Distributed Load

3 Key excerpts on "Distributed Load"

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  eBook - ePub

  Structure for Architects

  A Primer

  • Ramsey Dabby, Ashwani Bedi(Authors)
  • 2012(Publication Date)
  • Wiley

    (Publisher)

  CHAPTER 9 Load Distribution

  As we know from Chapter 3, the load path conducts various loads through a structure from their place of origin (roofs, walls, floors) through beams, girders, columns, foundations, and ultimately to the ground. Loads can be applied in several different ways.

  9.1 Point Loads

  Point loads occur at a single location. A beam framing into a girder, a column seated atop a girder, and the man standing on the log in Chapter 6 are examples of point loads. Appropriately enough, point loads are also called concentrated loads (Figures 9.1a and 9.1b ).

  Figure 9.1a Point Loads on Girder A and B

  Figure 9.2b Point Loads on Girder C

  9.2 Distributed Loads

  Distributed Loads are spread out over an area of floor or over a length of a member. Distributed Loads are expressed in weight per unit of area , such as pounds per square foot (psf); or weight per unit of length , such as pounds per linear foot (plf); or kips per linear foot (klf) (1 kip = 1000 lbs).

  To illustrate the distinction between a point load and a Distributed Load, let's use an example of a man, 6 ft tall and weighing 180 lbs, standing at the center of a 20 ft log spanning a ditch. When standing, his weight is a point load (Figure 9.2 ).

  Figure 9.2 A Point Load

  If the man lies down, however, his weight spreads out (i.e., is distributed) over a 6 ft length of log. His weight then changes from a point load of 180 lbs to a Distributed Load of 30 plf (i.e., 180 lb/6 ft) (Figure 9.3

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  eBook - ePub

  Bridge Engineering

  Classifications, Design Loading, and Analysis Methods

  • Weiwei Lin, Teruhiko Yoda(Authors)
  • 2017(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  The classifications of the load could be different according to the design specifications, but can be roughly divided into predominant (primary) load and subordinate (secondary) load. The load applied on bridge structures can also be classified as static load and dynamic load, as well as the concentrated load and Distributed Load etc.

  Taking the Japan design load as an example, four load systems were divided according to the Standard Specification of Highway Bridges of Japan Road Association. They are:

  1.  

  Principal loads (P )—dead load (D ), live load (L ), impact load (I ), prestressed forces (PS ), concrete creep (CR ), drying shrinkage (SH ), earth pressure (E ), hydraulic pressure (HP ), and buoyancy or uplift (U ).

  2.  

  Subordinate loads (S )—wind load (W ), temperature change (T ), and earthquakes (EQ ).

  3.  

  Special loads corresponding to principal loads (PP )—snow load (SW ), influence of ground displacement (GD ), influence of support displacement (SD ), wave pressure (WP ), and centrifugal force (CF ).

  4.  

  Special loads corresponding to subordinate loads (PA )—braking force (BK ), erection load (ER ), collision force (CO ), others.

  According to the bridge location and bridge type, the above mentioned loads should be selected appropriately during the structural design and analysis, but not necessarily consider all the loads. Major loads considered in the bridge design are discussed below.

  4.2 Dead Load

  Gravity loads of constant magnitudes and fixed positions act permanently on the structure. Such loads consist of the weights of the structural system itself and of all other material and equipment permanently attached to the structural system. In the bridge design, the dead load denotes the constant load in a bridge due to the weight of the members, the supported structure, and permanent attachments or accessories. To be specific, the dead load in a bridge include: (1) Facilities and additives (or accessories) on the bridge, such as guardrail, lamp standard etc.; (2) Self-weight of the deck system, such as deck, pavement, and pedestrian etc., (3) Self-weight of the floor system, such as stringer, transverse beam etc., and (4) Self-weight of the main girder or main structure system, including the floor beam etc. Although it is possible to determine (1) and (2) before the main structure design, it is hard to determine (3) and (4) before the final design of the main girder. Typical specific weights of different materials are summarized in Table 4.1

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  eBook - ePub

  Practical Reliability Engineering

  • Patrick O'Connor, Andre Kleyner, Patrick O'Connor, Patrick D. T. O'Connor, Andre V. Kleyner(Authors)
  • 2011(Publication Date)
  • Wiley

    (Publisher)

  The examples illustrate some of the advantages and limitations of the statistical engineering approach to design. The main difficulty is that, in attempting to take account of variability, we are introducing assumptions that might not be tenable, for example by extrapolating the load and strength data to the very low probability tails of the assumed population distributions. We must therefore use engineering knowledge to support the analysis, and use the statistical approach to cater for engineering uncertainty, or when we have good statistical data. For example, in many mechanical engineering applications good data exist or can be obtained on load distributions, such as wind loads on structures, gust loads on aircraft or the loads on automotive suspension components. We will call such loading situations ‘predictable’.

  On the other hand, some loading situations are much more uncertain, particularly when they can vary markedly between applications. Electronic circuits subject to transient overload due to the use of faulty procedures or because of the failure of a protective system, or a motor bearing used in a hand power drill, represent cases in which the high extremes of the load distribution can be very uncertain. The distribution may be multimodal, with high loads showing peaks, for instance when there is resonance. We will call this loading situation ‘unpredictable’. Obviously it will not always be easy to make a definite classification; for example, we can make an unpredictable load distribution predictable if we can collect sufficient data. The methods described above are meaningful if applied in predictable loading situations. (Strength distributions are more often predictable, unless there is progressive strength reduction, which we will cover later.) However, if the loading is very unpredictable the probability estimates will be very uncertain. When loading is unpredictable we must revert to traditional methods. This does not mean that we cannot achieve high reliability in this way. However, evolving a reliable design is likely to be more expensive, since it is necessary either to deliberately overdesign or to improve the design in the light of experience. The traditional safety factors derived as a result of this experience ensure that a new design will be reliable, provided that the new application does not represent too far an extrapolation.

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