Technical Talk with David Biggs

Bonding with Masonry 

By David Biggs

I hope you had a good summer! Masonry construction has been very active and brought about some new questions. 

Q: On a recent project, we used loadbearing CMU with brick veneer as a cavity wall. The interior walls were mostly CMU also. For aesthetic purposes, we located the exterior brick expansion joints on our architectural elevations. However, the mason contractor disagreed with our specification that required them to locate the movement (control) joints in all the CMU walls including the exterior backup walls. Why is our specification a problem for the contractor?

A: Unfortunately, this is a common source of conflict on some projects.

Just as you did, other architects have provided the exterior joint locations for aesthetic reasons. While aesthetics are important, there are other reasons for locating the joints that are code-based. Let’s start by reviewing the designer’s responsibility as stated in TMS 402-13, Building Code for Masonry Structureswhich is the referenced masonry standard in building codes.

“4.1.5 Other effects

Consideration shall be given to effects of forces and deformations due to prestressing, vibrations, impact, shrinkage, expansion, temperature changes, creep, unequal settlement of supports, and differential movement.” Without specifically saying the designer must show the joint layout, it is clear the designer must accommodate shrinkage, expansion and differential movement.

In TMS 602-13, Specification for Masonry Structures, Article 3.3.D.6, the contractor is to “Install movement joints.” It does notrequire that the contractor provide the layout and design of the joints.

 

Later in the Mandatory Requirements Checklist, we see more specific guidance to designers which states:

 

Section/Part/Article         Notes to the Architect/Engineer

“3.3 D.6 Movement joints         Indicate type and location of movement joints on the project

drawings.”

 

The checklist is also very clear that the movements joints are to be on the drawings, not in the specifications.

So, it seems that both the design and layout of movement joints are totally a designer’s responsibility. Not so fast! Here’s where the confusion starts. TMS 602 is a specification that designer’s may choose to reference directly or modify. Thus, it is not uncommon for architects to insert specific criteria into their project specifications that might not already be in TMS 602. One of the modifications many architects have used is similar to yours, and requires the mason contractor to locate the movement joints.

This is where the conflict develops! While the designer can modify the specifications, they can’t do so and modify the code itself. By mandating the contractor select the joint locations, the designer is not meeting the code since it’s the designer who needs to accommodate shrinkage, expansion and differential movement based upon Section 4.1.5. Therefore, my interpretation of the code is that designers need to provide the movement joint locations.

Further support for this interpretation is based upon how the locations of movement joints affect various performance aspects of the walls that only the designer can address. These include:

  1. The spacing and location of CMU joints affect the size of the joints and the type of the horizontal reinforcement. While TMS 402 does not say how to design the joints, TEK Notes from the National Concrete Masonry Institute (NCMA) provide guidance for CMU walls and Technical Notes from the Brick Industry Association (BIA) provide guidance for clay masonry.

Specifically, the NCMA TEK notes have empirical and engineered methods for sizing and spacing joints and selecting horizontal reinforcement for CMU to accommodate shrinkage only.

  1. The spacing of movement joints determines the length of, and the design of, the shear walls.
  2. The placement of movement joints can affect the design of masonry beams and lintels since the position of movement joints relative to an opening determines whether arching action can be used in the design.
  3. The locations of the movement joints can affect the axial design of the wall at concentrated loads. If a movement joint is placed near a concentrated load, the wall may not be able to distribute the concentrated load such that the wall can adequately provide sufficient support. If additional support is needed, a pilaster may be required near the movement joint.
  4. A movement joint placed close to an opening can result in an undersized jamb pier.
  5. Movement joints can assist in accommodating differential movement.

Hopefully, it is clear that locating movement joints is not just about accommodating shrinkage and contraction of the material. Joint placement can affect so much of the design that is not within the expertise of the contractor. Thus, the task of designing and locating movement joints is a design responsibility from the code and, in my opinion, should not be assigned to the mason contractor.

