The impact of corrosion on the behaviour and strength of wall ties in cavity brick walls

Steel wall ties are essential structural components of brick buildings. In brick veneer and cavity brick walls, the wall ties connect the external leaf of masonry to the internal load-bearing frame or internal masonry leaf, transferring lateral forces from wind and earthquakes. Corrosion of steel wall ties diminishes their effectiveness by reducing their cross-sectional area, compromising their strength when subject to lateral forces. The experimental work conducted for this study involved the compression testing of brick couplet, wall tie and timber subassembly systems replicating those used in a masonry veneer wall system with a timber backup frame. Each subassembly specimen included an artificially corroded wall tie with known section loss. Our findings reveal a decline in strength and a change in the failure mechanism of the wall tie as corrosion-induced section loss increases. This highlights the clear impact that the corrosion of steel wall ties has on the performance of brick veneer wall systems.

Sound Bridging By Wall Ties in Cavity Brick Walls: Theory and Experiment

Wall ties or masonry veneer anchors are a common part of brick veneer wall assembly construction. They serve to anchor the brick veneer to the backup wall, whether that is a wood-stud, steel-stud, or concrete block wall. They provide structural support and keep the brick veneer from moving. However, since wall ties are made from steel, they have a significantly higher thermal conductivity than the surrounding materials in the building envelope, and this difference may cause thermal bridging. The overall impacts of several common types of wall ties in residential and commercial construction were tested using a small-scale hot box apparatus under steady-state conditions. Each test panel was first tested without any wall ties and then subsequently tested with wall ties present. This procedure allowed for direct measurement of the impact of the wall tie while holding all other factors constant. In the case of a typical residential wall without continuous insulation, both types of wall ties tested were found to have no measurable impact on the overall thermal performance. In the commercial walls that contained continuous insulation, the walls ties were found to have a minor impact on the overall thermal performance. Wall systems with significant thermal mass, such as brick veneer, have better performance under dynamic thermal loading, which is not reflected in steady-state measurements. This paper focused on steady-state worst-case results, and future work will address dynamic performance.

The most common steel fixtures used in hollow-core masonry walls are wall ties that attach the brick veneer wall (outer wythe) to a concrete masonry wall or steel or wooden joists behind the wall. These fittings prevent wind pressure from defecting the outer wythe, typically clay brick. Corrosion of the wall tie in the mortar can crack the mortar and weaken the wall. Corrosion of the wall tie or the bracket in the cavity between the wythes can detach the inner and outer components and make the outer wall vulnerable to wind damage. Tests of a new coating system for wall ties is underway at Fort Stewart, Georgia, and inspections of specially constructed wall sections and exposure tests conducted on the ties indicate that the vitreous enamel coating performs satisfactorily.

Dynamic stiffness as an acoustic specification parameter for wall ties used in masonry cavity walls

Existing steel ties for masonry walls typically have a galvanic protection that may not provide adequate resistance to the corrosive forces experienced by wall ties. In contrast, fiber-reinforced plastic (FRP) materials exhibit high corrosion resistance and can thus provide an enhanced durability and a longer service life. Three types of wall ties constructed of FRP were developed: a unit tie, called a Strip Tie; and two adjustable ties; an Adjustable Angle Tie and an Adjustable Slide Tie. Two criteria were used to develop the FRP ties: compliance with current masonry design and construction code requirements and adaptation of readily available FRP products with little modification. Several tests were performed on the FRP ties: tension and compression tests on the ties themselves and bond tests of the ties embedded in the mortar joints of brick and concrete block prisms. Two different cavity widths (2 and 4 in.) were simulated in the tests. The tests performed reflect possible failure modes experienced by wall ties. Results indicate that the Strip Tie and Adjustable Slide Tie may be feasible alternatives as wall ties, while the Adjustable Angle Tie did not meet the target load requirements set forth in the program. A recommendation has been made to abandon the Adjustable Angle Tie design and to further pursue the Strip and Adjustable Slide Ties.

The most common steel fixtures used in hollow-core masonry walls are wall ties that attach the brick veneer wall (outer wythe) to a concrete masonry wall or steel or wooden joists behind the wall. These fittings prevent wind pressure from deflecting the outer wythe, typically clay brick. The ties consist of a wire bail or pintle that is imbedded in the mortar joints of the outer wall and a sheet metal bracket that is attached to the inner wall. The strength of the anchor to the outer wall depends on the strength of the bond from the pintle to the mortar. Corrosion of the pintle in the mortar can crack the mortar and weaken the wall. Corrosion of the pintle or the bracket in the cavity between the withes can detach the inner and outer components and make the outer wall vulnerable to wind damage. The bond to the mortar and the corrosion-resistance of the metal parts are critical to the structural integrity of the wall. By using a pintle and bracket that are coated with vitreous enamel and are manufactured with an outer reactive layer to improve the bond to the mortar, the parts where corrosion can weaken the wall are protected, and the security and durability of the hollow-core wall can be greatly improved. Tests of the new coating system for wall ties is underway at Ft. Stewart, Georgia, and inspections of specially constructed wall sections and exposure tests conducted on the ties indicate that the vitreous enamel coating performs satisfactorily.

