Important Question And Problems With Answer: Compression Members
1. What do you mean by compression members?
Compression members are the most common structural elements and it is
termed as columns, struts, posts or stanchions. They are designed to resist axial compression.
2. Name the modes of failures in a column.
Failure of the cross-section due to crushing or yielding
Failure by buckling, due to elastic instability
Mixed mode of failure due to crushing and buckling
3. Define slenderness ratio
It is defined as the ratio of effective length l of the column to the least
radius of gyration r of the column section.
4. Classify the columns according to the slenderness ratios.
Short columns - l/r <60
Medium columns - 60< l/r <100
Long columns - l/r >100
5. Distinguish column and strut
Columns are the vertical members which carry the loads to the beams,
slabs etc, generally they are used in ordinary buildings.
Struts are commonly used for compression members in a roof truss; it may either be in vertical position or in an inclined position.
6. What is meant by stanchions?
These are the steel columns made of steel sections, commonly used in buildings. 7. What is Post?
It is loosely used for a column, but in truss bridge girders, end compression members are called end posts.
8. What is a boom?
It is the principal compression member in a crane.
9. State the assumptions that made in Euler's theory.
The axis of the column is perfectly straight when unloaded.
The line of thrust coincides exactly with the unstrained axis of the strut.
The flexural rigidity EI is uniform
The material is isotropic
10. Why the lateral systems are provided in compound columns?
If the plates are not connected throughout their length of the Built up sections,
lateral systems may be provided, which act as a composite section. In such cases the
load carrying elements of the built-up compression member in the relative position,
without sharing any axial load. However when the column deflects, the lateral system
carries the transverse shear force.
11. Name the lateral systems that are used in compound columns and which is the mostly used one?
Lacing or latticing, Battening or batten plates, perforated cover plates.
Lacing or latticing is the most common used lateral system and the sections are flats, angles and channels.
12. What will be the thickness for the single and double lacing bars?
The thickness of flat lacing bars shall not be less than one-fortieth of the
length between the inner end rivets or welds for single lacing, and one-sixtieth of
the length for double lacing.
13. What is the purpose of providing battens in compound steel columns?
Batten plates consist of flats or plates, connecting the components of the built-up columns in two parallel planes. These are used only for axial loading. Battening of the composite column should not be done if it is subjected to eccentric loading or a applied moment in the plane of battens.
14. What is the thickness of a batten plate?
The thickness of batten plate shall not be less than one fiftieth of the distance between the inner most connecting lines of rivets or welds. This requirement eliminates lateral buckling of the batten.
15. Where the perforated cover plates are used and mention its advantages?
They are mostly used in the box sections, which consist of four angle sections so that the interior of column remains accessible for painting and inspection. Advantages:
They add to the sectional area of column and the portions beyond the perforation share axial load to the extent of their effective area.
There is economy and fabrication and maintenance
Perforations conveniently allow the riveting and painting work on the inside portion.
16. Name the types of column base?
Slab Base, which is a pinned base. Gusseted base, which is a rigid base.
17. State the purpose of column base?
The base of the column is designed in such a way to distribute the concentrated column load over a definite area and to ensure connection of the lower column end to the foundation. It should be in adequate strength, stiffness and area to spread the load upon the concrete or other foundations without exceeding the allowable stress.
18. Give the difference between slab base and gusseted base for steel columns.
Slab base is a thick steel base plate placed over the concrete base and connected to it through anchor bolts. The steel base plate may either be shop-welded to the stanchion,else can be connected at the site to the column through cleat angles. The column is faced for bearing over the whole area.
In a gusseted base, part of the load is transmitted from the stanchion through the gusseted base plate. The gussets and stiffeners support the base slab against bending and hence a thinner base plate can be used. The gussets serve for more or less uniform transmission of the force field from the column to the base plate. The gussets itself resists the bending as double cantilever beam supported on flanges of the column.
19. What is slab base and for what purpose is it provided?
The base plate connected to the bottom of the column to transfer over wider area is known as slab base. Column end is machined to transfer the load by direct bearing. No gusset materials are required.
20. When the slenderness ratio of compression member increases, the permissible stress decreases. Why?
The section must be so proportioned that it has largest possible moment of inertia for the same cross-sectional area. Also the section has approximately the same radius of gyration about both the principal axes.