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Introduction To Structures.docx
1. SSUD13-311 Introduction To Structures
Answers:
Task 1 – Structural Design Principles
Question 1.1:
For each of the structural principles pictured, Figure 1, detail structural considerations
required for buildings during their design and service life. Please detail in your own words,
what each of these different structural principles are? And what they related to specifically,
using examples? Please use provided space as a guide for approximate word limit.
Figure 1: Structural Principles
Robustness- the term robustness means that any given structures should be able to
withstand an accidental or a territorial attack which may pop unreasonably.
A typical example is a case when the balcony glasses are made of brittle glass which shatters
into pieces in case of minor accidents, the user may live in fear in that he/she may thought
that every time the glass breaks there may be insecurities and may completely avoid using
the balcony.
Strength- strength of a material/structure is its ability to withstand any loading subjected to
it.
Example
Suppose the same balcony is designed to withstand a loading of 7 members of the family, it
would collapse when a house party is held and additional of 15 run towards the balcony.
This therefore means that strength is a key principle when making designs of structures.
Serviceability–this is basically the conditions in which the structure will be usable. An
appropriate design for structures should be at a position of performing a satisfactory
service loads. By doing this, it too shouldn’t cause any problem to the user due to cracking,
2. vibration etc.
Example
Basing a reference to the same balcony. Suppose the balcony has some visible cracks that
may have been caused by certain loadings, the user may completely avoid using the balcony.
By doing this, the balcony would have failed to serve the initial purpose it was constructed
for. Serviceability is therefore a key principle to be considered while developing structures.
Stability- the term stability is used when there is no rigid displacement or deformation to
the internal members of the structures. Stability can either be external or internal. Internal
stability is majorly concerned with the members of the structures while external stability
basically refers to with the members of the structures.
Example
Suppose the same balcony collapse when subjected to some loadings, the user may be afraid
to visit the balcony as it wouldn’t be stable any more. This would renders it weak and it
would have failed it purpose.
Question 1.2:
Define the following material properties
Ultimate stress
Ultimate stress is referred as the maximum stress that any given material can withstand
under the applied load
Hardness
Hardness- This is the resistance a material offers to a localized plastic deformation.
Harness to ranges from super hard materials to hard materials to soft materials. Super hard
materials include diamond and boron-carbide.
Question 1.3:
What happens to a material after it experiences a stress greater than its yield strength? Your
answer might benefit from the inclusion of a drawing.
In any case a yield strength is exceeded the stress-strain curve would continue to rise to a
maximum point. This maximum point is called the tensile strength point or the ultimate
3. stress.
If more loadings is again subjects to it wouldn’t reach the yield point and the curve would
returns to its original state.
Question 1.4:
Describe three (3) critical considerations related to material selection taking particular
consideration for the relevance to a project and the associated mechanical properties of the
material.
Mechanical properties. This include strength, ductility, hardness, fracture toughness and
also the impact resistance. In the design for a particular material, it should be able to
withstand any loadings. Therefore strength and stiffness is important
Wear resistance of the material.
Wear is problem that occurs at the point of contact between the materials. In material
selection, one should consider selecting material with a greater wear resistant.
Corrosion. Corrosion do occurs when a material reacts with moisture content. It is therefore
important to take note when making a selection and therefore select material that do not
easily corrodes.
Cost- cost is very important factor to be considered in in designing of materials. Costs in
Question 1.5:
A 3 meter long steel CHS (Circular Hollow Section) column, whose external diameter is 296
mm and has a wall thickness of 3 mm, is subjected to a tensile axial force F of 175kN.
Calculate the axial tensile stress due to the applied load?
Question 1.6:
Describe each of the following load types and detail two things that an engineer must
consider when determining the value/size of each load type
Permanent Loads (dead loads)
Dead loads are described as structural loads of constant magnitude over time. This loads
include the self-weight of structural members such as plasters, ceilings, beams, columns and
even floors. Dead loads also include loads of fixtures that are permanently attached to the
structure.
4. To determine the size of the dead loads, an engineer should ensure a repetition of member
sizes, and weights since a constant changes may be witnessed. The process is repeated till
the final member size is obtained.
Live loads
Live loads are temporarily or moveable loads attached to structure. This loads include loads
on a building created by the storage furniture and equipment, impact and also occupancy.
Wind loads
Wind loads is an example of environmental loads. They are pressure exerted on structures
by the flowing winds. Wind flows and directions do varies making it difficult to make
predictions on the exact pressures applied by wind on the existing structures.