In a masonry veneer wall system, tie strengths and stiffnesses vary randomly and so are not consistent for all ties throughout the wall. To ensure an economical and safe design, this paper uses tie calibration experimental approach in accordance with the standard AS2699.1 to investigate the tie failure load under compression and tension loading. Probabilistic wall tie characterisations are accomplished by estimating the mean, coefficient of variation and characteristic axial compressive and tensile strength from 50 specimens. The displacement across the cavity is recorded, which resulted the complete load versus displacement response. Using the maximum likelihood method, a range of probability distributions are fitted to tie strengths at different displacement histogram data sets, and a best-fitted probability distribution is selected for each case. The inverse cumulative distribution function plots are also used along with the Anderson-Darling test to infer a goodness-of-fit for the probabilistic models. An extensive statistical correlation analysis is also conducted to check the correlation between different tie strengths and associated displacement for both compression and tension loading. Based on the findings, a wall tie constitutive law is proposed to define probabilistic tie behaviour in numerical modelling.

Seismic in-plane behavior of composite masonry walls strengthened with cold rolled steel sheets

Not counting domestic dwellings, it has been estimated that in Australia alone, some tens of thousands of masonry buildings and structures have exceeded their design life, with many of these being at risk of partial (or worse) collapse from falling or dislodged masonry. This has significant implications for human life but also for the urban environment and economic health of building owners, managers, and insurers and for local and national economies. This risk can mainly be attributed to the slow deterioration of masonry under atmospheric and other environments and the corrosion of so-called wall ties. Wall ties are relatively thin pieces of steel that tie the outer leaf of masonry walls to the stabilized inner leaf. The problem is likely severe for scenarios such as cyclonic and earthquake events, as they cause area-wide damage and the potential wide-spread loss of human life—losses that could be prevented by timely intervention. This paper reports on the in situ inspection of two case study buildings and the data obtained from controlled wall tie corrosion field trials, which are used to develop predictive models of structural response. These models will inform practical tools that will be developed for building assessment, cost-effective monitoring, and rectification, assisting in the management of existing masonry buildings.

Behavior of unreinforced multi-leaf stone masonry walls under axial compression: Experimental and numerical investigation

Out-of-plane behavior of U-shaped unreinforced masonry structures

Background: A 3-D finite element model of the internal masonry wall of a 103-year-Old Senate hall, Allahabad University, has been modeled using macro-modeling approaches. The masonry wall is an excellent example of Indo-Saracenic style architecture used by Britishers during the late 19th Century, which is a unification of the Mughal and Colonial architecture. Methods: Non-destructive Test (NDT) has been conducted to estimate is compressive strength and Young’s modulus of the wall. Compressive strength of the brick masonry and stone arch was estimated in the range of 10.5-12.5 MPa and 18.6-21.2 MPa, respectively, whereas Young’s Modulus was estimated in the range of 1800-5000 MPa and 5500-8000 MPa (outlier not considered). Finite Element model was prepared using the macro-modeling approach. Results: The gravity load analysis shows that the wall is stable, and its geometrical configuration is safe with maximum Von-Mises stress of 5.38 MPa and deformation of 2.27 mm. The results of the first six modes are presented. Further, in the absence of a recorded ground motion for the Prayag city, synthetic ground motion is simulated for 25th April 2015 Nepal earthquake (Mw) using a stochastic finite fault model. Conclusion: Evaluated behaviour of the internal masonry wall is shown in the form of acceleration, deformation and stress response.

SEISMIC BEHAVIOUR OF BUILDINGS WITH LARGE LIGHTLY REINFORCED WALLS ALONG THE PERIMETER

This paper presents a study about the behavior of brick masonry cavity walls. Three types of cavity walls were tested, each having two wythes of 4½ inches thickness and 2 inches cavity between the two wythes. The first type did not have any ties, the second types had I-shaped ties while the last one had Z-shaped ties. A total of six specimens were fabricated and tested, two specimens for each of the three wall types. The specimens were then subjected to constant vertical and horizontal cyclic loads in a displacement controlled environment. Lateral strength, stiffness and ductility of the walls were determined and compared. Test results show that use of ties increases stiffness of cavity walls, the increase being higher in the case of I-shaped ties compared to Z-shaped ties. The ductility also increased with I-type wall ties. However, wall ties have no effect on the lateral strength.