Anchor systems in masonry walls have been used for centuries, with the earliest dating back to the Romans and Greeks who used wrought iron to supplement conventional masonry. Other cultures and countries have used variations of metal anchors and straps in the construction of masonry, as well as for retrofit or repair applications to supplement and strengthen masonry systems after natural disasters such as earthquakes. There are numerous modern applications where lateral restraint anchors can be utilized to remediate masonry walls that are in service to accommodate additional applied forces or to strengthen masonry weakened by deterioration or excessive load. Often, lateral restraint anchors can utilize the existing interior framing of a building to re-establish a point of support at an intermediate height of the wall. This is accomplished through a combination of fasteners or anchors that secure straps, rods, or other tension elements to the masonry wall to resist lateral displacement. Many of these applications merge the basic premises of historic methods with techniques and modern materials of the twentieth century to provide a cost effective, aesthetically appealing, and preservation oriented method for remediating masonry. This merger of old and new techniques allows us to stabilize historic walls for modern service conditions allowing them to continue serving their intended function.
Keywords: Masonry, Repair, Restoration, Anchor, Lateral Restraint
1 Owner, Specialized Masonry Restoration, 3318 County Road, Melissa, TX 75454, USA alan@specializedmasonryrestoration.com
2 Senior Associate, Whitlock Dalrymple Poston & Associates, P.C., 10621 Gateway Boulevard Suite 200, Manassas, VA 20110, USA, epeterson@wdpa.com
Introduction
Many multiwythe masonry walls never have an opportunity to attain historic status. They are subjected to the elements as they carry the loads intended by their designers and constructors, and slowly, they weather and degrade. For the exterior walls of these buildings, often maintenance is forgone, and the damage sustained to the exterior surfaces over time, coupled with eccentric gravity loads from floor and roof systems of the building, will result in lateral displacement of the walls. As the movement progresses, it eventually reaches a point where it compromises the integrity of the structure. Other times, lateral loads from earthquakes, soil pressure, or other imposed loads can result in damage to a wall system that can equally compromise the structure. The typical repair options for multiwythe walls with significant damage or displacement entail partial reconstruction or other prohibitively expensive programs. Without more cost effective alternatives, many of these buildings remain unrepaired and continue to degrade until they become unstable or derelict, running a rapid path to demolition.
If these wall systems can be stabilized at intermediate points of support prior to becoming unstable, the rate of decay can be considerably reduced and the lifespan of many of these buildings can be extended. Over the years, the most effective method of providing lateral stability has been the installation of lateral restraint anchors. A lateral restraint anchor is a combination of anchors, bars, ties, cementations, grouts, and resins used to remediate and stabilize masonry or stone walls against lateral movement. These elements can arrest the natural tendency of exterior walls to bulge at floor lines, and when installed in conjunction with an aggressive tuck pointing program to reduce the natural weathering of the masonry, can prolong the service life of the building.
Historic Perspective
Stone and masonry anchors have been noted in history as far back as the Romans and Greeks for masonry repair. The anchors of ancient times were plain bars made of wrought iron, using metal straps to anchor buildings and ensure their stability. But it was the Dutch, as early as the 1550s, who used them the most. These Dutch iron anchors originated in the lowlands, on barns, houses, and mercantile buildings and were called Muurankers.
Muurankers were inserted during construction and were not intended for remediation. They served as part of the building’s primary frame to associate each transverse beam with the wall to resist lateral loads. As time went on, they became architectural features on the buildings, taking the form of the initials of the builder, the owner, or the marks of merchants to advertise their work. In the 1800s, Dutch foundries began making shorter anchors out of cast iron to create decorative symbols like rosettes and Fleur-de-lis to improve the aesthetic of the anchor. This system of anchoring exterior walls to the frame spread over Europe.
Patress plates and bars (patress derived from the Latin ‘pateras’, meaning a shallow bowl or plate) were anchors used extensively in Great Britain to provide lateral support to masonry walls. The iron patress bar was inserted through an exterior wall on one side of the building and threaded through the joists and framing and then returned through the opposite wall where plates were attached to the ends of the bars by a bolt. A turnbuckle attachment on the interior was used to apply tension to the bar in order to stabilize the building (Figure 1). These elements fell out of favor over time because the plates on the outside of the buildings corroded, leaving stains on the stone and brick masonry, detracting from the buildings’ aesthetic appeal.
Figure 1. Patress plates on a façade parapet
In North America, settlers utilized permutations of several building techniques in the construction of their houses and mercantile structures, but were influenced primarily by the European building techniques. Plates and anchor rods were used primarily for remediation of masonry failures and damage during storms or earthquakes. The star anchor was favored in eighteenth and nineteenth century brick construction and can still be seen throughout the southeast, particularly in areas like Charleston, South Carolina, where seismic activity is most prevalent. These anchors were commonly called earthquake washers and were used to stabilize structures weakened over time due to shifting soil conditions or earth movements.
