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Fall 2001 CODES AND SPECIFICATIONS UNDERSTANDING
AND STRUCTURAL DESIGN PROVISIONS - PART II
Provisions in 1998 California Building Code are discussed, rather than 1997 UBC Provisions, because it is a document based on 1997 UBC adopted in California and also covers State of California Amendments, which are applicable to public schools, community colleges, essential services buildings, and hospitals. Part I dealt with Material and Product Standards. Part III will cover Testing, Inspection, and Construction. Structural design requirements related to loads are covered in Chapter 16 of the 1998 California Building Code (CBC). Division I covers dead and live loads, and Division II covers snow loads. These two divisions provide gravity load considerations. Division III covers wind design and Division IV covers earthquake design. The provisions in Chapter 16 will not be covered in this article. It should be noted however, that even in areas where wind loads produce greater design demands on a structure, the detailing requirements and limitations of earthquake design as prescribed in Division IV of Chapter 16 shall govern. For public schools, community colleges and state owned or leased essential services buildings regulated by the Division of the State Architect (DSA), Chapter 16A instead of Chapter 16 governs. MASONRY CODE PROVISIONS All structural design provisions for masonry, including glass masonry, are given in Chapter 21. State of California Amendments are given in Chapter 21A. This article attempts to summarize some of the basic code provisions, rather than discuss the detailed provisions. Furthermore, only those sections of the code which are pertinent to seismic zones 3 and 4 are covered. Three methods of design are allowed, working stress, strength design and empirical design. Empirical design method is not addressed in this article, as it is applicable to design for wind loads in seismic zones "0" and "1." Stack bond for design purposes is defined in Section 2106.1.4, because the reinforcement requirements for stack bond masonry are different than those for non-stack bond masonry. These are covered in Section 2106.1.12.4 under item 2.4. Stack bond exists when less than 75% of the units in a vertical transverse plane have a lap less than: · One-half the unit height or · One-fourth the unit length WORKING STRESS DESIGN Section 2107 of CBC covers the requirements under this design method. Two choices in design are allowed. A. Use of one-half allowable stresses B. Use of full allowable stresses A. Use of One-half Allowable
Stresses 1. When "no special inspection" is
provided 2. Masonry compressive strength f
'm limited to 1500 psi 3. A letter of certification for unit
strength is not required B. Use of Full Allowable Stresses 1. "Special inspection" per Section 1701
is required 2. Masonry compressive strength f
'm 3. Letter of certification for unit
strength is required · Bearing walls — 6 inches · Columns — 12 inches (8 inches when one-half
stresses are used) REINFORCEMENT (Section
2107.2) 1. Maximum bar size — #11 2. Maximum ratio — 6% of cell area 12% of cell area with
splices 3. For Columns Minimum reinforcement ratio — 0.5% Maximum reinforcement ratio — 4% 4. For Walls Minimum ratio of horizontal and
vertical reinforcement combined — 0.002 Minimum in any direction — 0.0007 5. Allowable Tensile stress
(Fs) a. Deformed bars — 0.5 fy but, b. Wire reinforcement — 0.5 fy but, c. Ties, anchors and smooth bars — 0.4 fy but, 6. Allowable Compressive stress
(Fsc) a. Deformed bars in columns — 0.4 fy but, b. Deformed bars in flexural members — 0.5 fy but, c. Deformed bars in shear walls — 0.4 fy but, (See section 2107.2.11 item 2.3 for
other details) 7. Development Length (ld) for
deformed bars or deformed wires a. In tension — 0.002 dbfs b. In compression — 0.0015 dbfs Where db = diameter in inches fs = computed stress in
psi If smooth bars are used, the
development length as calculated above is
to be doubled. 8. Allowable Bond stress (u) a. Deformed bars — 200 psi b. Deformed bars without special
inspection — 100 psi c. Plain bars — 60 psi 9. Splices (Section 2107.2.2.6) Splice length is to be sufficient to
transfer allowable stress in
reinforcement, however, the minimum length
