STUDY ON STRESS PERFORMANCE AND FREE BRICKWORK HEIGHT LIMIT OF TRADITIONAL CHINESE CAVITY WALL

Preliminary communication Traditional Chinese cavity wall often suffers in-plane and out-of-plane damages in natural disasters like gales and earthquakes. However, the seismic and wind resistance of the cavity wall, an enclosure structure, are seldom studied in the engineering field. Instead, the disaster prevention and relief efforts are concentrated on the structural analysis and seismic damage of the main structure. Focusing on the bricklaying methods for 2 common types of cavity walls and 1 kind of solid wall, this paper designs a special loading device and uses it to examine the in-plane and out-of-plane stress performance of cavity wall and solid wall under the horizontal load. The results show that all out-of-plane damages have resulted from the flexural-bending failure of the bend; the cavity wall has far lower out-of-plane bearing capacity than the solid wall. Moreover, the free brickwork height limits of the cavity wall under the action of earthquake and wind load are deducted respectively, in reference to the schematic diagram of the internal force of the cantilever beam and on the basis of the measured flexural-tensile strength and shear strength. It is found that the out-of-plane performance controls the brickwork limits. The authors suggest that connecting structures should be installed on each floor if the cavity wall is to be connected with the main structure.


Introduction
Traditional Chinese buildings are masterpieces of architecture art, structure and construction.As a highlight in the history of world architecture, these buildings have promoted the advancement of global architectural technology and manifested the inestimable value of ancient Chinese culture [1][2][3].Hence, the maintenance and protection of ancient buildings and the representation of ancient architectural forms are in the limelight of the research of architectural technology [4,5].In both ancient and antique buildings, the brick wall is a critical loadbearing or enclosure component.The status quo of heritage buildings can be preserved only if the wall is safe and sound [6][7][8].Unfortunately, damaged walls are commonplace in ancient buildings owing to typhoons and earthquakes.The situation is particularly serious for buildings with cavity walls.With poor integrity and low bearing capacity, the cavity wall suffers heavy damages.In the case of an earthquake, the heritage buildings with cavity walls are bound to suffer major losses [9,10].
The seismic performance of the cavity wall was firstly studied the 1970s in China.The research pointed out that the shear strength of the wall was half of that of solid wall, but failed to provide the brickwork height limit [11][12][13].At present, the cavity wall design and construction of heritage buildings still follow the ancient technologies and standards.This calls for scientific, systematic theories on research, design and construction technology [14,15].One of the most practical ways to prevent the loss of cultural relics caused by cavity wall damages lies in evaluating the cavity wall performance of ancient and antique buildings, determining the height limits of the wall, and proposing rational construction measures according to the evaluation results [16,17].For the protection, design and construction of ancient and antique buildings with cavity walls, it is of great practical significance and engineering guidance to study the out-ofplane and in-plane stress performance of the cavity wall.
Focusing on the bricklaying methods for 2 common types of cavity walls and 1 kind of solid wall, this paper experimentally studies the internal force and deformation performance of these walls under in-plane and out-ofplane horizontal loads, respectively, analyzes the effect of bricklaying method, height-thickness ratio and heightwidth ratio on the in-plane and out-of-plane bearing capacity of the cavity wall, and arrives at the allowable brickwork height limits of the cavity wall under different seismic intensities and wind loads.The results lay an important theoretical basis for the protection, assessment, design and construction of ancient and antique buildings with cavity walls, and provide reference for the future research on the out-of-plane performance of the wall.According to written records [18], the ancient masonry mortar has the same compressive strength with the M5 mortar.Thus, the mix ratio of the mortar is set as 1:5.6:1.15.The sand is pre-filtered through a sieve (mesh size: 2.36mm).The mortar thickness in the wall is 10mm.The mortar specimens are produced under natural curing conditions.The wall mechanics test is carried out when the compressive strength of the specimens reaches about 5MPa.The strengths of the specimens are shown in Tab. 2.

Parameters of out-of-plane specimens
Two common bricklaying methods for the cavity wall are selected (Fig. 1): the cavity wall with header, and the cavity wall without header.The "header" stands for a brick laid flat.By contrast, a brick laid vertically is called a "shiner".The cavity wall with header is constructed by laying a layer of headers between every 1÷3 layers of shiners (Fig. 1(a)), while the cavity wall without header is built by laying shiners only (Fig. 1(b)).The third bricklaying method is used to construct the solid wall.Specifically, each layer of bricks is laid by inserting a rowlock between two stretchers.9 full-scale (1:1) specimens of different heightthickness ratios and height-width ratios are chosen to discuss the construction of the walls.The specific sizes and parameters are presented in Tab. 3.

Parameters of in-plane specimens
Four in-plane specimens are designed whose parameters are shown in Tab. 4.