Figure courtesy of NCMA TEK 10-2b using the Empirical Method

Q:In our office, a common question is whether to design reinforced masonry walls with horizontal bond beams or horizontal joint reinforcement. Can you provide any suggestions?

A: Many design firms work in various states and the answer can vary by region. While bond beams and masonry are generally part of many designs for the tops of walls and for openings, practice varies for how to reinforce the field of the walls. So, the short answer is that the choice can be determined by one or more of the following:

  1. regional practice,
  2. desired structural performance,
  3. controlling thermal movement.

Let’s consider each separately:

  1. Regional Practice:

In some western state, engineers design their walls fully grouted for seismic performance. In addition, they prefer bond beams for horizontal reinforcement. In the mid-west and eastern states, partial grouting is more common. In these areas, engineers often choose horizontal joint reinforcement unless there is a design condition that requires bond beams.

  1. Desired Structural Performance:
    1. The purpose of the horizontal reinforcement must first be established. If the sole purpose is for crack control, either bond beams or horizontal joint reinforcement may be used. If the horizontal reinforcement is to act as shear reinforcement, TMS 402-13, Building Code for Masonry Structuresallows either bond beams or horizontal joint reinforcement. However, Section 9.3.3.1(b) states “Joint reinforcement longitudinal wire used in masonry as shear reinforcement shall be at least 3/16 in (4.8 mm) diameter.” Research by Baenziger and Porter indicated that the 3/16” diameter is needed to provide strain capacity and prevent rupture.
  2. Section 9.3.3.4 further states “Joint reinforcement used as shear reinforcement— Joint reinforcement used as shear reinforcement shall consist of at least two 3/16 in. (4.8 mm) diameter longitudinal wires located within a bed joint and placed over the masonry unit face shells. The maximum spacing of joint reinforcement used as shear reinforcement shall not exceed 16 in. (406 mm) for Seismic Design Categories (SDC) A and B and shall not exceed 8 in. (203 mm) in partially grouted walls for SDC C, D, E, and F. Joint reinforcement used as shear reinforcement in fully grouted walls for SDC C, D, E and F shall consist of four 3/16 in. (4.8 mm) diameter longitudinal wires at a spacing not to exceed 8 in. (203 mm).” Note: The 3/16” diameter wire is notrequired if horizontal joint reinforcement is used as minimum prescriptive seismic reinforcement, not shear reinforcement.

These are significant issues because only 3/16 in. joint reinforcement may be used as shear reinforcement by code. Since many architects and engineers are accustomed to using horizontal joint reinforcement with 9-gauge (W1.7) wire for crack control, they must now adjust their designs to accommodate shear.

  1. While we generally think of masonry walls as spanning vertically, there are many situations where walls will span horizontally. One example might be an elevator or stair tower. If there is a missing diaphragm, spanning the walls horizontally is appropriate. For these designs, reinforced bond beams may be the only solution if joint reinforcement can’t provide the needed capacity.
Reinforced CMU wall with horizontal bond beams (courtesy of IMI, Sketchup 3D Warehouse)
  1. Controlling Thermal Movement

TMS 402-13, Building Code for Masonry Structures requires designers accommodate temperature changes. However, the specific methods are not defined in the code. The methods are provided by NCMA TEK Notes using either an Empirical Method (TEK 10-2C, Control Joints for Concrete Masonry Walls— Empirical Method) or an Engineered Method (TEK 10-3, Control Joints for Concrete Masonry Walls—Alternative Engineered Method). The table below is taken from TEK 10-3 and shows two options for 3/16 in. joint reinforcement as crack control. They could also be evaluated for shear reinforcement.

So, the choice of using bond beams versus horizontal reinforcement is more than accommodating crack control.

Thank you again for following this column. Remember, by bonding we get stronger! Send your questions to info@masonrymagazine.com, attention Technical Talk.

David is a PE and SE with Biggs Consulting Engineering, Saratoga Springs, NY, USA. He specializes in masonry design, historic preservation, forensic evaluations, and masonry product development.