Column and Strut

3 Key excerpts on "Column and Strut"

• 

  eBook - ePub

  Structural Design for Architects

  • A Nash(Author)
  • 2017(Publication Date)
  • Routledge

    (Publisher)

  Although the earliest temples were constructed of wood, the five orders are usually historically associated with some form of stone. The reason for the original presence of a column head, leaving aside the symbolism inherent in the five orders, could well have been based on considerations of a more practical nature. A column terminating beneath the entablature would have presented an untidy junction. It would probably have been more difficult to ensure that the load was supported near to the centre of the column – a necessary condition if tensile stresses were to be avoided.

  Functions of the column

  To describe a column in terms of its most obvious function, one need go no further than to state that it is there simply to transfer down to the base a load applied at its head. All columns, whether they belong to the primitive, Classical, Gothic, modern or any other architectural style, whether they be beautiful or ugly, have to justify their existence in this way. Yet a much clearer impression of their structural behaviour emerges when a column is regarded as a particular form of a strut – the name given to a compression member. A column is a vertical strut. A tie, being subject to a tensile force, will remain straight and in a state of tension until failure is reached. A strut, unless very short in relation to its cross-sectional area, will not remain in its original straight condition as it begins to fail. It will be noticeably bent, implying that it is subject to bending moments throughout its height as well as to axial compression. The bending moments so caused will, with increasing load, eventually lead to failure of the column by buckling. Bending can only be absent from a column, even under working loads, in the purely theoretical condition when the applied load does not waver from the axis in the journey from head to base.

  Columns in the elevations of a building are also called upon to transmit wind loads as horizontal uniformly distributed loads to the floors, which then act as horizontal reactions. This creates bending moments in the columns as though they were beams rotated through 90°. Where the construction is monolithic, as with reinforced concrete, it is impossible to avoid the transmission of bending moments at beam to column junctions. The same applies to some types of connection in structural steelwork and timber. It is essential, therefore, when looking at or thinking about any column built from any material, to understand that its function as a structural member in an apparent state of pure compression is dependent to a certain extent on its resistance to bending.

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  eBook - ePub

  Design of Structural Elements

  Concrete, Steelwork, Masonry and Timber Designs to Eurocodes

  • Chanakya Arya(Author)
  • 2022(Publication Date)
  • CRC Press

    (Publisher)

  Chapter 2 Basic technical and structural concepts

  DOI: 10.1201/9781003208037-2

  This chapter is concerned with general methods of sizing beams and columns in structures. The chapter describes how the characteristic and design loads acting on structures and on individual elements are determined. Methods of calculating the bending moments, shear forces and deflections in beams are presented. Finally, the chapter describes general approaches to sizing beams according to elastic and plastic criteria and sizing columns subject to axial loading.

  2.1 Introduction

  All structures are composed of a number of interconnected elements such as slabs, beams, columns, walls and foundations. Collectively, they enable the internal and external loads acting on the structure to be safely transmitted down to the ground. The actual way that this is achieved is difficult to model (predict), particularly in in-situ reinforced concrete construction which is considered to be continuous, and many simplifying, but conservative, assumptions have to be made. For example, the degree of fixity at column and beam ends is usually uncertain but, nevertheless, must be estimated as it significantly affects the internal forces in the element. Furthermore, it is usually assumed that the reaction from one element is a load on the next and that the sequence of vertical load transfer between elements occurs in the order: Ceiling/floor loads to beams to columns to foundations to ground (Figure 2.1 ).

  Figure 2.1 Sequence of load transfer between elements of a structure.

  It should also be remembered that structures are invariably subjected to lateral (horizontal) loads due to wind, for example. Walls and bracing elements are often provided to resist these loads and achieve lateral stability. For example, in the case of the concrete building shown in plan and elevation in Figure 2.2 , wind forces in the north–south direction will be resisted by the shear walls located at each end of the structure. For design purposes these walls are treated as relatively thin, deep cantilever beams. In steel structures, lateral stability can be achieved by providing diagonal cross members referred to as bracing, which is usually placed in the end bays of the frame (Figure 4.3). In both cases, the horizontal loads from wind pressure on the cladding will be transmitted via the floor slabs to bracing elements to foundations to ground (Figure 2.2

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  eBook - ePub

  Practical Concepts for Capstone Design Engineering

  • Frederick Bloetscher, Daniel Meeroff(Authors)
  • 2015(Publication Date)
  • J. Ross Publishing

    (Publisher)

  Figure 13.16 , the roof and all floors above the roof must be accounted for with the column. Hence, in large buildings, the columns carry very large loads. If a critical column collapses due to improper design, this may lead to a progressive failure of the floors above it, which in turn may result in a complete collapse of the entire structure.

  Generally speaking, columns can be designed as short/nonslender columns or as long/ slender columns . Columns can fail because of improper material design, such as yielding of the steel at the tension face, initial crushing of concrete at the compression face, or through buckling. Columns that fail due to initial material failure are referred to as short columns . As the height of the column increases, the column starts transitioning from a short to a long column. The probability of failure of a long column due to buckling increases due to its slenderness ratio property (

  klu

  /r ). The slenderness ratio is the effective length of the column (

  klu

  ) to the radius of gyration (r ). The height (

  lu

  ) is the unsupported length of the column; k is a factor dependent on the end support of the column and whether it is braced or unbraced (Nawy 2009). Referring to the ACI criteria for a concrete column, an unbraced column can be defined as a short column if its slenderness ratio is less than or equal to 22.

  The means to determine column loads is the same as for beams or slabs: find the tributary area and apply the loads, recognizing that each floor and the roof add loads. Reinforced concrete columns require significant calculations and are best covered in a reinforced concrete text. Steel is easier to work with, although the slenderness issue is common to both steel and concrete columns. Fire protection is a critical issue with steel columns, but selecting a column is straightforward, as follows:

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