Masonry Magazine April 1988 Page. 10
ACI/ASCE 530
The new design code is based on the assumption that all work is inspected and hence does not have two levels of allowable stress. Previously, a higher level of stress was permissible for work that had been inspected. Uninspected work was shown in testing to contain less than excellent workmanship, and thus a lower amount of stress was allowed for this type of construction. This is an important change in that it brings masonry design in line with concrete, steel and wood design. The level of inspection for different types of projects is left up to the designer. It is assumed that a small strip shopping center will require less inspection than a reinforced masonry high rise.
Empirical Design
Recognizing that many masonry buildings are very small and that these buildings do not require much design stress calculation to insure their safety, the committee maintained the concept of empirical design requiring no extensive load analysis. However, since modern masonry buildings are not built the same today as their counterparts forty or fifty years ago, the committee felt that some of the traditional empirical design rules needed to be made more restrictive. Some of these new restrictions are that empirical design cannot be used: 1) in Seismic Zones 3 and 4; 2) in areas where the basic wind load is over 25 psf.: 3) in buildings over 35 feet high where lateral load resistance is accomplished with masonry and; 4) in buildings which do not have a minimum number of shear walls. There are a number of other less significant changes included, most of which are the deletion of rules related to antiquated construction techniques.
Seismic Provisions
The new design code has an appendix which covers seismic design. This is another major change since neither the present BIA nor NCMA design code covers earthquake design. The seismic design provisions are based on those of the Uniform Building Code, but have been changed somewhat in the area of allowable stresses. This new design standard code along with its seismic appendix will be adopted by the Building Seismic Safety Council's NEHRP Recommended Provisions for the Development of Seismic Regulations for New Buildings.
Rational Designs
The new code continues to use working stress analysis for masonry. However, there are some significant changes in the design rules and information. Since the engineer is admonished to pay more attention to differential movements in masonry, the code lists a number of coefficients that can be used to predict drying shrinkage, moisture expansion, temperature movement and creep. Values for moduli of elasticity and rigidity are also given.
The area over which stress is measured, the net area, is redefined as the mortar bedded area. This results in a higher calculated stress than was obtained previously, so many of the allowable stresses had to be changed. At first glance it may seem that the code has been relaxed but this is not the case.
Slenderness of masonry walls has been read-dressed and is now measured by a traditional slenderness ratio, h/r, in which h is the unsupported height and r is the radius of gyration. Unlike the old codes, there is no limit on the slenderness ratio, but when it is over 99, the allowable stresses are reduced significantly.
As with empiricial design, there are a number of other changes in the way engineers will be allowed to design masonry walls with emphasis on composite and cavity walls, anchor bolt design, diagonal tension and shear transfer. These changes will have to be covered in seminars, and with design aids.
Now that the committee has released these documents it is important to make their contents available to designers, specifiers, contractors, and inspectors. This will be accomplished by means of a series of seminars that are being planned across the country. These seminars will begin as early as 1988. As part of the seminars, a design handbook and a set of notes is planned by the Council for Masonry Research. This handbook will not only give examples of design that complies with the standard but will contain tables that will assist designers by providing section properties, slenderness ratios and so forth as needed.
COMMONWEALTH OF PENNSYLVANIA
DEPARTMENT OF GENERAL SERVICES
FOR SALE:
1906 WOODBURY GRANITE
The Department of General Services, Surplus State Property, has approximately 5,500 pieces of 1906 Woodbury Granite, consisting of ballistar rail caps and bases; corners; stair treads and 3,500 1'9" square x 3" pavers. To obtain a complete inventory, please write to: Norman J. Kee, Chief, State Surplus Property, Department of General Services, 2221 Forster Street, Room G-48, Harrisburg, PA 17125 or call 717-787-4083.