Engineers therefore needs to know the relationship between the dynamic wind pressure
and wind velocity while designing of structures. This would enable them to known the
choice of structures to be built on different regions depending on the wind zone.
Question 1.7:
Limit state design is often referred to as a two-layered factor of safety approach. Explain
what this means, why it is done and how it is incorporated into the design and analysis
process?
Limit state design refers to design method used in the structural engineering.
It is done so that the capacity of load or the working load structure can be determined.
Limit state design is really important as it uses modern methods of design of structures
which also involves wide range of logical and technical considerations.
Question 1.8:
You are to design the size of a tensile cross bracing member. What must you know to be
able to determine the size of that member? You may provide an example to assist in
demonstrating your understanding.
To design the size of tensile across a bracing member, one must consider the corresponding
loadings on the structure on which the tension member forms part. Also the net area
required for the member.
5. Question 1.9:
A compressive member has a unique concern related to potential failure when compared to
tension members. Describe what this unique concern is? Then list four ways to increase the
compressive load capacity of a ‘long’ (or slender) column?
How to increase compressive load capacity
Reducing cement to water ratio
Altering the aggregates
Including admixture
By adjustment of cement type and quantity
Question 1.10:
A simplification of the structural process was detailed using the GLAD workflow. List and
describe each of the stages of the GLAD process for structural design
Question 1.11:
Describe two structural stability systems in detail including examples of their common
application in buildings?
Neutral equilibrium structure- this is a state by which the member remains in its initial
state while subjected to slight loading
Stable equilibrium structure. This occurs when a small perturbations do not cause large
movements as compared to that of a mechanisms.
Question 1.12:
Refer to the diagram of the 3D frame structure below. What would a suitable method be to
provide stability in the 3D frame from the load direction indicated at ‘A’? You may use
sketches to support your answer
Since the original frame’s support are hinged. The stability can be provided by the use of
braces which passes through the diagonals of the member as shown in the figure below.
Braces improves the stiffness of the member.
Task 2 – Reactions, Tributary Area And Beam Action
Question 2.1:
6. Find the reactions for the following simply supported beam.
Question 2.2:
Consider the concrete flat plate structure below. Respond to the following tasks:
Draw the tributary areas for each column.
What is the tributary area for C1, C5, C7 and C9?
What is the weight of the slab supported by column C9 (in kN)?
What is the weight of column C9 (in kN)?
What is the total weight of the structure?
Notes
Three story building
All dimensions are in meters.
Slab is 350mm thick
Floor to ceiling height is 3.4m – (the height of each column is 3.4m)
Columns are 400 x 400 mm
Unit weight is 25kN/m3
Question 2.3:
Consider the following beam which is experiencing a design load of 10kN. The beam is made
from a 290 x 35mm piece of timber with a Young’s Modulus of 9000MPa. The vertical
dimension of the timber beam is 290mm.
Equations:
What is the deflection of this beam?
a. Based on a deflection limit of span/300, is this acceptable? Justify your answer.
b. This deflection limit is not desirable as it causes visual distresses to the user of the
building and sometimes can lead to damage of parts of the building
c) If this rafter was not appropriate, what could the engineer do? Make three (3)
suggestions. For each suggestion, describe/detail how it would help with increasing the
beam’s performance. Then comment on which of the solutions would be the most viable and
justify your choice.
Question 3.1 (A)
Task 3 – Structural Systems
7. Question 3.1:
a. The diagrams below show the deformations of a simply supported concrete beam, a fixed
end concrete beam, a cantilever concrete beam and multi-span (or continuous) concrete
beam subject to point loads as shown.
Identify which parts of each beam are in tension and which parts are in compression. Label
these regions on the deflected shapes (on the right)
Draw on the beams (without deflection) where the steel reinforcement (for a normally
reinforced section) should be located for the loading given? The faint dotted line represents
the Neutral Axis of the beam.
Question 3.2:
Concrete can be designed to be under-reinforced or over-reinforced. What does this mean
and how does it influence the failure type?
Over-reinforced concrete do fails suddenly without any warning as a result of crushing the
concrete.
Under-reinforced concrete conditions occurs when the sections provides more warning
prior to the failure time.
Question 3.3:
Discuss the differences between a normal reinforced, pre-tensioned and post
tensioned concrete slabs. Consider the advantages, disadvantages, and construction
methodologies (6 marks)
Normal reinforced
Pre-tensioned
Post tensioned
Small sections constructed
8. Small sections are to be constructed
Size is not restricted
Whole concrete is useful in resisting external forces
Similar prestressed members are prepared
Products are changed according to structure
Question 3.4:
Why are the steel tendons in post-tensioned slabs usually draped so that they are close to
the top of the slab over supports and close to the bottom of the slab at the mid span?
