NAME: Chauncey Dale Pequiña
CE42 PRINCIPLES OF RC/PC DESIGN
Assignment 2 – Design Methods
1. What are the advantages of the strength design method as compared to the
allowable stress or alternate design method?
The strength design method, also known as the load and resistance factor
design (LRFD) method, offers several advantages over the allowable stress or
alternate design methods:
1. Safety and Reliability: LRFD considers variability in material properties, loading
conditions, and other factors, leading to a more reliable and safer design by
incorporating appropriate safety margins.
2. Realistic Load Consideration: LRFD incorporates realistic estimates of loads and
load combinations based on statistical analysis, providing a more accurate
representation of actual conditions.
3. Efficient Use of Material: By allowing higher stresses in the material under
certain conditions, LRFD often leads to more efficient designs with the use of less
material, resulting in cost savings.
4. Flexibility and Adaptability: The LRFD method is adaptable to different materials
and design scenarios, making it a versatile approach suitable for various
construction types and projects.
5. Consistency and Uniformity: LRFD promotes consistency across design practices
by standardizing load factors and resistance factors, contributing to a cohesive and
unified approach in engineering designs.
6. Incorporates Research and Advancements: LRFD evolves with ongoing research
and advancements, allowing for the integration of new findings and improved
design procedures, ensuring designs stay up-to-date and optimized.
7. Compliance with Modern Standards: Many modern design codes and standards
emphasize the LRFD approach, making it essential for compliance and regulatory
requirements in contemporary engineering projects.
2. What is the purpose of strength reduction factors? Why are they smaller for
columns than for beams? List all the strength reduction factors of NSCP Code
Section 421.2.1 / ACI Code Section 21.2.1.
Strength reduction factors, also known as φ-factors (phi-factors), are used in
structural engineering to account for the uncertainty in material properties,
construction quality, and load effects when designing structures. These factors are
applied to the nominal or calculated strength of structural components to
determine the design strength, which is the strength that can be safely used in
design to ensure the safety and reliability of a structure.
Strength reduction factors are smaller for columns than for beams because
columns typically experience more critical loading conditions and are considered
more critical for the overall stability and safety of a structure. Columns are
subjected to compressive forces, which can lead to sudden and catastrophic failure
if not designed conservatively. Beams, on the other hand, are typically subjected
to bending moments, and their failure tends to be more ductile and predictable,
making it possible to use larger strength reduction factors.
NSCP Code Section 421.2.1:
φ for flexure in reinforced concrete beams: 0.90
φ for shear in reinforced concrete beams without stirrups: 0.75
φ for shear in reinforced concrete beams with stirrups: 0.85
φ for axial compression in reinforced concrete columns: 0.70
φ for axial compression in reinforced concrete walls: 0.75
ACI Code Section 21.2.1 (ACI 318-14, which is a common reference):
φ for flexure in reinforced concrete beams: 0.90
φ for shear in reinforced concrete beams without shear reinforcement: 0.75
φ for shear in reinforced concrete beams with shear reinforcement: 0.85
φ for axial compression in reinforced concrete columns: 0.65
3. What are the basic assumptions of the strength design theory?
The Strength Design Theory, also known as Load and Resistance Factor Design
(LRFD) in the United States, is a widely used method in structural engineering for
the design of structures and their components. It is based on a set of fundamental
assumptions that form the basis of the design approach. The basic assumptions of
the Strength Design Theory are as follows:
Load Variability: This theory assumes that the loads acting on a structure (e.g., dead
loads, live loads, environmental loads) are variable and subject to uncertainty.
Different types of loads have different probabilities of occurrence and magnitudes.
The design should account for these variations to ensure safety.
Material Properties: Material properties, such as the strength of concrete, steel, or
other construction materials, are considered to be variable. There is inherent
variability in material strength, and strength design takes this into account by using
appropriate reduction factors (φ-factors) to ensure that the structure can withstand
variations in material strength.
Resistance Factors: The design method assumes that structural components are
capable of withstanding loads greater than the expected maximum loads.
Resistance factors, denoted as φ-factors or Ω-factors, are applied to the calculated
or nominal strengths of materials and structural elements to ensure a margin of
safety. These factors are less than 1 to account for uncertainties in material
properties and construction quality.
Reliability and Probability: Strength design is probabilistic in nature. It is based on
statistical principles and aims to achieve a high level of reliability by ensuring that
the probability of failure of a structure is acceptably low over its intended lifespan.
Ductile Behavior: The design theory assumes that structural failure, if it occurs, will
be ductile rather than brittle. Ductile failure allows for warning signs and
deformation before ultimate failure, enhancing the safety of the structure.