Modern Applications
There are many modern applications where lateral restraint anchors can be utilized to remediate masonry walls in service in order to accommodate applied loads or to strengthen masonry walls weakened by deterioration or excessive load (such as an earthquake). Many of these applications merge the basic premises of the above mentioned historic methods with techniques and modern materials of the twentieth century to provide a cost effective, aesthetically appealing, and preservation oriented method for remediating masonry.
The most common structures that may benefit from these techniques are nineteenth century two and three story mercantile structures commonly found in the small town centers of the United States. Many of these structures share common walls between buildings with variable roof heights and eccentric loads on each side of the wall (Figure 2).
Figure 2. Separation and displacement of common wall.
Uneven weathering and the lateral thrust of roof and floor systems can result in lateral displacements and distress at building perimeters. This lateral movement can become visible at interior walls where plaster and molding begin separating at ceilings (Figure 3). Many of these buildings utilize large rough hewn non-dimensional timbers set in fire cut sockets of the masonry or on timber framing elements with mortise and tennon joinery. This type of horizontal framing can provide a robust diaphragm when the framing elements are tied together and confined or restrained laterally. For these applications, lateral restraint anchors can be installed to utilize the existing interior framing system as a point of support at an intermediate height of the wall. The reduction in unbraced length serves to re-establish the stability of the wall and resist further displacement.
Figure 3. Separation of common wall floor system on interior of building. Repair Strategies
The selection of the proper lateral restraint system depends on several factors, including the existing construction of the floor and wall systems, the condition of the masonry and timber, the access to the interior and exterior sides of the wall, and the extent or severity of the existing displacement. While there are a number of different lateral restraint anchor configurations that may be employed, this paper focuses on the overriding principals of the repair strategies.
As with any retrofit lateral restraint system, the first step in the repair design is the development of the design loads. Lateral loads from seismic and wind forces can be developed directly from the applicable local code provisions. Additionally, the gravity loads resisted by the wall system including dead, live and snow loads must be generated, combined appropriately with other loads, and applied taking into account the appropriate eccentricities that generate internal moments and shears in the masonry. The joist seat details must be evaluated to determine distance from the centroid of the wall system to the line of action from the bearing seat. It is also important to incorporate any observed out of plane displacement in the wall, which will often times increase the thrust at the lateral restraint anchor due to the increase in the internal moment in the wall.
After the static forces in the wall are resolved, the lateral restraint is modeled as a new support element at an intermediate point in the wall. The type of lateral restraint system used is often based on the magnitude of the lateral thrust to be resolved and the construction or configuration of intermediate floor and roof systems. If the lateral restraining force to be resolved by the anchor system is relatively low, (on the order of 200-250 lbs per foot) it is often suitable to utilize the existing interior framing system to provide the necessary restraint. However, depending on the construction of the floor system, more flexible framing configurations may require more complex computations by modeling the support as a lateral spring versus a point of fixity in order to accurately resolve the resisting force generated.
As the lateral restraining force increases, it becomes more difficult to resolve the stresses generated by adhesive or expansion anchors in the existing brick masonry and shear fasteners in members of the framing system. Depending on the configuration of the building, higher loads may require the resolution of the forces by anchoring to opposite or interior walls independent of the floor system and the use of patress plate type elements to distribute anchor forces in the existing masonry walls.
Generally, it is advantageous to configure lateral restraint anchors to resist both tension and compression. A reversal of displacement is possible after repairs are performed. Also, the installation of noggings or blocking between framing elements and shims or wedges between the flooring and wall system allows an even transfer of forces into the framing.
For walls running perpendicular to the primary framing joists, the joists can be utilized in direct tension as a strut between opposite walls. Joists must be continuous to employ this strategy. Otherwise, modification of splices and joints must be performed to transfer load across the joists. In one configuration, an anchor plate or angle is affixed to the masonry wall, using either anchors fixed in an epoxy resin or a mechanical anchor, midway between each of the joists. If the floor system is accessible from the underside, a stainless or galvanized tension strap can be affixed to each side of the joists with a series of shear fasteners to transfer the load from the wall into the joists (Figure 4). Spaces between joists and walls can be filled with mortar or resin to engage the joists in compression. Fastener spacing, stagger and interaction with plates on opposite sides of the joist must be taken into account to insure the maximum tensile force can be obtained in the tension straps.
Figure 4. Retrofitting of masonry walls using straps and anchors (Top – utilizing straps between joists, Bottom – straps secured to top of joist under flooring)
Conversely, if access to the sides of the joists is limited, more substantial strapping plates, similar to earlier muuranker style connections, can be affixed to an angle on the top side of the joist and then fastened to the masonry wall using a similar masonry anchor. The area of the upturned strap leg should be sized to serve as a bearing surface against the masonry (Figure 5).
