HORIZONTAL SPANS

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HORIZONTAL SPANS PRINCIPLES

  • Span:

    • Defined as the amount of space between two structural supports.

  • Bending Moment:

    • An external moment that induces a portion of a structure to spin or bend.

  • Resisting Moment:

    • An internal moment produced by a force couple to maintain equilibrium in the structure; it is equal and opposite to the bending moment.

  • Deflection:

    • The deviation of a spanning member from a true course under a transverse force.

    • Increases with load and span.

    • Decreases with increased stiffness of material or section.

  • Bending Stress:

    • The largest bending stress occurs at the surface farthest from the neutral axis.

    • Comprises both compressive and tensile stresses, created at a cross-section to counteract a transverse force.

STRUCTURAL LAYERS

  • Each layer consists of one-way spanning elements supported by the layer below, requiring alternate span directions in successive layers.

  • Layer 1 (Surface-forming Layer):

    • Materials may include:

    • Structural wood panels

    • Wood or steel decking

    • Precast concrete planks

    • Cast-in-place concrete slabs

    • Function: Determines the size and spacing of Layer 2 joists and beams based on load-carrying and spanning capability.

  • Layer 2 (Support Layer):

    • Comprising larger linear elements such as:

    • Wood and light-gauge steel joists

    • Open-web joists

    • Beams

  • Layer 3 (Support for Joists and Beams):

    • Involves girders or trusses, or alternatively, use a series of columns or bearing walls to carry Layer 2 elements.

HORIZONTAL SPANNING SYSTEMS

  • The choice of materials and structural systems is influenced by:

    • Material Type:

    • Wood one-way systems

    • Steel one-way systems

    • Concrete systems (one-way or two-way systems)

    • Bay Proportion:

    • Two-way spans are typically square.

    • One-way spans can be rectangular or square.

    • Span Direction and Length:

CONCRETE SPANNING SYSTEMS

  • Concrete Elements Include:

    • One-Way Slabs

    • Joist Slabs

    • Flat Plates

    • Flat Slabs

    • Two-Way Slabs with Beams

    • Waffle Slabs

    • Precast Concrete Slabs

STEEL SPANNING ELEMENTS

  • Steel Beams and Girders:

    • Typical configurations:

    • Beam span should generally be calculated as span divided by 20.

    • Steel beams calculated as span divided by 15.

    • Beam width is typically 1/3 to 1/2 of the beam depth.

  • Open-Web Steel Joists:

    • Beam span typically calculated as span divided by 24.

    • Spacing between joists usually ranges from 2' to 10' (approximately 0.6 to 3 m), with common practice in large buildings being 4' to 8' (1.2 to 2.4 m).

OPEN-WEB STEEL JOISTS

  • Require reinforcing from concrete or masonry bearing walls to counter lateral movement.

  • Bridging Types:

    • Horizontal or diagonal bridging is mandated to prevent lateral shifts of joist chords.

    • Spacing of bridging typically ranges from 10' to 20' (3050 to 6095), depending on joist span and chord size.

  • Joist spacing commonly ranges from 4' to 10' (1220 to 3050).

  • Maximum joist span suggested is 24 times the joist depth.

STEEL SPANNING SYSTEMS

  • One-Way Beam System:

    • Beam span can range from 6 m to 12 m.

  • Trussed System:

    • Girder joists (LH and DLH series) support a variety of open-web joists.

  • Triple-Layer System:

    • Comprises major trusses, primary beams, and secondary beams.

  • Light-Gauge Steel Joists:

    • Cold-rolled formed, spaced depending on load; space increments can be 405 mm, 610 mm, or 1220 mm.

HORIZONTAL SPANS WOOD SPANNING SYSTEMS

  • Solid Wood Beams:

    • Available in nominal increments of 2" (51 mm) from sizes 4x8 to 6x12, with actual dimensions being slightly less.

    • Depth estimation: span/15.

    • Width is roughly 1/3 to 1/2 of the depth.

    • Glue-laminated beams can span greater distances compared to solid beams.

  • Beam Specifications:

    • Glue-laminated beam: Spans > 60' (18 m)

    • LVL and PSL beam: Spans > 10' (3 m)

ASSIGNMENT

  • Task: Explain the horizontal spans of wide span building structure precedents.

REFERENCES

  • Ching, Francis D.K. (2014). Building Structures Illustrated, 2nd Edition. John Wiley and Sons, Inc., New York.