To investigate the temperature field and residual bearing capacity of the sandwich wall panels with GFRP skins and a wood-web core under a fire, three sandwich walls were tested. One of them was used for static load test and the other two for the one-side fire tests. Besides, temperature probe points were set on the sandwich walls to obtain the temperature distribution. Meanwhile, the model of the sandwich wall was established in the finite element software by the method of core material stiffness equivalent. The temperature distribution and performance reduction of materials were also considered. The residual bearing capacity of specimens after fire exposure were simulated considering the effects of web spacing, wall panel thickness and fire exposure time. Because the sandwich wall panels were stressed by eccentric compression after a fire, the residual compressive strength of the wall panel after the fire can be calculated through the eccentric loading analysis. Compared with the numerical results, it can be concluded that the effectiveness of calculation method of residual bearing capacity after fire exposure was proved.

Fibre reinforced polymer (FRP)-wood sandwich structural components have been widely adopted in the civil engineering due to their advantages of low energy consumption, corrosion resistance, fast construction, convenient maintenance and thermal insulation. FRP-wood sandwich structural components are usually formed by bonding the upper and lower high-strength skins and the middle light-weight core materials. The skins can bear tensile and compressive loads applied by bending moment, and the core materials can bear shear force [

In the past ten years, a number of experimental and theoretical studies on the mechanical properties of sandwich components at room temperature have been conducted. However, with the increase in the building fires in recent years, the fire resistance of sandwich components has caused the social extensive concern [

It is well known that wood is a kind of easily flammable material. The temperature rises, the physical and chemical changes occurred with the increase in temperature. Shen et al. [

However, most of the studies focused on the material properties subjected to a fire, while it is hardly to find some references to introduce the investigations on the fire resistance of sandwich components under fire exposure, which has become a serious lack of scientific basis for the use of such components in building structures. Therefore, the performance degradation mechanism subjected to a fire and residual bearing strength should be evaluated. This is the motivation of this study. In this paper, the GFRP web reinforced-Douglas fir sandwich wall panels were taken as an example for the one-side fire tests. Then, the variation distribution of the internal temperature field and residual bearing capacity were studied and analyzed, which provides a reference for the subsequent fire resistance design of sandwich components.

A total of three web sandwich specimens were produced in the tests. One of them was a control specimen, named Control, which was not subjected to a fire, while the other two specimens subject to a 60 min one-side fire, named HW110 and HW90, and then the residual strengths were tested after the fire exposure. For all the specimens, the length was 1800 mm, the width was 350 mm, the thickness of a skin was 5 mm, the web thickness was 5 mm and the web spacing was 70 mm. But the core thickness was different, the core thickness of Control and HW110 were 110 mm, the core thickness of HW90 was 90 mm. The Douglas fir was adopted as the core material. The component schematic is shown in

Step i: Douglas fir wood was cut according to the designed size along the grain direction. In the meantime, to completely and uniformly combine the resin with the core material, the Douglas fir wood should be grooved and punched before the resin was imported.

Step ii: Douglas fir was wrapped with GFRP mats, and nails were driven into the core material by an air gun so that GFRP can be fixed on it.

Step iii: The specimens were wrapped by the vacuum bag. Then, the release cloth, flow guide cloth, and flow guide tube were installed.

Step iv: The vacuum bag was evacuated, and the unsaturated polyester resin was introduced to fully combine the fiber and core material.

Step v: After reaching the curing time (24 h), the specimen was cut according to the required dimension.

