Seismic Analysis Of Partially-Grouted Reinforced Masonry .

Seismic Analysis of Partially-Grouted Reinforced MasonryWalls Constructed Using Masonry Cement Mortar(PCA Index No. 03-12)byA.A. Hamid and F.L. MoonDrexel UniversityDepartment of Civil, Architectural and Environmental Engineering3141 Chestnut St.Philadelphia, PA 1910412 April 2005

EXECUTIVE SUMMARYSince the establishment of ASTM standard C91, masonry cement mortars have becomewidely used for masonry construction in low to moderate seismic regions. However, theMasonry Standards Joint Committee Code prohibits the use of masonry cement mortarsin lateral force resisting systems for structures that fall into Seismic Design Categories D,E, and F. The general objective of this study is to examine the appropriateness of thisrestriction in light of past research and, if necessary, propose additional research requiredto fill any existing knowledge gap.Since information about the specific impact of the physical and mechanical properties ofmasonry cement mortars on the seismic response of masonry shear walls is limited, thisreport primarily focuses on research that has identified the influence of mortar on thebehavior of masonry assemblages. In particular, the influence of mortar type is examinedin terms of in-plane pier response, out-of-plane wall response, and the response ofmasonry assemblages under axial compression, flexural tension, bed joint shear anddiagonal tension. The primary gap identified through this literature review was the lack ofexperimental research that addressed the response of reinforced masonry shear wallsconstructed with masonry cement mortar. To establish a comprehensive and efficientresearch program to fill this gap, the available literature related to the behavior ofpartially grouted reinforced masonry shear walls was also reviewed and key factors thatinfluence response were established.Based on this literature survey, it is the authors’ opinion that the use of masonry cementmortar instead of portland cement lime mortar will not have a detrimental effect on thestrength and deformation capacity of grouted and partially grouted (up to a spacing of 48in.) reinforced masonry walls subjected to in-plane and out-of-plane loadings. However,it must be emphasized that the evidence supporting this conclusion for in-plane loading islimited to investigations of assemblage behavior and a single in-plane pier experimentalstudy. As a result, the report concludes with an outline of a proposed in-plane testingprogram aimed at filling this knowledge gap.ii

TABLE OF CONTENTSEXECUTIVE SUMMARY . ii1. INTRODUCTION . 11.1 Background .11.2 Reinforced Masonry Walls . 21.3 Development of Mortar . . 41.4 Seismic Provisions Related to Mortar Type . 51.5 Objective and Scope . 61.6 Outline of Report . 62. EFFECT OF MORTAR TYPE ON THE PROPERTIES MASONRYASSEMBLAGES 72.1 Introduction . 72.2 Compressive Strength . 72.3 Bond Strength . .92.4 Diagonal Tensile Strength . 112.5 Summary . . 123. EFFECT OF MORTAR TYPE ON THE PROPERTIES OF MASONRYWALLS . . 143.1 Introduction 143.2 In-plane Behavior . . 143.3 Out-of-plane Behavior 173.4 Summary 204. EFFECT OF GROUTING ON THE PROPERTIES OF MASONRYASSEMBLAGES . . 224.1 Introduction 22iii

4.2 Compressive Strength . 224.3 Bed-joint Shear Strength 244.4 Tensile Strength 254.5 Summary 275. IN-PLANE BEHAVIOR OF PARTIALLY GROUTED MASONRY SHEARWALLS . 295.1 Introduction 295.2 Review of Past Research 295.3 Summary 456. CONCLUSIONS . 477. REFERENCES . 48iv

1. INTRODUCTION1.1 BackgroundMasonry is one of the oldest construction materials employed by man as evident from thehistoric remains of the Egyptians and the Greeks. The first masonry was a crude stack ofselected natural stones often with earthen mortar packed between them. This type ofmassive masonry could resist large compressive forces and was quite durable, althoughits tensile strength was poor. As a result, traditional masonry buildings exploited theweight of the floors and the massive walls to offset tensile stresses that arose due toeccentric vertical and lateral loads. Due to its constructability and substantial durability,construction using this type of masonry (termed UnReinforced Masonry [URM]) waswidespread throughout the 19th Century in the United States. However, a series ofearthquakes around the turn of the century, including the 1886 Charleston, 1906 SanFrancisco, 1925 Santa Barbara, and the 1933 Long Beach earthquakes, clearly illustratedthe seismic vulnerability of URM structures. These events prompted the 1933 passage ofthe California Field Act, which banned the use of URM for public buildings in California.This ban was subsequently adopted by other western states; thus, URM construction waseffectively halted west of the Rocky Mountains.This restriction has led engineers and builders to seek a more ductile, earthquake resistantform of masonry construction for high seismic regions. Along with economic concerns,this impetus resulted in the development of reinforced masonry, which typicallycombines high strength manufactured concrete and clay masonry units along withgrouting and reinforcing steel (Figure 1.1) to more efficiently resist tensile stresses andprovide a more ductile and reliable system. Throughout the last 70 years, this type ofconstruction has been used extensively in high seismic regions throughout the UnitedStates and, based on past performance, is widely considered as one of the mostearthquake resistant structural systems.1

