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Feature

The Masonry Standards Joint Committee (MSJC) is in the process of completing a new edition of masonry standards for design and construction and final approval of these revised standards is expected in early 2002. These standards entitled "Building Code Requirements for Masonry Structures (ACI 530/ASCE5/TMS 402)" and "Specification for Masonry Structures (ACI 530.1/ASCE 6/TMS 602)" along with their companion commentaries are typically referred to as the MSJC Standards, or the "Code" and "Specification," respectively.

The MSJC has made numerous changes to the four documents (Code, Specification, Code Commentary and Specification Commentary). Many of these changes are minor corrections, updates, and editorial improvements and do not warrant including in this short summary description; whereas, several other changes are substantial and are summarized briefly in the sections to follow. Some of these significant topics include a new strength design chapter, integrated seismic design requirements, modifications for allowable flexural tension design values, wind speed threshold for empirical design, criteria for veneer supported by wood, clarifications for empirically designed shear walls, removal of drip ties, removal of "when required" provisions, quality assurance provisions for prestressed masonry, grout demonstration panels, and protection requirements for reinforcement.

Strength Design Provisions
For the first time, a chapter on strength design of masonry is included in the MSJC. This new chapter provides a major design advancement in the Code. Previously, strength design had not been included in the MSJC, but other sources such as the NEHRP (National Earthquake Hazards Reduction Program) and the IBC (International Building Code) had included such provisions, especially in conjunction with earthquake design. The MSJC has worked for many years on various forms of limit state design criteria and has now included strength design as a means of inelastic computation. Many sources of research over the past few years have contributed a wealth of new data to provide a firm basis of inelastic criteria for masonry design.

Even though a philosophy of strength design has been added for masonry design, the MSJC retained and improved the other masonry provisions for allowable stress design, veneer criteria, empirical design, prestressed design, and glass unit design. The rationale for including strength design was to provide for an improved model for inelastic system performance, especially for earthquake-induced loads. The rationale includes a better compatibility with the inelastic load criteria put forth by the ASCE 7-98 (Minimum Design Loads for Buildings and Other Structures) and the IBC documents.

In addition to the inelastic material and system criteria, the new masonry strength design chapter also includes strength provisions for flexure, shear, bond, anchorage, and other system components. Criteria are provided for cross section design properties, including maximum strains and other material provisions. Strength for maximum bending and the associated shear and bond criteria are included. These strength design criteria have been correlated to the latest version of the ASCE 7 load criteria.

Seismic Design Requirements
Prescriptive seismic design requirements in the MSJC have been substantially revised to be compatible with both the IBC and ASCE 7-98. Many of the changes to the seismic requirements were needed to update terminology. Previously seismic design requirements were applied to various seismic zones based on where a building was located. Later, seismic requirements were applied to buildings based on their assigned Seismic Performance Category (SPC) that was based on both the location of the building relative to areas of known seismic activity and on the importance of the building. Recently that term was abandoned by the seismic design community, and it has been replaced by a more accurate descriptor of Seismic Design Category (SDC), which is based on the location of the building, its importance, and the soil conditions supporting the structure. To recognize the change in terms, the MSJC requirements have been updated to now apply to the various Seismic Design Categories (SDC). SDC A, B, C, D, E, and F rather than the previously used Seismic Performance Categories (SPC A, B, C, D and E).

In addition, shear wall definitions consistent with the those in ASCE 7 -98, Appendix A.9.11 and the IBC have been have been added into the Code. These limitations allow designers to coordinate the prescriptive detailing and design requirements of the MSJC with the Response Modification Factors (R Values) and other seismic coefficients from ASCE 7 or the IBC. Because of the higher risk associated with moderate to high SDCs, restrictions are placed on the use of some shear wall types in specific SDCs. In addition, minimum reinforcement requirements are established in areas of moderate to high seismic risk. Phillip Samblanet describes some of these shear wall types in TMS Responds Volume 1, No. 1 as related to the IBC. The following also briefly describes each shear wall type:

  • Empirically Designed Shear Walls — As the name implies, empirically designed masonry shear wall must comply with the requirements of Chapter 5 of the Code, which covers empirical design requirements for masonry. Empirically designed shear walls are not required to contain any reinforcement. The use of these shear walls is limited to structures assigned to SDC A.

  • Ordinary Plain (Unreinforced) Shear Walls — Ordinary plain shear walls are designed to meet the requirements of Section 2.2 (allowable stress design of unreinforced masonry), Section 3.3 (strength design of unreinforced masonry), or Chapter 4 (prestressed masonry design). These shear wall types are not required to contain any reinforcing steel. The use of these shear walls is limited to structures assigned to SDC A or B.

  • Detailed Plain — (Unreinforced) Shear Walls Detailed plain shear walls are designed essentially the same as ordinary plain shear walls with the additional requirement of a minimum amount of prescriptive reinforcement(Note 1). The use of these shear walls is limited to structures assigned to SDC A or B.

