Seismic Design And Reinforcement Solutions For Masonry (Brick-Concrete) Structures

Inherent Seismic Defects of Masonry Structures

Different from ductile reinforced concrete frame and steel structures, masonry structures rely on brickwork and mortar to bear vertical and horizontal loads. Their core seismic weaknesses are obvious. First, masonry materials are brittle with extremely low tensile and shear strength, easily producing diagonal cross cracks under horizontal seismic force. Second, traditional masonry buildings lack effective integral restraint, leading to separation between walls, floors, and roofs during earthquakes. Third, irregular layout, excessive wall openings, and insufficient wall thickness further aggravate seismic risks, causing partial or overall collapse of masonry buildings in moderate and strong earthquakes.

The core seismic design goal for masonry structures is to improve structural integrity, enhance wall ductility, and restrict crack development. Unlike frame structures that adopt energy dissipation or damping devices, masonry seismic resistance focuses on confinement reinforcement, integral connection, and standardized structural detailing to avoid brittle failure.

1. Basic Layout Optimization for Seismic Resistance

Reasonable architectural and structural layout is the primary premise of masonry seismic design, which can fundamentally reduce seismic response and avoid structural weak points. First, maintain planar and vertical regularity of the building. Avoid asymmetric plane layout, excessive torsion, and sudden vertical stiffness changes. The number of building floors and height must be strictly limited; low-rise masonry buildings within 3 to 6 floors have stable seismic performance, while high-rise masonry structures are strictly prohibited in high-intensity seismic zones.

Second, control the size and position of wall openings. Excessively large doors and windows openings will weaken wall shear capacity and reduce the effective bearing area of masonry. Openings should be evenly arranged to avoid continuous weak sections on the same wall. In addition, maintain uniform wall thickness and continuous wall arrangement, reduce staggered walls and disconnected vertical walls, and ensure uniform transmission of seismic loads in all directions.

For site conditions, masonry buildings are suitable for rigid Class I and Class II sites. Soft soil, liquefiable soil, and steep slope unfavorable sections will amplify seismic waves, easily causing foundation uneven settlement and wall damage. For buildings on unfavorable sites, foundation reinforcement and seismic isolation measures must be adopted in advance.

2. Core Seismic Measures: Structural Columns and Ring Beams System

The most effective and classic seismic scheme for masonry structures is the confined masonry system composed of reinforced concrete structural columns and ring beams. This system solves the defect of poor integral performance of pure brickwork and is the key to achieving "no collapse in major earthquakes" for masonry buildings.

Structural columns (constructional columns) are arranged at wall intersections, wall ends, and the corners of door and window openings. They confine the surrounding brickwork, effectively restrict the development of seismic cracks, prevent wall out-of-plane collapse, and improve the ductility and bearing capacity of masonry walls. The reinforcement ratio and concrete strength of structural columns must meet seismic specifications, with dense stirrup arrangement at column tops and bottoms to enhance local confinement performance.

Ring beams are continuously arranged along the floor and roof elevation of masonry buildings, forming an integral closed restraint frame with structural columns. Ring beams connect scattered masonry walls and floor slabs into a unified whole, ensuring synchronous deformation of the structure during earthquakes, avoiding relative displacement between walls and floors, and greatly improving the overall seismic rigidity and integrity of the building.

In low and medium seismic intensity zones, the combination of structural columns and ring beams can meet the basic seismic fortification requirements of ordinary residential buildings. In high-intensity zones, the spacing of structural columns should be reduced appropriately to form a denser confinement system and enhance the seismic redundancy of the structure.

3. Wall and Foundation Seismic Reinforcement Measures

Wall masonry quality and foundation stability directly determine the seismic performance of masonry structures. In terms of masonry materials, medium-strength Type S seismic mortar is preferred, with stable bonding performance and good seismic adaptability. Avoid low-strength lime mortar for seismic buildings. Strictly control the masonry process, adopt full mortar joint masonry, eliminate empty joints and incomplete joints, and ensure effective bonding between bricks to improve wall shear resistance.

For masonry walls with insufficient seismic capacity or existing old masonry buildings, targeted reinforcement measures can be adopted. Common methods include steel mesh cement mortar surface reinforcement, high-ductility concrete wall reinforcement, and additional boundary restraint reinforcement. These measures can significantly improve the shear strength and crack resistance of original walls without changing the original building layout.

Foundation seismic design cannot be ignored. Masonry buildings have large self-weight and poor foundation deformation adaptability. In seismic zones, strip foundations are preferred to ensure uniform foundation stress. For weak foundations, foundation grouting reinforcement or integral foundation beam measures should be adopted to prevent uneven foundation settlement and structural tilt caused by earthquakes.

4. Advanced Seismic Schemes: Base Isolation for Key Masonry Buildings

For important masonry public buildings such as rural schools, clinics, and emergency command buildings in high-intensity seismic zones, conventional confined masonry measures are insufficient to meet high-level seismic safety requirements. Base isolation technology is the optimal advanced seismic solution for masonry structures.

Different from traditional passive reinforcement that improves structural rigidity, base isolation sets a isolation layer with lead rubber bearings (LRB) or high-damping rubber bearings at the building foundation. It extends the natural vibration period of the masonry building, avoids resonance with ground seismic waves, and isolates most seismic energy from transmitting to the superstructure. The upper masonry structure maintains nearly rigid deformation during earthquakes, with almost no cracking or collapse damage.

Compared with frame structures, masonry buildings have lower height, smaller overturning moment, and no tension failure risk of isolation bearings, making them extremely suitable for base isolation transformation and new construction. Isolated masonry buildings can achieve ultra-high seismic performance, realizing normal use after moderate earthquakes and no collapse under strong earthquakes.

5. Forbidden Design and Common Seismic Mistakes

In masonry seismic design, some inappropriate designs will seriously weaken seismic performance and must be strictly avoided. Do not adopt large-span unsupported brick walls, random wall demolition and opening, and discontinuous structural columns and ring beams. Avoid excessive floor overhang and asymmetric load distribution, which will increase structural torsion effect. In addition, do not use dry-laid bricks and low-quality mortar, and prohibit random reduction of wall thickness and reinforcement ratio of seismic components.