| Introduction
Most structural tests of full-scale masonry walls use air bags or other loading devices to apply simulated wind loads to the exterior face of the wall, while tests of water resistance have focused on permeability of the veneer. Neither approach is appropriate for walls intended to function as pressure-equalized rain screens.
This report, Part 5 of a series, describes full-scale wall tests that use static air pressure for loading in conjunction with a spray rack, with or without cavity pressure equalization, to evaluate wall performance, with the load applied either to the face of the wall or to a barrier within the wall, and with water penetration driven by an air pressure difference, or not, as desired. The previous tests characterized performance of individual components: ties, brickwork, studs, and connections in isolation.
Research Program
For these tests, a test apparatus was developed to subject a section of brick veneer wall, with a 5.2 m x 2.6 m interior wythe or panel of stud wall, and an exterior brick wythe 5.2 m x 2.8 m, to combined effects of static air pressure up to 12.5 kPa and uniform water spray at a rate of 5.6 L/ min*m2, to simulate the effects of steady wind load and rain. The apparatus collects water from the bottom of the wall cavity and records the rate of flow. It allows pressure inside the cavity to be equalized to the exterior or to the ambient pressure.
Five different walls were tested, four with steel stud backup and one with concrete block masonry backup. None included insulation or space for cavity insulation. A different type of tie was used for each wall.
Wall 1 - BVSS supported at top and bottom, 25 mm cavity, through the web bridging.
Wall 2 - BVSS supported on four sides, 25 mm cavity, face bridging.
Wall 3 - BVSS with back-to-back studs, supported on four sides, 50 mm cavity, no bridging, interior air barrier supported with steel hat and J-sections on the interior.
Wall 4 - BVSS supported on four sides, 50 mm cavity, through the web bridging, with a large window.
Wall 5 - BV with concrete block backup, supported on all four sides, 50 mm cavity.
All of the steel study walls used the same studs and spacing (0.93 mm x 92 mm @ 406 mm, or back to back @ 812 mm). By conventional design procedures, deflection would be the first limit state reached, L/360 @ 2 kPa, or L/720 @ 1 kPa. 190 mm concrete blocks were used for Wall 5, capable according to current design procedures of carrying 1.2 kPa assuming load sharing with the veneer, or 0.84 kPa assuming no load sharing.
In addition, they offer the following good-practice recommendations, based on detailed observations of the full-scale tests, as well as lessons learned from the previous four phases:
Provide 50 mm minimum clear cavity.
Clean mortar droppings out of the cavity.
Tool the mortar joints.
Provide full head joints.
Minimize furrowing of bed joints.
Minimize mortar fins.
Properly attach tracks to structure.
Keep knock-out holes away from mid-height.
Use four-screw clip angles to secure through the knockout bridging.
Splice joints in bridging.
Use two screws for stud-to-track connections (inside and outside).
Provide a vertical movement joint (and make it stiff).
Provide at least double studs at openings.
Select stiff ties that do not become flexible at extreme adjustments.
Place line of action of tie force as close to centroid of stud section as possible (e.g. use side mounted ties).
Do not locate ties near knockout holes.
Use hot-dip galvanized or stainless steel ties.
Provide additional ties at openings.
Support air barrier to carry the full wind load to the studs.
Seal the air barrier.
Provide air barrier continuity at openings.
Fully seal top movement joint in brick veneer (or shelter it behind flashing).
Permit unprotected vents only at bottom of cavity.
Keep weep holes and vents clear.
Make flashings continuous, with sealed laps and end dams.
Integrate flashing with backup wall design to ensure water cannot get behind flashing.
Research Consultant: Prof. Robert G. Drysdale, P.Eng and Michael J. Wilson, Department of Civil Engineering and Engineering Mechanics, McMaster University. |