Experimental loading apparatus 2.2.1 Out-of-plane loading apparatus
The horizontally distributed load resulting from the wind load and earthquake appears vertically along the wall [19][20][21].Simulating such a load in the test is no easy feat.If surface loading is applied, it is difficult to maintain the uniform contact between the loading surface and the horizontally deformed wall, which will distort the subsequent loads.The conventional jack loading, however, lacks sufficient control accuracy under small load value of each level because the out-of-plane bearing capacity is relatively low and the rated load of the jack is far greater than the load required [22].In order to maintain high control accuracy, the project team designs a special loading device, divides the distributed surface loads into horizontal linear loads at an interval of 0.5 m along the vertical direction, and applies the horizontal loads manually with steel blocks.The loading device and the loading method are shown in Fig. 2    ,67 0,67 Note: 1, 2 stand for the building method I for the cavity wall and building method II for the cavity wall respectively, 3 for the building method for the solid wall; there are three specimens for each group, A, B, C show the impacts of height to breadth ratio and height to thickness ratio.The length of the two solid ends is 200 mm.

In-plane loading apparatus
In view of the large load of the in-plane test, the existing out-of-plane loading frame is modified for inplane loading (Fig. 4).The loading device is a jack.The in-plane wall mainly withstands the horizontal seismic action [23][24][25].As an inertial force, the seismic action is related to the mass distribution and is thus uniformly applied across the wall.The cavity wall is empty in the middle and solid on both ends.Unlike the solid wall, the cavity wall features uniform load distribution under in-plane loading: most of the loads are borne by the solid wall section at the loading end.As the load is transferred to the other end, the cavity wall section may undergo local instability, resulting in test failure.Thus, a special loading measure is badly needed.The project team comes up with the following measures: level up the top of the wall with epoxy resin, paste the capping beam, and apply the jack's horizontal loads on the beam.Since the steel beam is much stiffer than the cavity wall, the horizontal force can be applied evenly on the top of the wall via the epoxy resin bonding surface.

Test contents 2.3.1 Out-of-plane experiment
In this test, the quasi-static method is adopted to simulate the seismic action applied onto the wall during an earthquake.Before the wall is cracked, the load is controlled and applied level by level.Each level of load is applied in one loading cycle.The magnitude of load level is reduced when the wall is about to crack.The moment of cracking signifies that the wall has reached the ultimate load.The loading blocks apply load onto the wall via the beams of the loading frame.The number of loading blocks is recorded in the test.

Loading system
During the test, the load is applied level by level.The theoretical cracking load of the wall is calculated in advance.In the out-of-plane loading test, the load is increased by 49 N (5 kg) in each level; after reaching 80% of the designed limit load, the load is increased by 9.8 N in each level.The number of weights grows with the load (Tab.5).In the in-plane loading test, the load is increased by 4 kN in each level; after reaching 80% of the designed limit load, the load is increased by 1kN in each level (Tab.6). 3 Results and discussions 3.1 The building height limit under the impact of earthquakes The inertial force of the earthquake is correlated with mass.The seismic action is expressed as a horizontal uniform force (Eq.( 1)) because the mass of the wall is evenly distributed along the vertical direction.The simple diagram for calculation shown in Fig. 5. Eq. ( 2) is the calculation formula of the bending moment of the wall under seismic action.
. 2 As the maximum permitted bending moment [M] can be obtained by the experiment data, the wall should meet the condition: where m is the mass of the wall (kg); g is the gravitational acceleration (m/s 2 ); H is the height of the wall (m); α is the horizontal seismic impact coefficient (see the Code for Seismic Design of Buildings [24]); γ = 1.3 is the partial coefficient of seismic action; M is the bending moment under seismic action (kN•m); [M] is the allowable bending moment obtained based on the test data.
The test results of each group are substituted into Eq.( 2) to get the brickwork height limits of the wall under the action of earthquakes of different intensities.The calculated results are shown in Tab. 7.
As can be seen from Tab. 7, when the height of the wall is the same, the wall is less likely to hold together as the width expands and the proportion of cavity section grows.For cavity wall without header, the average strength and allowable height are reduced significantly by 25%; for cavity wall with header, the two parameters are increased to a certain extend.Considering the discretization of mortar, it is safe to conclude that the width has a minimal effect on the mechanical properties of the wall when headers are used.
Comparing the different bricklaying methods, the two types of cavity walls have basically the same allowable height.Since the seismic fortification intensity of most regions in China is below 7, the height limit is set as 4m without any lateral constraint, that is, the single-layer cavity wall will not suffer out-of-plane collapse when an earthquake of the fortification intensity takes place.