shall be: a. Bars in tension — 40db b. Bars in compression — 30db Splice length needs to be increased by
30% if bars are spaced 3 inches or
less. Other details on splices are given in
Sections 2107.2.2.6 and 2107.2.12. 10. Hooks All details given in Section
2107.2.2.5. MASONRY 1. Allowable Compressive stress in flexure
(Fb) — 0.33 f 'm but
not greater than 2000 psi. 2. Allowable Axial compressive stress
(Fa) for walls varies
based upon slenderness of the wall
(Section 2107.2.5). e.g., for 8-inch fully
grouted wall, when Fa = 0.1875 f
'm Fa = 0.1225 f
'm Fa = 0.0625 f
'm where h' = height of wall in
inches r = radius of gyration. (See
Table 21-H-1) For columns, capacity
contribution of steel needs to be added to
masonry capacity. ALLOWABLE SHEAR STRESSES
(Fv) a. Flexural members with no
reinforcement b. Flexural members with
reinforcement c. For shear walls, see section
2107.2.9 for details. In general, the
allowable shear stress depends upon M/Vd
ratio. ALLOWABLE BEARING STRESS
(Fbr) a. Bearing on full masonry — 0.26 f 'm b. Bearing on one-third or less area of
masonry element — 0.38 f 'm For walls with STRENGTH DESIGN Section 2108 of CBC covers the provisions for strength design. When designing under strength design method, special inspection of masonry during construction as stipulated in section 1701.5 item 7 is required. Minimum sizes (nominal not actual) of members shall be as follows: 1. Beams Width > 6 inches Depth > 8 inches 2. Piers Width > 6 inches < 16 inches Length > 3 times width < 6 times width Clear Height < 5 times length 3. Columns Width > 12 inches Length > 12 inches < 3 times the width REINFORCEMENT (Section
2108.2.2) 1. Maximum bar size — # 9 2. No more than 2 bars in a cell of a
wall 3. For columns Longitudinal reinforcement ratio > 0.5% Lateral reinforcement ratio > 0.18% 4. For piers > 0.07% Transverse reinforcement ratio > 0.15% 5. Specified yield strength
(fy) < 60 ksi 6. Development length (ld) The basic required embedment length is
a function of cover of reinforcement,
diameter of bar, yield strength of bar and
compressive strength of masonry. (See
Section 2108.2.2.6) Maximum
ld — 65
db 7. Splices (Section 2108.2.2.7) Minimum lap length — 12 inches Maximum lap length — 65 db Mechanical or welded splices shall
develop 1.25 fy 8. Hooks MASONRY ( f
'm) Minimum — 1500 psi Maximum — 4000 psi 1. Maximum usable compressive strength — 0.85 f 'm 2. Shear strength (Vm) Depends upon M/Vd ratio and f
'm of masonry. For the highest f 'm
» 4000
psi, Vm varies from »
76 Ae to 151 Ae Saying it differently, the maximum
shear stress masonry can sustain varies
from 76 psi to 151 psi. It is important to recognize that design strength is calculated by multiplying nominal strength with a strength reduction factor. For ease of use, the following table of strength reduction factors (F) is prepared:
For out of plane wall design with axial load
< 0.04 f 'm minimum nominal
thickness is EXISTING MASONRY Existing masonry for structural purposes is allowed with some restrictions. If the existing masonry does not meet the requirements of reinforced grouted masonry, its use is not allowed. Strengthening of existing masonry, which does not meet the requirements of reinforced grouted masonry is allowed by shortcreting or other concrete structural system. Detail provisions are given in Section 2114 A of CBC.
This issue of "Masonry Chronicles" was written by Dr. Vilas Mujumdar, Executive Director of Concrete Masonry Association of California and Nevada. |
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This
is Part II in a series of three articles to
understand and locate codes and specification
provisions related to concrete masonry design and
construction. The purpose of these articles is not
so much to provide specific design guidance, but to
direct a design engineer to various provisions,
primarily in the 1998 California Building Code.
Many other codes and standards such as ASTM,
UBC-Standards and Masonry Standards Joint Committee
(MSJC) provisions are also referenced where
appropriate.
1500
psi 
4000 psi
,
,
,
but not greater than 50 psi
but not greater than 150 psi
ratio greater than 30, analysis is
required, which considers axial loads,
variable moment of inertia, effect on
stiffness and fixed end moments, effect of
deflections on moments and forces and
effect of duration of loads.