ARCH 3323 - Test 2 - Chapter 4 Short Answer

What is the difference between a ductile material and a brittle material?

A brittle material is one for which ultimate strain is less than or equal to 0.5%. A ductile material is one for which the ultimate strain is more than 0.5%. Strain is the relative change in length, defined as the change in length divided by the original length. SO materials that have lower strain don't deform as much and are brittle. Materials that have a high strain, deform more and are ductile.

What is meant by a "structurally efficient" cross sectional shape in bending? Note: "in bending" means that the member is used as beam or slab.

A structurally efficient cross sectional shape is one where the stresses are most evenly distributed across the cross section. For example a I shaped cross section is more efficient than a rectangle cross section because of the way bending stresses distribute.

What is the difference between bearing strength and compressive strength?

Bearing strength is the resistance of the stress at the contact between an element and its supporting element. Compressive strength is just the resistance in an element due to a load.

Explain why shear strength of brittle materials is low

Because of the deformation of an element under shear, tension is introduced along one diagonal and compression along the other diagonal. Brittle materials are weak in tension and therefore are also weak under shear stresses.

When a member buckles, it bends. Explain the difference between buckling and bending.

Bending occurs when loads are perpendicular to the axis of the member and occur under all magnitudes on members. Buckling occurs when loads are axial and increase to a certain critical load. Buckling happens abruptly unlike bending.

What is the relationship between a material's brittleness and its tensile strength?

Brittle materials are strong in compression but weaker in tensile strength. This is common for most brittle materials.

Being a brittle material, concrete is weak in shear. Therefore, shear reinforcement is used in concrete beams. Sketch the profile of this reinforcement and provide the term by which it is identified.

Concrete is reinforced with steel to resist tensile and shear stress. The longitudinal steel resists tensile stress due to bending and the steel stirrups are used to resist shear stress.

Explain the difference between an elastic and a plastic material.

Elasticity and plasticity are converse. Elastic materials are completely recoverable after deformation when the load is removed and conversely plastic materials are not recoverable after deformation when the load is removed.

List the following materials (aluminum, concrete, steel, and wood) in terms of their ductility beginning with the most ductile and ending with the least ductile.

From most ductile to lease ductile the materials are steel, aluminum, wood, and concrete. Steel is very ductile while concrete is a brittle material.

In the stress-strain diagrams of materials, which axis (X-axis or Y-axis) is typically used for stress and which for strain?

In a stress strain diagram the x-axis is used for strain and the y-axis for stress.

Is low-carbon steel an elastic or a plastic material?

Low carbon steel is an elastic-plastic material. It is elastic up until its yield point and then it becomes plastic.

9 The saw blades used for cutting masonry, concrete and stone are made of diamond-tipped steel. Why is that so?

Of the common materials, diamond has the highest E value of 170x10^6 psi. This is why it is used to cut other materials.

Among the commonly used structural materials (steel, concrete, masonry and wood), which has the highest modulus of elasticity and which has the lowest modulus elasticity?

Steel has the highest modulus of elasticity(the stiffest). Wood has the lowest modulus of elasticity(least stiff).

For most materials, their strength is directly related to their modulus of elasticity, i.e., the greater the strength of the material, the greater its modulus of elasticity. Is there a material whose modulus of elasticity is not related to its strength but is constant, i.e., it has a fixed value regardless of the material's strength? If so, name the material and provide its modulus of elasticity value.

Steel has the same modulus of elasticity no matter its strength. Its modulus of elasticity is always E=29x10^6 psi. The modulus of elasticity of steel is based on its straight line portion of the stress strain diagram.

What does the term "strength" of a material imply and how does it differ from a material's ultimate stress?

Strength and ultimate stress are the same thing. Both terms refer to the stress at failure of a material. It is the stress(the internal resistance to external force) at the moment of failure.

2 If a concrete test cylinder measuring 6 in. x 12 in. breaks when the load on it reaches 169.5 k, what is the compressive strength of concrete? Express the strength in psi.