  • Ordinary Reinforced Shear Walls — Ordinary reinforced shear walls are designed to meet the requirements of either Section 2.3 (allowable stress design of reinforced masonry) or Section 3.2 (strength design of reinforced masonry). These shear wall types are required to contain the reinforcing steel determined necessary by design or the minimum prescriptive reinforcement(Note 1), whichever is greater. The use of these shear walls is limited to structures assigned to SDC A, B, or C.

  • Intermediate Reinforced Shear Walls — Intermediate reinforced shear walls are designed essentially the same as ordinary reinforced shear walls with the additional requirement that the maximum spacing of the vertical prescriptive reinforcement shall not exceed 48 in. (1219 mm). The use of these shear walls is limited to structures assigned to SDC A, B, or C.

  • Special Reinforced Shear Walls — Special reinforced shear walls are designed essentially the same as intermediate reinforced shear walls with the additional requirement that the maximum spacing of the vertical prescriptive reinforcement shall not exceed 48 in. (1219 mm), onethird the length of the shear wall, or one-third the height of the shear wall. Also, the minimum cross-sectional area of the vertical reinforcement shall be one-third of the required horizontal reinforcement. Special reinforced shear walls may be used in any SDC (A, B, C, D, E, or F).
Table 1: Requirements for Masonry Shear Walls Based on Shear Wall Designation
Shear Wall Designation Prescriptive Reinforcement Requirements SDC Allowed
Empirically Designed Shear Wall None SDC A
Ordinary Plain (Unreinforced) Shear Wall None SDC A and B
Detailed Plain (Unreinforced) Shear Wall Note 1 SDC A and B
Ordinary Reinforced Shear Wall Note 1 SDC A, B and C
Intermediate Reinforced Shear Wall Note 2 SDC A, B and C
Special Reinforced Shear Wall Note 3 SDC A, B, C, D, E and F
Note 1 Vertical reinforcement of at least 0.2 in.2 (129 mm2) in cross-sectional area (equivelent to a No. 4 (M#13) bar is required at corners, within 16 in. (406 mm) of each side of openings larger than 16 in. (406 mm) in either the horizontal or vertical direction, within 8 in. (203 mm) of each side of movement joints, within 8 in. (203 mm) of the ends of walls, and at a maximum spacing of 10 ft (3.05 m). Horizontal reinforcement of either W1.7 (MW11) joint reinforcement spaced not more than 16 in. (406 mm) or bond beam reinforcement having a cross-sectional area of not less than 0.2 in.2 (one No. 4 bar) and spaced not further than 10 ft (3.05 m) apart is required. Horizontal reinforcement mustl also be placed at the top and bottom of wall openings and within 16 inches of the top of the wall.

Note 2 Same as Note 1, except that the vertical reinforcement is required to be spaced at no more than 4 ft (1.22 m) apart.

Note 3 Same as Note 2, except that the maximum spacing of the vertical reinforcement may not exceed one-third the shear wall length or one-third the shear wall height. Also, the minimum cross-sectional area of the vertical reinforcement is required to be one-third of the required horizontal reinforcement.

Allowable Flexural Tension Modifications
Recent research supported revisions to the allowable flexural tension values for grouted unreinforced masonry elements when subjected to flexural tension perpendicular to the bed joints as show in Table 2.

Table 2: Allowable Flexural Tension, psi (kPa) Perpendicular to Bed Joints for Fully Grouted Hollow Unit Masonry
Mortar Types
Portland cement/lime or Mortar cement Masonry cement or air entrained portland cement/lime
M or S N M or S N
65 psi (previously 68 psi) 63 psi (previously 58 psi) 61 psi (previously 41 psi) 58 psi (previously 29 psi)

Empirical Design Modifications
The threshold limit of wind speed for empirical design was changed to be compatible with the new provisions of ASCE 7. That is, previous provisions of the MSJC prohibited empirical design of masonry in regions where the design wind pressure exceeded 25 psf (1197 MPa). The revised limitation is changed to a wind speed limit of 110 miles per hour (145 km/h) three second gust.

Shear wall spacing requirements for empirically designed building were clarified so that the maximum length-to-width ratios for diaphragm panels must follow the values in Table 3.

Table 3: Diaphragm length-to-width ratios for Empirically Designed Masonry
Floor or roof diaphragm Construction Maximum length-to-width ratio of Diaphragm panel
Cast-in-place concrete 5:1
Precast concrete 4:1
Metal deck with concrete fill 3:1
Metal deck with no fill 2:1
Wood 2:1

Support of Veneer by Wood Construction
Although not applicable to all forms of masonry construction, masonry veneer having an installed weight of less than 40 psf (195 kg/m2)and a height not exceeding 12 ft (3.7 m) is now permitted to be supported on wood members. This form of construction, often realized as a cost-effective solution in wood-framed residential structures, was previously not permitted due to perceived concerns over fire safety and cracking resulting from excessive deflection. The height limitation of 12 ft (3.7 m), in conjunction with deflection limitations, addresses these concerns.