Figure 5. Muuranker style of retrofit wall repair utilizing straps along the top of the joist
When the primary framing joists are running parallel to the wall, it is more difficult to develop a reliable tensile force to resist the tendency for outward displacement of the masonry. Much like with the patress plates and earthquake washers of earlier years, if access can be obtained to opposite walls of a building, a threaded rod can be fed through the wall and into the floor framing to secure opposite walls to one another. Either decorative, non-corrosive patress plates can be installed on the exterior, or to minimize the impact to the appearance of the building, the threaded rod can be affixed with an epoxy resin within the masonry walls. The hole in the masonry is then filled with a tinted mortar or resin plug to match the exterior. This repair approach is particularly useful in the event the rigidity of the floor system is inadequate in tension to utilize as bracing, but can be supplemented with blocking and wedges to provide compressive restraint (Figure 6).
Often with older buildings, the floor system will have rough hewn timbers possessing sufficient lateral bending strength to provide resistance to the outward thrust. If so, discrete threaded anchors can be installed passing through multiple joists and affixed to the timbers with washers and nuts to secure the rod in place. The rod is then affixed in the masonry with epoxy and wedges or noggings installed between the joists to distribute the loads into the flooring system. Alternatively, a strapping system with associated blocking can be secured above or below the joists with shear screws to develop the lateral resistance (Figure 7). Of critical importance with this design is establishing a load/deflection relationship between the flooring system and the wall to determine how much lateral support can be derived from the flooring. Also, connections in the wood framing must be analyzed to ensure that the load transfer from the anchors into the framing does not exceed dowel or tendon capacities. Figures 6 and 7 depict the design and construction of such a strengthening technique.
Figure 6. Alternatives for lateral wall ties utilizing existing transverse floor elements.
Figure 7. Installation of tension straps in joists parallel to wall.
Material selection of the elements utilized for the masonry anchors is of critical importance with regard to exterior wall systems. Most multiwythe masonry walls are constructed as mass walls that do not manage water through the use of water resistive barriers and flashings, and as such, when exposed to the environment would be expected to take on significant volumes of water within the section of the wall. Therefore lateral restraint systems utilized in exterior walls must have some method of corrosion protection to provide a durable and lasting repair. Normally, steel components embedded in either epoxy resin or cementitious grout would be adequately protected from corrosion. However, it is often very difficult to ensure complete encasement of the embedded wall elements because they are installed either “blind” or with single sided access. Inadequate coverage and localized exposure of these elements to moisture can then result in corrosion and the expansion of the iron oxides can disrupt the integrity of the wall around the anchor. It is the recommendation of this author that either hot-dipped galvanized or stainless steel elements be utilized for exterior wall applications to reduce the potential for localized corrosion. Since most repairs of this type are labor intensive, the additional material cost for utilizing such materials is most often negligible with respect to the overall project costs.
A fundamentally important component of any such repair program involves the verification of the design assumptions in the exisiting materials through quality assurance testing of the repairs. Depending on the repair approach and the condition of the existing wall and framing system, it is normally recommended that initial material testing of the masonry (brick and mortar) be performed to estimate the anticipated anchor capacity. Additionally, in-situ tensile pull-out tests should be performed on anchors, fasteners and adhesives to verify the capacity as installed is consistent with the design. This is particularly true of epoxy resins that rely on the bond to the in-place materials to achieve the necessary tensile capacity. Tests should be performed in multiple locations with varying existing conditions. Load capacities should be conservatively estimated to account for future weathering of the masonry systems.
Conclusions
The use of lateral anchors to stabilize exterior masonry walls is a relatively inexpensive method of repair if it can be applied before masonry displacement becomes excessive and the walls become unstable. The materials used and the methods employed are easily utilized by most specialty repair contractors. Structurally, the addition of intermediate support conditions is one of the most efficient methods for providing supplemental lateral restraint in a masonry wall, and the existing framing system often provides more than enough support to restrain further movement. By using corrosion resistant components and by performing required maintenance on the floor framing systems and the masonry walls (such as pointing or mortar joints), the repairs can easily extend the service life of a building without the need for major reconstruction or modification of the load bearing systems of the building. Utilizing such an approach is both cost effective and provides a sustainable design methodology for the repairs. By minimizing the removal of the existing masonry, the historic character of the building is preserved, while insuring that such valuable structures stay in service for many years to come.
References
Reynolds, P., Muurankers – Wall Anchors, Reproduced at http://www.lowlands-l.net/history/reynolds_muurankers.php
Hillman-Crouch, Barry, Historic Ironwork Repairs in Timber Framed Buildings, http://www.dowsingarchaeology.org.uk/Ironwork/iron-index.htm
“Technical Information Note 8 – Installation of Lateral Restraint to External Walls at Pitched Roof Level”, Cheshire West and Chester Building Control Consultancy, March 2009