Page 134 - Design of Reinforced Masonry Structures

3.28 CHAPTER THREE

The MSJC-08 Code [3.2 (Section 1.8.6)] specifies values of the coefficients of creep in
terms of long-term deformation per unit compressive stress in masonry as follows:
−7
Clay masonry: k = 0.7 × 10 per psi (3.15)
c
−7
Concrete masonry: k = 2.5 × 10 per psi (3.16)
c
It is noted that this approach of specifying deformation due to creep is different than that
of the Canadian code which specifies creep coefficients as a multiple of the corresponding
elastic deformation—2 to 4 times the elastic strain for clay masonry and 3 to 5 times the
elastic strain for lightweight concrete masonry. Research indicates that creep of concrete
(1) is somewhat less than for ordinary concrete made from similar aggregates because of
lesser portland cement and water contents and (2) lightweight aggregate blocks creep more
than dense aggregate blocks [3.47, 3.48].
Example 3.3 illustrates calculations for creep deformation in a masonry wall.

Example 3.3 Creep deformation in a concrete masonry wall.
Determine deformation due to creep in the wall described in Example 3.1.

Solution:
For an 8-in. nominal wall, b = 7.625 in.
Height of wall = 12 ft
Cross-sectional area per foot length of wall = (7.625)(12) = 91.5 in. 2
3
For a fully grouted wall (grout weight = 140 lb/ft ) constructed from medium-
weight CMUs.
Self-weight of wall per foot of height = 78 lb (Table A.19)
Total weight of wall = (78)(12) = 936 lb
Calculate the average compressive stress in the wall. The entire wall weight does
not contribute to creep. The bottommost layer supports the entire wall weight (so the
compressive stress is maximum at this level), whereas the top layer supports none.
Therefore, it is reasonable to average the weight of wall over its height.
Average self-weight of wall = ½ (936) = 468 lb
Live load on the wall = 1200 lb/ft length
Total compressive load on the wall = 468 + 1200 = 1668 lb
1668
Compressive stress in the wall due to total load = = 18 2 . lb/in 2
91 5 .
−7
Coefficient of creep for concrete masonry: kc = 2.5 × 10 per psi [Eq. (3.16)]
= 6.55 × 10 in.
−7
Total creep deformation = (2.5 × 10 )(18.2)(12)(12)
−4
REFERENCES

3.1 NCMA TEK 9-1 (1994). Mortars for Concrete Masonry, National Concrete Masonry Association,
Herndon, VA.
3.2 MSJC (2008). Building Code Requirements for Masonry Structures, (TMS 602-08/ACI 530-
08/ASCE 6-08/), American Concrete Institute, Farmington Hills, MI; Structural Engineering
Institute of the ASCE, Reston, VA; and the Masonry Society, Boulder, CO.