Reading #7: Spans Fernandez, Andy
In Ching’s BCI book, he discusses spanning elements and their relationship to their structural importance. A major spanning element is the arch. Arches span spaces using the compressive strengths of brick or stone while depending on the resistance of thrust by the wall. The size of the masonry units dictates the size of the arch. Another spanning tool is the lintel. Lintels span spaces of 4’-6’ respectively. They are made of one of four ways, steel angle supports, Reinforced brick filled with grout spans (8”-12”), concrete lintels with cement grout and precast lintels made of brick or concrete.
Other spanning elements include wood floor joists, decking, sub flooring, wood planks and steel open web joists. Floor joists span from 12”-24” O.C. They can span 10’-18’ distances. In comparison, steel joists range from 12’-60’ spans. Wood Joists are part of a structural subsystem and with the addition of sheathing (wood subflooring) becomes a strong structural diaphragm to transfer loads to shear components. Joists must be specified according to loads, spacing, species and defection allowance. Bridging is required with joist of 6” or deeper depth at every 8’.
Wood sub flooring is a structural component and consists of wood panels of plywood/OSB or wafer board. These panels are less susceptible to shrinkage than lumber. The panels are laid perpendicular to the joists with nails and glue (eliminates squeaks and creeps), have smooth surfaces for finished flooring and serves as an impact load resistant. Decking is usually laid over the sub flooring. Decking can be single (weakest), double or a continuous (strongest) span.
Wood plank and beams are used to distribute loads evenly by spacing elements in a grid pattern that aligns with other load bearing structure units. This system can be used as heavy timber construction if it uses fire resistive exterior walls (brick/ concrete). Disadvantages of the wood plank system include sound transmission and lack of concealment for MEP’s and insulation.
Wood beams are supported by load bearing structural elements such as masonry walls, timber, steel and concrete columns.
Steel beams and joists are also spanning units. Similar to wood framing, steel members come in nominal sizes and shapes. Steel open web joists can span distances from 12’-60’ respectively at 2’-10’ O.C. spacing. Other deeper steel joists (LH & DLH) can span from 36’-144’. Steel beams can be connected with steel angles, stiffer palters, bolts and/or welds. Similar to lumber, steel beams and joists must increase its depth to span longer distances. Some steel beams were made to alleviate this problem. One good example is the castellated beam which is deeper and longer but not heavier. Steel joists works well with rectangular one-way systems, masonry bearing walls, steel beams and suspended ceilings. These joists can support decking of wood, precast panels or concrete fill toppings. Bridging is also needed at 10’-20’ O.C. Connections to steel must resist shear and moment deflections. In summary, all spanning elements must be calculated in proportion to the distance spanned and load carrying capacity. They must be paired with vertical elements for load distribution and equilibrium.