Factor of Safety: Unlike older design methods, such as the Allowable Stress Design
(ASD), which use a factor of safety applied to loads, strength design applies factors
to both loads (load factors) and resistances (resistance factors). The safety of the
structure is ensured by comparing the load effects (factored loads) to the design
resistances (factored strengths) with appropriate safety margins.
Limit States: Strength design considers different limit states, such as ultimate limit
states (ULS) and serviceability limit states (SLS). ULS relates to the safety of the
structure under extreme loads, while SLS addresses issues like deflection, vibration,
and cracking under normal service loads.
By incorporating these fundamental assumptions, strength design theory aims to
provide a rational and reliable approach to structural design, ensuring that
structures are safe and capable of withstanding the uncertainties associated with
loadings and material properties. It has become the standard design method in
many countries due to its ability to optimize structural performance while
maintaining a high level of safety.
4. Why does the NSCP / ACI Code specify that a certain minimum percentage of
reinforcing be used in the beams?
The NSCP (National Structural Code of the Philippines) and ACI (American Concrete Institute)
codes specify a minimum percentage of reinforcing steel in concrete beams to ensure the
structural integrity, safety, and durability of the beams. Concrete is strong in compression but
weak in tension, and reinforcing steel adds the necessary tensile strength to resist bending and
shear forces. These minimum requirements help prevent cracking, promote ductile behavior in
the event of overloading, distribute loads more evenly, enhance durability, ensure code
compliance, and provide a consistent and reliable basis for structural design. In essence, these
specifications are crucial for safeguarding the structural and long-term performance of
concrete beams and, consequently, the safety and reliability of the structures they support.
5. Distinguish between tension-controlled and compression-controlled beams.
Tension-controlled and compression-controlled beams differ in their primary mode of
failure and the associated design considerations. Tension-controlled beams primarily fail
in tension, where the concrete in the tensile zone undergoes cracking and deformation,
while the compressive zone remains relatively intact. The key design concern for tensioncontrolled beams is ensuring sufficient tensile reinforcement to prevent brittle failure and
achieve ductile behavior, typically achieved by specifying an adequate minimum amount
of reinforcement. Compression-controlled beams, on the other hand, primarily fail due to
crushing of the concrete in the compressive zone, and their design focuses on achieving
adequate compressive strength through proper concrete mix design and confinement of
concrete with stirrups or ties, rather than emphasizing tensile reinforcement. The behavior
and design requirements for these two types of beams are fundamentally shaped by their
respective modes of failure.
6. Explain the purpose of minimum cover requirement for reinforcing bars specified by
the NSCP / ACI Code. List all the minimum cover requirements of NSCP Code section
420.6.1 / ACI Code Section 20.6.1.
The minimum cover requirement for reinforcing bars specified by the NSCP (National
Structural Code of the Philippines) and ACI (American Concrete Institute) Code serves to
ensure the long-term durability and structural integrity of reinforced concrete structures.
This requirement mandates a specified minimum distance between the outer surface of
the concrete and the surface of the reinforcing bars. The primary purposes of this minimum
cover are to protect the reinforcing steel from corrosion caused by environmental factors,
provide fire resistance, ensure proper bonding between the concrete and the steel for load
transfer, extend the service life of the structure by reducing deterioration risks, and
maintain the desired aesthetics and surface quality of the concrete. Adhering to these
cover requirements is essential for the safety, performance, and longevity of concrete
structures.
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NSCP Code Section 420.6.1 (Philippines):
Minimum cover for concrete cast against and permanently exposed to earth: 75 mm (3
inches).
Minimum cover for concrete cast against and permanently exposed to weather: 50 mm (2
inches).
Minimum cover for concrete cast against and temporarily exposed to weather: 25 mm (1
inch).
Minimum cover for concrete cast against forms that will be removed: 15 mm (0.6 inch).
ACI Code Section 20.6.1 (American Concrete Institute):
Minimum cover for concrete exposed to earth or weather: 1.5 times the bar diameter, but
not less than 25 mm (1 inch).
Minimum cover for concrete not exposed to weather and sheltered from earth contact: 3
times the bar diameter, but not less than 25 mm (1 inch).
Minimum cover for concrete not exposed to weather but subject to contact with the
ground or other aggressive environments: 50 mm (2 inches).
Minimum cover for concrete surfaces that will be permanently exposed to view and that
are not exposed to weather or in contact with the ground: 19 mm (3/4 inch).
Please keep in mind that these requirements are based on the versions of the codes
available up to my knowledge cutoff date in September 2021. Building codes can be
updated, so it's essential to consult the latest code edition and any local amendments or
requirements when designing and constructing concrete structures.
7.
Determine the values of provided, min, max, c, Ԑt, Ø, and ØMn