Post tension slab is a combination of conventional slab reinforcement and additional
protruding high-strength steel tendons, which are consequently subjected to tension after
the concrete has set. This hybridizationov helps achieve the formation of a much thinner
slab with a longer span devoid of any column-free spaces
Question 3.5:
Discuss the structure presented below in relation to the system through which stability is
provided for both wind directions pictured. Provide detailed information for both the Cross
Wind and Longitudinal Wind. Please also comment on the ability for stability to be
“transferred” to other bays throughout the structure
9. Longitudinal wind
Question 3.6:
Inside any given structural system: a) list and describe three (3) potential causes of
vibration in the structure and b) list and describe three (3) potential solutions to vibration
within structures
Causes of vibration
- Structural resonance- this is a type of failure which occurs with smaller reciprocating
pumps or compressors.
- Dynamic forces generated by compressors
- Dynamic forces generated by pumps
- Dynamic forces generated by engines
To control vibration,
- The resonance need to be completely avoided
- By limiting vibrations of the structural member
Question 3.7: What Is Meant By Concrete Cover And Why Is It Important?
Concrete cover is referred to as the least distance between the surface of the embedded
reinforcement and the outer surface of the concrete.
Importance
Concrete cover do protects the steel from any harmful influences such as fire and aggressive
solutions.
Task 4 – Soil
Question 4.1:
a) A pad footing has to carry a design load of 450 kN and is founded in soil with a design
allowable bearing pressure of 200 kPa. What is the minimum required diameter of a
circular pad footing to resist this load? Show your workings.
First, area is to be calculated
10. b) During excavation to prepare the pad footing, unexpected material of lower strength is
encountered (100 kPa maximum). Describe two options that could be used to continue with
the preparation of a suitable footing. How would your proposed solution assist the
identified problem?
Question 4.2:
Illustrate (draw) three typical modes of failure mechanisms of retaining walls and describe
them in detail including potential causes of failure.
Overturning Failures
Causes of failure
-inadequate width of the base
-increase in the design loads as compared to that anticipated during the design.
-due to increase of the fill height with time
Sliding failures
11. Causes of failure
- The friction forces that exists between the base material and the rock
- Resistance that is provided by the cohesion in the rocks surfaces.
Bearing Failures
Causes of failures.
- Non-uniform pressure under the base of the walls.
- Soil stress under the base
- The vertical force acting on the soil.
Question 4.3:
Choose three (3) of the ground improvement techniques from the list below. Describe your
three chosen ground improvement techniques and give examples of their application.
12. Surcharge loading
Compaction
Soil mixing
Grouting
Soil reinforcement
Cement/lime stabilization
Surcharge loading- this type of loading results on the objects that add loadings to the
protective system. This loads are also referred to as vertical loads that is mostly used to
backfill the soil above the top of the walls.
Examples include live load ie, which results from highway and parking lot.
Compaction
Soil compaction basically occurs when the soil particles are pressed together therefore
reducing pores spaces present between them.
When the soil is compacted, there is a great reduction in for the rates of water infiltration
and drainage process.
Examples include; a typical example is compression of sediments in water bodies over a
period of time. This results to formation of sedimentary rocks.
Grouting
Grouting is the process of mixing sand, water and cement or chemical in order to fill the
gaps. This mixture is mostly used in filling gaps and repairs in the concrete crack. The
method is also used to seal and fill gaps purposely for waterproofing. Also for soil
stabilizations.
Question 4.4
Described and detail the composition of soil including detailing how different types of soils
are classified?
Clay soil- clay soil is composed of 60% clay minerals, 30% quartz and chert, 5% feldspar,
4% carbonates, 1% organic matter.
13. Sandy soil- sand soil is composed of 35% sand and less than 15% silt and clay. Sand is
basically small pieces of eroded rocks with some gritty texture.
Loamy soil- loamy soil is basically composed of almost equal amount of sand and silts with
little less clay.
Question 4.5
Described the two different types of foundation system (shallow and deep footings). What is
it that distinguishes between the two primary types of systems? Provide details of examples
of each? (3 marks)
Shallow foundation is a foundation which distributes loads from the buildings into layers of
the ground.
Deep footings- this is a type of foundation that transfers buildings loads to the earth farther
down from the surface then the shallow foundation does to a subsurface layer.
Shallow footings
Deep footings
Shallow foundation is considered cheaper
This types of foundation is more expensive than the shallow foundations
Depth is approximately 3 meters
Depth is greater than that of shallow foundations