The compressive strength of concrete that failed at 169.5 k load is 5990 psi. This is found by dividing the load at failure(169.5 k) by the area of the cylinder(28.27 square in.) 1 ksi = 1000 psi

A concrete cylinder specimen is tested by loading it in compression from its top. The cylinder is restrained from expanding laterally (not allowed to bulge horizontally under the load). A second identical cylinder is tested in compression similarly but without any lateral restraint, i.e., this cylinder is allowed to bulge laterally. Will both cylinders fail at the same or a different value of load? Explain your answer

The cylinders will fail at different value of loads because one is allowed to bulge while the other has a lateral load restricting it from bulging. When lateral confinement is imposed, the cylinder will be able to bear a larger compressive strength than if no lateral confinement is imposed. The compressive strength of concrete increases as the confining stress on it increases.

What is the difference between an I-section beam and an open-web I-section beam, called a castellated beam?

The difference between an I section beam and a castellated beam are the openings in the center of the castellated beam causing a higher efficiency. The castellated beam is more efficient because it takes more material from the center and places it at the extremes where the stresses are higher.

What is the yield stress of grade 60 reinforcing steel? Of A-36 structural steel?.

The grade or A number stand for its yield stress. So a grade 60 would have a yield stress of 60 ksi and the A-36 structural steel has a yield stress of 36 ksi.

What is the modulus of elasticity of a material whose stress-strain relationship is a vertical straight line? Name the material with such a stress-strain relationship.

The modulus of elasticity of a material whose stress strain relationship is a vertical line is infinity. This means that the material is completely rigid because strain is infinity. There is no material that is infinitely rigid.

What is the neutral axis of a structural member and in which situation do we refer to it— when the member is in axial compression, axial tension or bending?

The neutral axis or the neutral plane is present when there is bending. The neutral axis is the part of the member between the compressive stresses on one side of the member and the tensile stresses on the opposite side of the member. This is the part of the member that will retain the original dimensions and experiences no stress.

Does the slope of the stress-strain diagram relate to any property of the material? Explain

The slope of the stress strain diagram gives the magnitude of the modulus of elasticity of the material. The greater the modulus of elasticity the more stiff a material is and the less it will deform under a given stress.

Why do we use the term "bearing strength of soil" and not "compressive strength of soil" when, in reality, the soil is under compression from the load of the structure being supported.

The soil is under compression but is laterally restrained from mass of the adjoining soil. Because the soil directly under the footing is confined by a large mass of the surrounding ground, the compressive strength of the soil is refereed to as the bearing strength of the soil. It is essentially the compressive strength plus the effect of confinement.

A concrete column 20 ft high shortens to 19 ft 11-1/2 in. under a load. Determine the strain in the column.

The strain in the column is 0.002. This is obtained from dividing 0.5 in by 240 in.

5 What is the unit of measure for modulus of elasticity in U.S. System of units? In SI System of units?

The unit for the modulus of elasticity is the same as stress because strain has no units. So the unit for E is psi or ksi for the US system and pascals in the SI system.

What is the unit of measure for strain?

There are no units for the measurement of strain. Because strain is the change in length divided by the original length, units are canceled.

Why is an I-section beam structurally more efficient than a beam of rectangular cross section whose cross sectional area is the same as that of an I-section beam?

This I shape configuration places most material at the top and bottom of the beam where stresses are the greatest. Having that material evening distributed across a rectangular shape is inefficient because the stresses are not evenly distributed across the member; they are higher at the top and bottom. The I section can withstand a greater load than the rectangle of the same area.

Explain two methods by which the buckling of a member may be prevented.

To reduce chance of buckling, you can increase the cross sectional area of a member(reduce its slenderness) or brace it at intermediate points along its length or height. Usually the bracing approach is taken in structures because increasing cross sectional area is uneconomical.

1 What is the shape and dimensions of the specimen typically used for determining the compressive strength of concrete?

To test the compressive strength of concrete, we use a cylinder with a diameter of 6 in. and a length of 12 in. This standard is used to test concrete's compressive strength and make it easier to compare different types of concrete.

Explain what yielding of a material means.

Up to the yield point stress and strain are proportional. Beyond this point, the material deforms without a change in load and becomes plastic. Yielding is the excessive deformation that happens after the yield point of a material. In structures, we don't want this to happen so we stay well below that point of yielding.