Masonry Cavity Wall: Restrained vs Unrestrained Slenderness in a Warehouse
A single-storey warehouse has 8 m high external walls of 190 mm concrete blockwork, unreinforced, with a 50 mm cavity and 100 mm facing brick. The walls are supported at the base by a reinforced concrete ground beam and at the top by a steel roof structure. There are no intermediate restraints, but the internal partitions are built tight to the walls at 3 m height in one bay. The designer has used a slenderness ratio based on the full 8 m height and is concerned the walls fail the limit. The site is in a moderate wind zone (basic velocity 24 m/s) and the blockwork has a compressive strength of 7.5 N/mm².
1. Evaluate the slenderness of the wall and determine whether the top and base supports can be treated as full restraints.
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Slenderness for an unreinforced masonry wall is the effective height divided by the effective thickness. The effective thickness of a cavity wall is taken as the greater of the two leaves or a value derived from their combined stiffness; here the 190 mm blockwork leaf governs, so the effective thickness is about 190 mm. If the wall is restrained top and bottom, the effective height is typically 0.75 times the clear height for a wall restrained against rotation at both ends, giving about 6 m and a slenderness of roughly 31. If it is unrestrained, the effective height is the full 8 m and slenderness is about 42.
The judgement is whether the steel roof structure provides a genuine lateral restraint. A roof that is merely bearing on the wall does not restrain rotation; a roof diaphragm tied to the wall does. Check for a positive connection - holding-down straps or a ring beam. If the connection is absent, treat the wall as unrestrained and the slenderness may exceed the limit for the stress level, requiring either a thicker leaf, piers, or intermediate restraint. The partition at 3 m is not a designed restraint and should not be counted unless it is detailed as one.
2. If the wall is found to be over-slender, what are the practical options and how does each affect the load path?
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The options are: increase the effective thickness (use a 215 mm block or a 140 mm leaf with a tied cavity), add intermediate restraint (a ring beam or windpost at mid-height), reduce the effective height by adding a structural floor or roof at a lower level, or introduce reinforcement.
Each changes the load path. A thicker leaf increases self-weight and therefore the vertical stress, which improves the slenderness capacity but adds load to the ground beam. A windpost transfers horizontal wind load to the foundation and roof, so the post and its base connection must be designed for the full wind pressure on the tributary width - for 8 m height and, say, 4 m spacing, that is a significant moment at the base. A ring beam ties the wall to the roof diaphragm and makes the top restraint genuine, but it must be continuous and anchored. Reinforcement in the blockwork changes the section from unreinforced to reinforced masonry and brings in a different design method. The cheapest option is usually a genuine top restraint, but only if the roof structure can take the horizontal reaction.