When designing masonry panels with openings, the 'Reduced Fixity' method can be employed as one method of evaluating the bending moment capacity of an enter.
Restrictions on usage:
BS 5628 Pt1 Appendix D allows the form of a panel with just a little opening assuming the framing of the opening is capable of doing acting stiffly and carrying the loads around the outlet as if it wasn't there. However, the code gives no guidance on the proportions of a 'small' opening.
In the MasterSeries, the size associated with a small opening is assumed to be no larger than 0.3L x 2.3H where L and H are the gap and height for the overall panel. An opening up to 8.5L x 0.5H can be tolerated but a cautionary note will be printed in the calcs. Any opening above this size may not be allowed under the lower fixity method. See 'Concrete Masonry Designers' Handbook'
Panel edges are assigned a greater level fixity as follows
Free edge is -100%
Pinned edge is 0%
Continuous edge is 100%
These are then input into the yield line equations give the values in the BS5628 folding tables. When openings are introduced the 'Concrete Masonry Designers' Handbook' recommends these kinds of degrees of fixity are reduced in proportion to the number of the length of the opening on the overall panel length.
Eg Fixity =((Opening Length x Free fixity) + (Remaining Length x Edge fixity))/ Overall length
5m wide by 3m high panel with 1m wide by 1.2m high opening. The panel is pinned simple and sides with a nonstop edge at the base.
the fixities became:
Top (1.0 x -100% 4.0 x 0%)/5 = -100%/5 = -20%
Btm (1.0 x -100% 4.0 x 100%)/5 = 300%/5 = 60%
Left (1.2 x -100% 1.8 x 0%)/3 = -120%/3 = -40%
Right (1.2 x -100% 1.8 x 0%)/3 = - 120%/3 = -40%
These percentage fixities are then accustomed determine the right inner and outer leaf bending moment coefficients by inserting them into formulae which correlate with the values given in the diagrams of Table 9 of the BS. As soon as capacities of the inner and outer foliage is then calculated based on these coefficients and rrn comparison to the actual bending moments your panel.
When 2 openings are specified, their areas are combined for your purposes of evaluating the reduced partial fixities on each edge for the panel.
The bending moment capacity of the panel without openings additionally evaluated for comparison purposes. In order that it can be seen that the panel will often have failed you'll have an openings, indicating that altering the opening sizes won't affect the design, but other remedial action should be taken to increase the panel's capacity.
'Concrete Masonry Designers' Handbook ' by J.J. Roberts, A. Tovey, W.B. Cranston, A.W. Beeby et al
The author of this article, Tommy White, can be a Chartered Civil Engineer and is a Senior Developer of the MasterSeries Structural Analysis, Design Drafting and Detailing computer program. MasterSeries provides a complete solution for the structural office, from analysis to design, drafting and detailing in steel, concrete, composite, connections, masonry, timber and retaining walls. All MasterSeries programs have been developed in-house, by one team of structural engineers, ensuring a substantially higher degree of integration than any other structural engineering software obtainable today.