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Design of a Concrete Staircase Stringer Beam to BS8110

Introduction

Stringer beams are structural members that supports a floor or a deck along its longitudinal direction. They are used to convert distributed loadings from a slab into point loads and are mostly inclined secondary beams stemming from primary beams or supports. They are also very useful in staircases where the thickness of the waste would be very large relative to the span of the flight due to deflection requirement.

Stringer beams when utilized in staircases can be designed either with two edge beams (simply supported) or with a central beam (double cantilever). Figure 1 illustrates a simply supported staircase on two edge beams and a staircase supported on a central stringer beam

Fig 1: Section through Stringer beams

Figure 2.0 illustrates the second flight of a staircase required to span 7.5m between supports. Adopting a staircase spanning longitudinally supported by beams at the supports is uneconomical as the waste of the stair would be very large resulting in a huge volume of concrete and quantity of steel required to control deflection. Stringer beams can thus be adopted in order to reduce bending moments and deflection requirement. For this case a central stringer beam (double cantilever) type will be designed

Fig 2: Plan of Considered Flight

The staircase is idealised as a T- section therefore the stringer beam is designed as a flanged beam and the waste of the staircase as spanning transversely (like a cantilever). Transverse beams will be required to support the stringer beam. These transverse beams are designed as carrying point loads from the stringer beam at midspan.

Design Data

  • Concrete grade, Fcu = 25N/mm2
  • Fy = 410N/mm2
  • Fyv = 250N/mm2
  • Concrete cover = 25mm
  • width of stairs = 1500mm
  • Risers = 150mm
  • Tread = 300mm.

Design of Flight

Loading
a.\quad Dead\quad loads\\ \qquad 150mm\quad concrete\quad waste\quad \\ \qquad \qquad \qquad \qquad =0.15\times 24\quad =3.6kN/{ m }^{ 2 }\\ \qquad Finishes\quad say\qquad \qquad \quad =1.5kN/{ m }^{ 2 }\\ \qquad Ballustrade,\quad say\quad \qquad =0.5kN/{ m }^{ 2 }\\ \qquad steps\qquad \qquad =\qquad \frac { 0.15 }{ 2 } \times 24\quad =1.8kN/{ m }^{ 2 }\\ \qquad slope\quad factor\quad =\frac { √({ 300) }^{ 2 }+({ 150) }^{ 2 } }{ 300 } \quad =1.12\\ \qquad Dead\quad loads\quad =\quad (3.6+1.5+0.5)\times 1.12+1.8\quad \\ \qquad \qquad \qquad \qquad =8.07kN/{ m }^{ 2 }\\ \qquad b.\quad Imposed\quad load\quad \quad =3.5kN/{ m }^{ 2 }\\ Design\quad load\quad =\quad 1.4{ g }_{ k }+{ 1.5q }_{ k }\quad \\ \qquad \qquad \qquad =1.4(8.07)+1.5(3.5)\\ \qquad \qquad \qquad =\quad 17kN/{ m }^{ 2 }\\ \\

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Fig 3: Stringer Beam/Staircase Elevation

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Published by Omotoriogun Victor

A dedicated, passion-driven and highly skilled engineer with extensive knowledge in research, construction and structural design of civil engineering structures to several codes of practices

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