Building height limits under different wind loads
The cavity wall may also get damaged under the action of wind.Thus, it is necessary to deduct the brickwork height limits under different wind pressures.
For general buildings, the standard wind load vertically applied onto the building surface is calculated as follows: where W k is the standard wind load (kN/m 2 ); w 0 is the basic wind pressure (kN/m 2 ); β z is the wind vibration coefficient at height z; μ s is the wind load shape factor; μ z is height variation coefficient of wind pressure.
According to the Load Code for the Design of Building Structures, μ s =0.8, μ z =0.74, β z =1 and γ=1.4.The designed wind loads under different wind pressures are listed in Tab. 8. Tab. 9 shows the wind load q at the top of different walls.
For the wind load acting on the wall, the root bending moment is calculated by the inverted triangle method.The root bending moment calculation model is illustrated in Fig. 6, and the wall root stress is expressed in Eq. ( 5).
. 3 The wall should meet the following conditions: There is: where [M] is the allowable bending moment obtained based on the test data; H is the height limit.Substitute the test results to calculate the brickwork height limit of each wall under the action of wind load (Tab.10).
It can be seen from Tab. 10 that the height limit is merely 2.5 m for a cavity wall without connecting structures on the top under the direct impact from the 50year return period wind.The limit is lower than the height of general buildings.Therefore, connecting structures or anti-wind columns should be installed on each floor if the cavity wall-enclosed building is located in a region with large wind pressure.This measure is of great necessity for the prevention of out-of-plane damages.

Height limit controlled by in-plane shear strength under the impact of earthquakes 3.3.1 Shear strength
According to the stepped damages in the in-plane test, the shear strength formula and the shear strength of the test are as follows: .
Similar to the calculation method for seismic action, the bottom shear method is adopted for calculating the shear strength: where α is the horizontal earthquake influence coefficient, ρ is density of the wall, A is cross-sectional area, H is height, and g is acceleration of gravity.The brickwork height limits under different seismic intensities are calculated based on the wall strengths measured by the test (Tab.12).It is learned that the height limits are far greater than the allowable out-of-plane values.Thus, the limit wall height is dependent on out-ofplane stress. (9)

Suggested height limits
Aiming to control the minimum height limit and taking account of the wall mass discretization and possibility of overload, the safety coefficient of seismic action is increased 1.5 times; the partial coefficient of wind load is set as 1.4 and the wind pressure height coefficient is temporarily set as 10m.No additional safety coefficient is needed because the temporary value is relatively large and the wind load is unidirectional, i.e. no instantaneous collapse will occur under the cracking-induced vibration.Tab. 13 lists the recommended height limits.
Tab. 13 shows the brickwork height limits of cavity walls with no buttress column or connecting structure.Once the height exceeds the limit, connecting structures should be installed.From the data in Tab. 13, it is inferred that the height limit in regions with seismic fortification intensities of 8 and 9 is lower than the height of a singlestory house.Hence, the cavity wall is not recommended for regions with seismic fortification intensities of 8 and above, provided that no connecting structure is installed.
While the basic wind pressure is above 0.4 kN/m 2 in most regions in China, the height limit under wind load is lower than the height of a single-story house.If there is no connecting structure, the cavity wall faces the risks of cracking and tilting.To sum up, the enclosure walls of ancient buildings should be protected with independent support structures like buttress columns.The antique columns should be made of wood or reinforced concrete, and should be firmly rooted in the foundation.

Conclusion
This paper conducts the experimental study and theoretical analysis on the out-of-plane and in-plane stress performance of the cavity wall in antique buildings.In total, 13 groups of cavity walls are subjected to the tests on out-of-plane stress and in-plane stress performance.The research mainly aims at disclosing the effect of bricklaying method, height-width ratio and widththickness ratio on the ultimate bearing capacity and deformation properties of the cavity wall.The following conclusions are drawn through the relevant experimental research and theoretical analysis: the height limit of the continuously laid brickwork no shorter than 2 m can be controlled according to Tab. 13; if the height exceeds the limit, connecting structures or anti-wind columns should be installed.For the cavity wall brickwork, the out-ofplane load condition is more dangerous than in-plane load condition, and is the control factor of height limit.The wind load has a more obvious impact on the cavity wall enclosure structure than the seismic action.It is recommended to set up an independent anti-wind support structure for cavity wall enclosure structure.

2 Experimental methods 2 . 1
Design and production of specimens 2.1.1Experimental materials Grey bricks used in this study were bought from Changzhou Long Yun Antique Building Materials Corporation Ltd.Their physical properties are shown in Tab. 1.

Figure 1
Building method I: planar graph and elevation graph (b) Building method II: planar graph and elevation graph Two building methods of specimen

Figure 2 Figure 3
Figure 2 Loading apparatus

Figure 4
Figure 4 In-plane loading apparatus

Figure 5
Figure 5 Calculation diagram of the wall

Figure 6
Figure 6 Calculation diagram under the impact of wind loads

Table 1
Physical property of grey bricks

Table 2
Strength of mortar blocks under the same curing conditions

Table 3
Design of Out-of-plane specimens

Table 4
Design of in-plane specimens Number of specimens K2 are cavity bricks; K3 is cavity bricks laid flat and S1 is the solid wall.
Breadth B (mm) Height H (mm) Thickness h (mm) Height to thickness ratio H/h

Table 5
Loading of out-of-plane specimens

Table 6
Loading of in-plane specimens

Table 11
Shear strength of masonry measured in in-plane tests

Table 7
Height limits under different earthquake intensities

Table 8
Wind loads under different basic wind pressures

Table 9
Wind loads at the top of walls

Table 10
Height limit of buildings under wind loads

Table 12
Height limits under earthquake

Table 13
Suggested height limits