For specimens HW110 and HW90, the fire surface and the two sides were coated with ultra-thin fireproof coating. The brand of fireproof coating was Sika Pyroplast Wood T with topcoat. The coating thickness was 3 mm. Meanwhile, to ensure the one-sided fire test, the aluminum silicate panels were installed on the side and backfire surface of specimens, as shown in

The K-type thermocouples were arranged along the thickness of the wall panel to investigate the temperature distribution. The thermocouple depth was successively increased by 20 mm. The positions are summarized in

GFRP web | Hole depth(mm) | Wood core | Hole depth(mm) |
---|---|---|---|

TW1 | 5 | T1 | 0 |

TW2 | 10 | T2 | 5 |

TW3 | 30 | T3 | 10 |

TW4 | 50 | T4 | 30 |

TW5 | 70 | T5 | 50 |

TW6 | 90 | T6 | 70 |

TW7 | 105 | T7 | 90 |

- | - | T8 | 105 |

- | - | T9 | 107 |

GFRP web | Hole depth(mm) | Wood core | Hole depth(mm) |
---|---|---|---|

TW1 | 5 | T1 | 0 |

TW2 | 10 | T2 | 5 |

TW3 | 30 | T3 | 10 |

TW4 | 50 | T4 | 30 |

TW5 | 70 | T5 | 50 |

TW6 | 85 | T6 | 70 |

- | - | T7 | 85 |

- | - | T8 | 88 |

In this study, the tensile and compressive performance of GFRP skins (

Specimen | Tensile strength | Compressive strength | Elastic modulus | Poisson’s ratio | ||||
---|---|---|---|---|---|---|---|---|

Average value (MPa) | Discrete coefficient | Average value (MPa) | Discrete coefficient | Average value (GPa) | Discrete coefficient | Average value | Discrete coefficient | |

Standard specimen | 220.3 | 0.0344 | 233.6 | 0.159 | 13.5 | 0.474 | 0.084 | 0.07 |

Specimen | Shear strength(MPa) | |||||||
---|---|---|---|---|---|---|---|---|

Longitudinal section | Cross-section | |||||||

Average value | Standard deviation | Average value | Standard deviation | |||||

Standard specimen | 2.628 | 0.689 | 4.943 | 2.167 | ||||

Specimen | Compressive strength(MPa) | |||||||

Grain direction | Cross-section | Longitudinal section | ||||||

Average value | Standard deviation | Average value | Standard deviation | Average value | Standard deviation | |||

Standard specimen | 43.005 | 5.65 | 2.34 | 0.927 | 3.034 | 0.474 | ||

The test set-up was MTS universal loading test machine, as shown in

The fire tests were conducted at the Structural Fire Laboratory of Nanjing Tech University, China, as shown in _{0} is the temperature in the test furnace at the beginning of the test, in °C;

The ultimate load capacity of Specimen Control was 2446 kN. As shown in

The fire test was carried out for 60 min.

A FEM model was adopted based on the commercial finite element program, ABAQUS, to numerically study the behavior of the sandwich panels. The model consisted of two types of elements: four-node shell (S4R) for the skin, and eight-node reduced integration element (C3D8R) for the core. The boundary condition of panels is hinged support at one end and roller support at another. The two types of elements were assumed to have perfect bonding through the resin.

To simplify the numerical modeling, the equivalent modulus of the web reinforced core material was adopted [

The core material was an elastoplastic material during the mechanical analysis process, regardless of the creep and relaxation behavior of the material.

The core material was considered as orthotropic material considering the modulus equivalent processing.

The Hashin failure criterion was adopted for the skins, and the equivalent core material was applied to the maximum stress criterion. The formula for Hashin failure criterion is as follows [

Fiber tensile failure:

Fiber compression failure:

Fiber shear failure:_{i} is the main direction stress; _{ij} is the main direction shear stress; _{ij} is the shear strength; _{ij} is the shear modulus.

