each other in the outer walls the other columns can abrosb the load of the failed element and the structure in total would not collapse.

Question 36

Describe the difference between static and dynamic loads.

Answer 36

Static loads are for example dead loads or live loads, they are loads that are considered to not change significantly with time. Dynamic loads change rapidly with time and involve acceleration and inertia effects. In a static problem, the response due to static loading is displacement only. in a dynamic problem, the response is displacement but also velocity and acceleration.

Question 37

Which range of frequencies of wind-induced vibration can affect the motor skills of the occupants?

Answer 37

Vibrations at less than 1 Hz and especially between 0.1-0.5 Hz can cause nausea and vibrations at more than 1 Hz affect the motor skills.

Question 38

How can you evaluate the response of a floor system under human induced vibrations?

Answer 38

First you should determine the characteristics of the loading, you can describe the standard walking load of a person as a series of consecutive steps and each step is given by a polynomial function. Next, determine the dynamic properties of the floor. Eigenfrequency, modal mass and damping value. Lastly you make a comfort assessment of the floor using OS-RMS method.

Question 39

Describe the one step-root square mean (OS-RMS) method used for the comfort assessment of a floor.

Answer 39

With this method you calculate a value, called the OS-RMS value which represents the response of a floor which is brought into vibration due to a person walking on that floor. You obtain this value from measurements or simulations and a standard walking load function for a person with given weight and walking pace. With this value you can then classify your floor.

Question 40

How many classes of perception are included in the OS-RMS method and how do you use the design chart for the vibration assessment?

Answer 40

6 classes, A-F. In the chart upper and lower limits are given for OS-RMS value in each class. Then, depending on the usage of the floor you can see which class is recommended or not for that floor. For example for residential buildings classes A-D is recommended.

Question 41

Describe the basic principles of seismic design.

Answer 41

You have to make sure the structure resist horizontal and vertical shaking through proper load paths. There should be redundancy, meaning there should be multiple load paths and lateral force-resisting elements if one component fails Also symmetry is important to think about, since regular, symmetric buildings perform better in earthquakes by avoiding the torsional effects. Irregular structures concentrate stresses which is very unfavourable in case of earthquake. Base isolation and damping is another important thing to think about. You can have base isolators to reduce the seismic forces and dampers to absorb energy and reduce the vibrations. Foundation design is also very important, since earthquake load loads the structure by shaking at foundation level Also ductility and energy dissipation. Ductile materials such as steel can deform without sudden failure and so absorb seismic energy.

Question 42

What are the fundamental requirements in the Eurocode 8 for the seismic design of a structure?

Answer 42

Structures must be designed in such a way that, in the event of earthquakes, human lives are protected, damage is limited and structures that are important for civil protection must remain operational. The fundamental requirements can be divided into two categories: - No collapse requirement: the structure must withstand the design seismic action without local or global collapse. It should retain structural integrity and reidual load bearing capacity after the event. For ordinary structures this requirement should be met for a reference seismic action with 10% probability of exceedence in 50 years. ie with 475 years return period. - Damage limitation requirement: The structure must withstand a more frequent seismic action without damage. The structure should not have permanent deformations and its elements should retain its original strength and stiffness with no need for structural repair.

Question 43

What are the main differences between a brittle and ductile behavior of a structure? Give some examples.

Answer 43

Brittle behavior: - low deformation before failure - sudden failure or rupture - low energy absorbion. they cannot disspiate much energy before breaking Ductile behavior: - high deformation before failure, which gives warning of impending failure - high energy absorbtion, meaning they can disspiate seismic energy effectively - failure mode is yielding, plastic deformation or slow fracture

Question 44

Describe the basic concepts of capacity design.

Answer 44

The basic concept is to maximise the energy dissipation and prevent catastrophic collapse. This can be done by use of ductile elements and connections. Also by use of strong-column weak-beam concept, meaning beams should yield before columns to prevent a progressive collapse. Plastic hinges should form in beams to allow the structure to sway without collapsing.