MASONRY STRENGHTENING AND ANCHORS

 

PECULIAR TECHNOLOGY FOR MASONRY STRENGHTENING

Over the last recent years a greater and greater attention to the existing building heritage and to the activities aimed to the preservation of both buildings with high architectural value and those belonging to the category of traditional buildings has been developed. Among the restoring interventions of existing buildings there are often strengthening works, in which one provides the embedment of metal anchors into masonry or wood: stitching of cracks, bonding between reinforcement metal anchors and the existing structures, tie-rod or reinforcement ring put to contain the action of pushing elements such as arches and vaults, connection between floor slabs and outside masonry walls, embedding of anchorages inside the walls in order to increase their resistance and the global behaviour of structures also for seismic upgrading. Modern researches, materials and operating technics permitted to develop a special technology that, remembering the functioning of traditional strengthening systems, guarantees the best results from technical point of view, complying with the existent and with the constructive logic of the building.

G-BOS, BOSSONG ANCHORS FOR CONTROLLED INJECTION
The system is composed by a strong element, a high strength stainless steel bar -AISI 304 or AISI 316- (ft nom 750 N/mm2 - fy nom 650 N/mm2 ), with full thread along the entire length. It is provided of a sock with the function of controlling the coaxially carried injection of the grout. These two elements, together with the grout, are fundamental to make integral the reinforcing element to the masonry. Besides allowing good injection operations, avoiding unexpected and often damaging spread of grout in voids and cavity that can be present in the existing walls, the sock guarantees the adherence of the injected material to the substrate throughout its entire length and a homogeneous distribution of stress thanks to both the adherence of injected material and the mechanical interlock that the injected grout develops with masonry. The line of anchors for controlled injection is complete with a range of accessories in AISI 316 stainless steel, couplers, turnbuckles, nuts, to fulfil the different needs of each project; Bossong can provide un-tensioned steel reinforcements, pre-stressed tendons, vertical anchors and anchors with every degree of tilt.

DIATONI AND DIATONOS, ARTIFICIAL TRANSVERSAL CONNECTORS
Ancient masonry building are usually made with stone or brick masonry multi leaf walls. The presence of transversal connectors, made of large stones placed transversally, necessary to create connections between masonry leaves, is an important parameter to assess the masonry quality and weakly connected masonry leaves needs to be strengthened, with the insertion of new transversal connection, to mitigate their seismic vulnerability. Compared with usual retrofitting techniques the use of artificial connectors made of a stainless rod inserted into a grouted fabric sleeve allows an immediately effective reinforcement: by injecting with pressure the grout, it is possible to increase bonding with masonry and restore the stress state in the area around the hole. The use of socked injected anchor as transversal connection also guarantees minimum impact on existing masonry thanks to reduced size of borehole diameter and to full control of injected grout that is completely contained by the fabric sleeve with no spread in void and cavity within the masonry.
The effectiveness of the artificial connectors can be improved by introducing a pre-tensioning procedure. Thanks to the post-tensioning force, the reinforcement can act as an “active system,” able to be engaged even for service loads and for low-intensity seismic activity, providing extra tensile strength to the masonry material.
A further improvement can be obtained by providing a mechanical interlock between the new connector and the masonry substrate: the hole can be countersunk at both ends and the fabric sleeve, thanks to its flexibility expands, molding itself into the shape of the hole, thus providing mechanical as well as chemical bond. The effect of pre-tensioning and countersinking produces a confinement of the masonry material and increase the level of connection between leaves.

 

SEISMIC DESIGN AND QUALIFICATION FOR ANCHORS

A correct design in seismic conditions considers how the building reacts during the earthquake. This means takes in account the displacements and deformation in its structural elements that cause the opening of cracks in the concrete components. For this reason, all anchors designed to transfer seismic loads must be suitable for use in cracked concrete and their design must be based on the assumption that the cracks in the basic material may have a cycle opening and closing all along the earthquake. This is in brief the principle on which the EOTA European standard (Organization for Technical Assessment) are based for the qualification and the design methods of post-installed anchors (mechanical and chemical) in seismic area:

-           ETAG 001 -Annex E: guideline for assessment of metal anchors (mechanical and chemical) under seismic action.
-           EOTA TR045: design of metal anchors for use in concrete under seismic actions up until the Eurocode 1992-4 become effective.

Specifically, two test protocols are identified depending on the seismicity of the area and the class of importance of the building on which work has to be performed (please see the table below). The levels are:

-           C1: only for non-structural uses and for levels of low seismicity.
-           C2: for structural uses and non-structural uses and for all levels of seismicity.

The C1 test protocol includes everything that is recommended already in the United States regulatory document ACI 355-2 and provides 10 tests to be performed with tensile and shear seismic simulation for every diameter of the anchor to certify. On the other hand, the test protocol C2 is stricter and provides a minimum of 30 tests per diameter and tests in which the anchors are qualified for the load cycling and in cracks up to 0,8 mm in width by simulating the inversion of the moment through the active compression of the concrete around the anchor. As you can see country with seismic importance as Italy are almost completely in the category C2. For this reason, Bossong S.p.A. was one of the first companies in Europe to focus on this qualification for its products in order to offer reliable and safety solutions also in earthquake situations.

Bossong range of products selected for fixing in seismic area has, as top products, two chemical anchors:
EPOXY21, pure epoxy resin ETAG 001-5 Option1 Seismic AnnexE C2. Leed tested, Fire resistance test, InPP Underwater test, COV etc.
V-PLUS, vinylester resin styrene free ETAG 001-5 Option1 DTA Sismique ReBar. Leed tested, Fire resistance test, InPP Underwater test, COV etc.
In Bossong range of products you can find also mechanical anchors specific for fixing in seismic area as SZ e B-Zplus ETAG 001-2 Seismic C1 e C2. Swiss Shock approval, Fire resistance test, ICC-ES approval etc.

 

[di Michele Taddei]