The residual post-fire bearing capacities of the sandwich wall panels were simulated. The tested temperature distributions of the specimens were input the numerical model. According to EN1995-1-2 [

In this simulation, a total of 6 specimen models were established for simulation according to the three parameters, namely web spacing, wall panel thickness and fire exposure time. The parameters of the specimens are listed in

Specimen | Specimen size | _{s} |
_{w} |
|||
---|---|---|---|---|---|---|

HW110 | 1800 | 350 | 110 | 5 | 5 | 70 |

HWS110-50 | 1800 | 350 | 110 | 5 | 5 | 50 |

HWS110-105 | 1800 | 350 | 110 | 5 | 5 | 105 |

HWT110-30 | 1800 | 350 | 110 | 5 | 5 | 70 |

HW90 | 1800 | 350 | 90 | 5 | 5 | 70 |

HWT90-30 | 1800 | 350 | 90 | 5 | 5 | 70 |

Note: _{w} is the web thickness, _{s} is the skin thickness, and

To validate the accuracy of the proposed model, the ultimate strength of HW110 at room temperature was numerically calculated using the mentioned model.

Material properties and corresponding temperature distributions of HW110, HWS110-50, and HWS110-105 were substituted into the simulation for reduction calculation, as shown in

HW110 and HW90 after 60 min fire tests were eccentrically compressed and destroyed. It can be found that HW90 was more prone to bending due to its wall thickness which was smaller than HW110, and the carbonization failure area on the fire side of it was larger than that of HW110. From the load-displacement curves in

Meanwhile, the ultimate bearing capacity of HW110 at room temperature is 2446 kN. After 30 min fire exposure, the residual bearing capacity was 1210 kN, and it was 628.928 kN under 60 min fire exposure. Compared to the compressive bearing capacity of HW110 at room temperature, the compressive bearing capacities decreased by 50.5% and 74.3% after 30 and 60 min fire exposure, respectively. The conclusion can be drawn that components are obviously affected by the fire exposure time.

For sandwich wall panels after fire exposure, due to the different degree of material decomposition and carbonization on the fire surface and backfire surface, the loads applied to the fire-damaged components become eccentric compression. Therefore, the eccentric load analysis and calculation of were undergone. The residual compressive bearing capacity of the sandwich wall panel was theoretically calculated by analyzing the cross-section of the sandwich wall panel after the fire and the degrading of the corresponding material strength reduction. The theoretical and finite element calculation results are shown in

Specimen number | FEM results (kN) | Theoretical results (kN) | Difference (%) |
---|---|---|---|

HW110 | 814.11 | 736.41 | 9.5 |

HWS110-50 | 766.46 | 618.022 | 19.34 |

HWS110-105 | 861.3 | 789.83 | 8.23 |

HWT110-30 | 1161.7 | 1210 | 3.99 |

HW90 | 577.02 | 628.928 | 8.25 |

HWT90-30 | 1060 | 1100.4 | 3.67 |

In this paper, sandwich wall panels with GFRP skins and a wood-web core are taken as the research objection. The conclusions are summarized as follows:

The results show that GFRP skins and webs can provide the effective confinement to the wood core. Hence, the web reinforced sandwich wall panels can achieve much higher bearing strength.

The internal temperature-time variation rule of the sandwich structure was tested by using the internationally accepted ISO-834 standard heating curve and measured with the thermocouples. The result shows that although the thermal conductivity of GFRP webs and Douglas fir core materials are different, the temperature-time distribution of them is roughly the same.

The model of the sandwich wall panel was established by the finite element software. The residual bearing capacity after the fire of 6 components were analyzed taking web spacing, panel thickness and fire exposure time as parameters and combining with the temperature distribution law obtained in the fire tests. Specific conclusions are as follows:

Improving the web spacing within a certain range can effectively increase the residual bearing capacity of members. When the web spacing was changed from 50 to 70 mm, the residual bearing capacity of a member increased by about 20%. However, the residual bearing capacity of them is only increased by about 6.7% when the web spacing changes from 70 to 105 mm.

When the thickness of the component is reduced by 18%, the strength of it decreases by 14.6%, and the reduction in the thickness of the wall panel has a large influence on the residual strength of the specimen.

When the burning time is different, the residual strength of the component burning for 30 min is 74.96% higher than that of burning for 60 min. By comparing the dead load specimens with those exposed to fire for 30 and 60 min, the residual bearing capacity decreased by 50.5% and 74.3%, respectively. It can be seen that the influence of fire exposure time is considerably important.