Masonry Magazine January 1974 Page. 42
New Masonry Wall Support System Protects
Employees, Cuts Construction Delays
By C. Gregory Veith and John G. Williams
Article 11.9 of the American Standard Building Code Requirements for Masonry of the National Bureau of Standards, U.S. Department of Commerce, states: "Masonry work in locations where they may be exposed to high winds during erection shall not be built higher than ten times their thickness unless adequately braced."
The National Concrete Masonry Association's (NCMA) Specification for the Design and Construction of Load-Bearing Concrete Masonry states in Article 4.7.1: "Adequate precautions shall be taken to prevent damage to the wall during erection by high winds or other causes."
The Brick Institute of America (formerly Structural Clay Products Institute) Building Code Requirements for Engineered Brick Masonry states in Article 5.13.1: "Members of brick masonry in locations where they may be exposed to high winds during erection shall be adequately braced until provision is made for the prompt installation of permanent support at the floor or roof level immediately above the story under construction or until such time the masonry has attained sufficient strength to resist such forces."
The Occupational Safety Health Act (OSHA) states in Section 1926.7000 (a) by reference to Section A10.9 of the American National Standard Safety Requirements for Concrete Construction and Masonry Work. Article 12.5: "Masonry walls shall be temporarily shored and braced until the design level strength is reached to prevent collapse due to wind or other forces."
Why do all of these different governing bodies have the stipulation that masonry walls should be adequately braced during erection or until final construction stages are reached? A simple analysis of a non-reinforced masonry wall, 20' high, consisting of 8" concrete block and 4" brick will give the answer. The formulas (1) are for an engineering analysis of a 12" composite wall using type S mortar and an ultimate strength f of 2000 psi. The section properties of the structure are as follows: A 88.5 sq. in., I/C 235 in. cubed. P = 72
C. Gregory Veith studied civil engineering at Illinois Institute of Technology and the University of Illinois, and is a registered structural engineer. He is a principal of Wiesinger-Holland Ltd., Chicago, consulting structural engineers. John G. Williams is co-owner with James P. Maroney of Wil-Mar Mason Contractors, Wheaton, III. Williams and Veith are developers of this unique wall bracing system.
lbs. per sq. ft. of surface area. F. is the allowable tensile stress in 28 days and the factor 1.33 is the allowed increase in stress due to wind loading. The conversion of wind load from lbs. per sq. ft. to mph is based on the following formula and illustration (2). The 5 psf horizontal load capacity of the wall is far below the minimum 15 psf loading prescribed by the codes; thus the code requirement for bracing is obvious.
Once a wall has failed, what are the consequences? The most important would be death or injury to personnel. OSHA regulations state that you, as an employer, are responsible for the job site safety of your employees. Next would be property damage. Walls have fallen on scaffolds, automobiles, equipment, and even on trains. Although most contractors protect themselves with insurance, the cost of this insurance rises with each occurance.
Third would be delay in construction. The general contractor has scheduled other trades according to a preset time table to insure completion by a specified date. If a wall should fail, all schedules are voided, causing havoc with other subcontractors and the owner.
Last are the financial effects. Everyone involved in the project is affected: the architect, engineer, subcontractors, general contractors, insurance companies, and the owner. Insurance premiums rise, and the availability of such insurance decreases. However, the owner is the most seriously harmed because he still must bear the cost of interim financing and lost income from non-use of his building.
Now the question arises, what type of bracing should be used? What distance should braces be spaced along the wall? What is meant by the word "adequate" as used in the codes?
Let's start by answering the last question first. It is our opinion, and the basis of all our research, that the word "adequate" means "keep the wall standing until its supporting elements are in place." This is a broad statement and needs some rational limitations. The best information on wind loading comes from the building codes themselves. The completed wall was originally designed for a certain lateral load requirement; therefore, it is logical to brace the wall for that same load.
The second question can be answered by using engineering analysis. The spacing of braces cannot be greater than the ability of the wall to span between them. The wall, its properties, and the formulas (3) are the same as the former example (1). However, the tensile capacity has doubled since the bending is now parallel to the bed joint. (The P/A portion of the formula was omitted because it is not applicable here.) The 16-0 spacing was selected for several reasons, among them is the fact that control joints are usually located at 40-0 centers, and three braces at 16-0 leaves only a 4-0 projection from the last brace to the free edge at the joint. The 16'-0 brace spacing used is adequate for even the moderately severe wind areas in the country. However, the wall is only 3 to 7 days old at the time of bracing, reducing its tensile strength by about 15. Accordingly. its load capacity would be reduced to 18 lbs. per sq. ft., still acceptable to most code provisions.
The type of brace to be used has always been a problem. Contractors have been bracing walls in various ways for decades, even though they realized the available systems were not adequate. Traditional systems consist of a series of 2 X 10 scaffold planks placed against the wall and secured to the ground with more 2 X 10 planks and 2 X 4 stakes. The shape