Allows More Productive Construction More Economically
With internal wall bracing as standard operating procedure, Burggrabe Masonry experienced zero days of lost time due to high wind events in 2017.
From retail fire walls to auditoriums, Burggrabe Masonry constructed many projects, different in scale and size. Masonry walls were built using standards complying with the Internal Wall Bracing portion of the Wall Bracing Standard set forth by MIOSHA (Michigan OSHA).
The Standard Practice for Bracing Masonry Walls Under Construction (Standard Practice), developed by the Council for Masonry Wall Bracing and published by the Mason Contractors Association of America, is based on these fundamental principles:
- Life safety is primary goal.
- Acknowledgement that walls are vulnerable to collapse during construction.
- Basic premise:
– Design walls to resist specific wind speed
– Typical 40 mph design wind speed for temporary bracing
– Monitor wind speeds continually on site
– Evacuate areas close to walls, referred to as restricted zone, prior to actual wind speeds reaching specified wind speeds.
The Standard Practice also establishes engineering criteria that govern structural design of temporary bracing systems.
How Internal Bracing Works
Unlike other wall systems, masonry can be designed to be self-supporting during construction, eliminating the need for temporary external bracing and associated costs (even for extreme wall heights). Bracing walls under construction is required primarily for life safety of workers and others on the construction site as well as to preserve progress of construction in high wind events.
Very frequently, reinforced walls are specified with an amount of reinforcement that is also sufficient for internal bracing purposes. Allowable internally braced wall heights for reinforced 8″ CMU is typically in the range of 15′ to 25′, whereas the range for 12″ CMU is typically 20′ to 35′. Greater heights can be achieved with heavier and/or double reinforcement. However, a fixed base is a requirement to maintain a stable wall and foundation under wind load. This is called fixity. Fixity can be achieved with full lap splices, including at the dowels extending from the foundation.
Importance of a Fixed Base
Except for walls with frequent intersecting walls and/or corners (as described for shafts), masonry walls function structurally as a freestanding vertical cantilever during construction. The only way for such a wall to be stable under wind load is to develop fixity at the base. Without a fixed base, the wall will simply rotate at its base and collapse, regardless of how strong or heavily reinforced it may be above the base.
Short dowels extending out of the foundation are unable to develop the required fixity of reinforced masonry. While some degree of fixity is developed just by the mortar alone (unreinforced behavior), the strength of this connection is of modest magnitude. Even for walls to be braced by external braces, until the brace is actually installed, the wall will require a fixed base for stability. In essence, most masonry walls spend at least some of their lives as an internally braced wall.
Per MIOSHA, bracing plans, including designs and specifications, created using accepted engineering practices as laid out in Standard Practice for Bracing Masonry Walls Under Construction must be available on the job site. Check with other state OSHA offices for specific state requirements.
Job Site Coordination
First, coordination with the concrete contractor is paramount to ensure that vertical dowel rebar is properly placed within the foundation to maximize its effectiveness for the wall bracing system.
Coordination with other trades is the key to effectiveness of the system. When erecting tall walls that extend beyond bearing points, it is imperative to coordinate with the appropriate contractor to have all the elements for that bearing level installed prior to continuing building the wall. It is imperative to build tall walls, when possible, with corners and not just straight runs. To help support the tall wall, any adjacent intersecting wall should be installed at the same time.
Passing the Test
In 2017, August was the only month that southwest Michigan didn’t have a wind event producing gusts up to 40mph. The average wind gust for the other 11 months was 48.45 mph.
This gust speed alone is why proper installation and implementation of wall bracing means and methods is truly imperative for the health and safety of all individuals working within the restricted zone.
Todd Dailey, PE, is president of Dailey Engineering in Onsted MI. Before starting his own consulting firm in 1993, he was employed by Bechtel Power Corporation performing nuclear power plant design and for Dow Chemical doing facility design. He is a registered professional engineer in Michigan and Ohio with more than 30 years’ experience in structural engineering and building design. Dailey has served as an Adjunct Professor for the Civil Engineering Department at Lawrence Technological University teaching masonry structural design, has been an active member of the Masonry Institute of Michigan (MIM), and is proud to be an original member of the Generic Wall Design Committee (where he obtained his real education in masonry). Dailey received his Bachelor of Science degree in Civil Engineering from Michigan Technological University. firstname.lastname@example.org | 517.467.9000