Figure 1.1 Reinforced Fully Grouted Concrete Block Masonry Wall (Drysdale et al.1999)1.2 Reinforced Masonry WallsIn general, both clay and concrete masonry construction have been widely used in thepast; although, in current practice concrete masonry is more common. Depending on thedistribution of vertical and horizontal reinforcement (location and spacing) the MSJCcode (2005) classifies reinforced masonry into the following three categories: Ordinary reinforced masonry shear walls Intermediate reinforced masonry shear walls Special reinforced masonry shear wallsFigure 1.2 shows reinforcement details for these three designations.In addition to these MSJC definitions, a more general way to classify masonry shearwalls exists. Walls with reinforcement spaced at large intervals (greater than 48 in.vertically and horizontally) are referred to as partially, lightly or nominally reinforcedmasonry, whereas walls with more closely spaced reinforcement are known simply asreinforced masonry. A partially reinforced masonry wall is considered to consist ofreinforced strips of masonry with URM spanning between them (Figure 1.3), similar to“confined masonry” commonly used in Europe and South America.2

(a) Ordinary Reinforced Masonry Wall(b) Intermediate Reinforced Masonry Wall(c) Special Reinforced Masonry WallFigure 1.2 MSJC code (2005) reinforcement details for masonry shear walls3

In addition to the distribution of reinforcing steel, reinforced masonry walls can also bedistinguished based on the extent of grouting. Partially grouted masonry walls typicallyonly have grout placed where reinforcement is located whereas fully grouted masonrywalls have grout placed in every cell. From a construction standpoint, partially groutedmasonry (with grouted cells spaced at larger than 24 in.) is more efficient. For example, itenables easier installation of services (in ungrouted cells), faster construction, materialsavings, and reduced weight (resulting in reduced seismic loads) compared with fullygrouted walls. However, from a behavior standpoint, fully grouted walls are superiorsince grouting has been shown to reduce the inherent variability in masonry and alsoimprove the tensile and shear strength.Figure 1.3 Partially reinforced masonry walls (Drysdale et al. 1999)1.3 Development of MortarEarly mortars were primarily used to fill cracks and provide uniform bedding formasonry units. Such mortars were typically composed of clay, bitumen, or clay-strawmixtures (Dysdale et al. 1999). Following centuries of lime mortar use in masonryconstruction, portland cement-lime mortars were developed to be suitable for particularapplications in the late 1800’s and early 1900’s (Speweik 1995). More recently, masonrycement mortars have been introduced in the market. Masonry cement is primarilycomposed of portland cement or blended hydraulic cement and plasticizing materials. Inaddition, other materials are often added to improve properties such as workability,4

setting time, and durability. Aside from these improved properties, the primary advantageof masonry cement is that it is proportioned in controlled conditions which greatlyenhance its uniformity. This alleviates the need for on-site mixing of portland cement andlime and results in savings in construction time as well as more reliable mix proportions.Since the establishment of the ASTM standard C91 for these products in 1932, masonrycement mortars have become widely used in masonry construction in regions of low tomoderate seismicity (Speweik 1995).1.4 Seismic Provisions Related to Mortar TypeCurrently, the Masonry Standards Joint Committee (MSJC) Code (2005) prohibits the useof MC mortars in the construction of lateral force resisting systems for structures that fallinto Seismic Design Categories (SDC) D, E, and F. Two factors have likely contributedto establishing and maintaining this ban: (1) MC mortars were not common in highseismic regions where seismic codes were developed and (2) research into the behaviourof ungrouted (solid or hollow) masonry assemblages has shown that MC mortarstypically display lower bond strength than PCL mortars. Clearly the first factor is socialin nature rather than related to the actual seismic performance of shear walls constructedwith masonry cements. The second factor, while well established, may not have as largean influence on seismic performance as expected. Consider that for SDC D, E, and F, allmasonry is required to be reinforced and either partially or fully grouted (the MSJC Coderequires a maximum horizontal and vertical reinforcement spacing of 1220 mm [48 in.]).According to past research, as the extent of grouting and reinforcement increases, theinfluence of mortar type and mortar unit bond diminishes (Drysdale et al. 1999). Thisphenomenon is attributed to the continuity across the weak bed-joint plane, and theadditional load path provided by the grouted cells and reinforcement. As a result, it isgenerally accepted that for fully grouted masonry this provision is overly restrictive;however, it is unclear at what level, if any, of partially grouted reinforced masonry thisrestriction is appropriate.5