Removal of Drip Ties
The use of wall ties with drips (bends intended to inhibit the migration of moisture from one masonry wythe to other) has been eliminated from the Code due to their reduced load-carrying capacity. This removal was also due in part to the recognition that such ties do not perform their intended function of reducing moisture migration when improperly installed.

Protection Requirements for Reinforcement
Corrosion protection provisions, which were previously covered by the Specifications, have been added to the Code. Different requirements apply for joint reinforcement, wall ties, and anchors depending upon their intended use or exposure conditions. When these elements are exposed to earth, weather, or are installed in masonry exposed to a mean relative humidity exceeding 75 percent, they are required to be manufactured from stainless steel, epoxy coated, or hot-dipped galvanized. As a minimum, all other joint reinforcement and wall ties are required to be mill galvanized. When not exposed to earth, weather, or are not installed in masonry exposed to a mean relative humidity exceeding 75 percent, anchors need not be coated.

Quality Assurance for Prestressed Masonry
As part of the continuing effort related to the integration of prestressed (post-tensioned) masonry design into the Code (first introduced in the 1999 edition), the 2002 Code includes new quality assurance (QA) provisions for Level 2 (moderate) and Level 3 (rigorous) programs. The intention of these new QA provisions mimics the inspection requirements of conventional masonry construction. Hence, for projects requiring verification of grout placement, this requirement would also extend to prestressing grout. Similarly, when inspection of reinforcement location is mandated, this would also apply to the prestressing tendons.

Grout Demonstration Panels
The MSJC Specification has limited the maximum grout pour height or lift height based on the minimum grout space dimensions. These limitations were established through past field experience, and have helped ensure the adequate filling and consolidation of grout within masonry construction. As a new option, designers and contractors may now deviate from these prescribed limitations if it can be established, through the use of a demonstration panel, that the resulting finished construction is sound. This modification to the Specification has the potential to dramatically reduce the cost of grouted masonry construction by eliminating multiple-lift grouting or low pour heights.

Other Specification Changes
A major difference between the 2002 Edition of the MSJC Specification and previous editions of the Specification is the removal of the when required provisions that appeared in previous editions of the Standard. The Committee either modified or deleted those when required provisions to reflect minimum construction requirements, which is the intent of the MSJC Specification. Cold weather construction requirements have also been revised as Don McMican describes in TMS Responds, Volume 1, Number 3. Included in these revisions are protection procedures that need to be implemented when grouting in cold weather. Other notable revisions to the Specification and the Specification Commentary include:

  • Addition of veneer anchor requirements

  • Updating of ASTM C 270 mortar proportion and property specification tables to include mortar cement.

  • Addition of a discussion in the Specification Commentary to reinforce the fact that field sampling and testing of mortar is conducted under ASTM C 780 and that it is used to verify consistency of materials and procedures, not mortar strength.

Conclusion
As described briefly above, the new 2002 editions of the Code, Specification, and the associated Commentaries will provide numerous improvements over the 1999 edition. These changes have undergone rigorous consideration, review and sometimes debate, not only by being balloted using three societies' rules (those of ACI, ASCE and TMS), but also by having been scrutinized by the Technical Activities Committees of ACI and TMS, and by a three-month long public review. The resulting provisions represent the consensus of the Masonry Standards Joint Committee, and should meet the needs of the entire masonry design and construction communities for years to come.

The 2002 MSJC standards are expected to be released this spring. The price for the 2002 MSJC Standards (TMS order code TMS 0402-02) has been set at $93.50 Retail (nonmember) and $70 for members. To place an order for a copy of the MSJC Standards, please call the TMS office.

Disclaimer

This document is intended to provide explanation of typical and not-sotypical questions regarding masonry design, construction, evaluation and repair. It is intended for masonry design professionals, architects, engineers, inspectors, contractors, manufacturers, building officials, students, and others interested in masonry. It is not intended to cover every aspect of the discussed topics, but rather to focus on key issues that should be considered and addressed. This document should not be used as the sole guide for designing, constructing, evaluating or repairing masonry. It is imperative to refer to relevant building codes, standards and other industry-related documents. As such, TMS assumes no liability for any consequences that may follow from the use of this document. In addition, the opinions, ideas and suggestions given herein are those of the respondent, and not necessarily those of The Masonry Society.

This document is produced bimonthly by: The Masonry Society 3970 Broadway, Suite 201-D Boulder, CO 80304-1135 Phone: (303) 939-9700 Fax: (303) 541-9215 Website: www.masonrysociety.org

Oversight: TMS Design Practices Committee, William A. Wood, chair Editors: Edwin T. Huston, Vilas Mujumdar, Phillip J. Samblanet and William A. Wood

Questions, ideas, suggestions and differing opinions may be sent to TMS for consideration for inclusion in future issues of TMS Responds.






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