In the parallel beam approach (PBA), secondary or 'rib' beams pass over primary or 'spine' beams to form a grillage of members. The primary beams are placed in pairs so that they pass either side of the columns and are attached by large steel-brackets which transfer the shear forces into the column (see Figure 4.12). Secondary beams are
4.12 Parallel beam approach showing service zones
designed to span the greater distance because they can develop composite action with the slab.
The PBA system enables continuity of the beams to be achieved without the high cost of moment-resisting connections. Beam lengths are only limited by handling and transportation requirements. This improves efficiency for long-span applications and can save erection time and costs as the piece count is significantly lower than for conventional construction. Economic comparisons have shown that the PBA system can be advantageous for certain floor layouts for highly serviced buildings. Whilst it may appear that the PBA system would lead to deeper floor construction, their depth remains shallow because of the continuity of the beams, and the overall depths are comparable with the other forms of construction noted earlier.
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