Cyclic Behaviour of Heat-Damaged Beam−Column Joints Modified with Nano-Silica, Nano-Titanium, and Nano-Alumina

Round 1

Reviewer 1 Report

Dear Authors. Interesting research, congratulations. However some information was not given clearly enough in my opinion and this must be corrected before publication.

1) You used rebars with diameters of 16 and 10 mm, meanwhile in Table 1 there are results for only one type of reinforcement? Which one? What about the other type? Did you enter equal parameters for main bars and stirrups in the mathematical models?

2) In the studies you conducted, it is difficult to clearly define the failure criterion? What conditions did you use? Why is it that with the incremental model for Control and N2SITI specimen is 50kN, but for N2SIAL is 40kN?

3) If I understand correctly, you assumed uniform heating of the models to 720C? Has this been confirmed by any measurement? I have a doubt that the two-hour heating allows to reach such temperature in the core of the sample? Most likely, the thermal degradation of concrete was overestimated in the numerical models.

4) Not all variables in formulas (2) and (3) are explained in the text?

5) The FEM meshing of the models (Fig. 9) has no regularity of reinforcement. How was the contact between concrete and reinforcement modeled?

6) It is not clear to me how the Ductility Index was calculated, especially how over the yield deformation was determined?

7) I do not think that Figure 9 shows any kind of cracki pattern (page 18). Maybe zone, where cracking can be expected.

Author Response

Reviewer 1

 

Dear Authors. Interesting research, congratulations. However some information was not given clearly enough in my opinion and this must be corrected before publication.

• You used rebars with diameters of 16 and 10 mm, meanwhile in Table 1 there are results for only one type of reinforcement? Which one? What about the other type? Did you enter equal parameters for main bars and stirrups in the mathematical models?

Table 1 is modified, including the other diameters. The mathematical model was implemented based on the values given in Table 1.

• In the studies you conducted, it is difficult to clearly define the failure criterion? What conditions did you use? Why is it that with the incremental model for Control and N2SITI specimen is 50kN, but for N2SIAL is 40kN?

Lateral loads were applied to the joint specimens until failure, where the hydraulic actuator automatically stopped increasing the load when the specimen reached failure. For the ABAQUS model, loads were also applied until the specimen reached failure. It should be noted that the stress-strain curve of concrete material prepared with nano-silica and nano-titanium is different than that for concrete prepared with nano-silica and nano-alumina. The stress-strain curves were obtained from the test results of the cylinders made from concrete prepared with nano-silica and nano-titanium and that for concrete prepared with nano-silica and nano-alumina. The experimental stress-strain curves were then implemented into ABAQUS to simulate the material model. The cylindrical compressive strengths of plain concrete and concrete prepared with nano-silica and nano-titanium are greater than that measured for concrete prepared with nano-silica and nano-alumina, resulting in increasing the load carrying capacity of the control and N2SITI joint specimen (50kN), while the load carrying capacity of joint specimen N2SIAL is 40kN.

• If I understand correctly, you assumed uniform heating of the models to 720C? Has this been confirmed by any measurement? I have a doubt that the two-hour heating allows to reach such temperature in the core of the sample? Most likely, the thermal degradation of concrete was overestimated in the numerical models.

The heat curve was designed based on the ASTM-E119 [16], as shown in Figure 3. Regarding the ABAQUS model, the material models of plain concrete, concrete prepared with nano-silica and nano-titanium, and concrete prepared with nano-silica and nano-alumina were simulated into ABAQUS based on the test results obtained from the experimental stress-strain curves of concrete cylindrical and tensile reinforcement tests. The concrete cylinders, cylinders modified with nanomaterials, and the reinforcement bars were placed into the furnace and were subjected to a temperature of 720C according to the heat curve shown in Figure 3. The heat-damaged cylinders and reinforcement were then tested, and the obtained stress-strain curves were then implemented into ABAQUS to simulate the material model.

• Not all variables in formulas (2) and (3) are explained in the text?

The following notations are added.

= full depth of column.

= the horizontal shear force in the joint

= tension force of the top beam reinforcement

shear in column calculated based on Mn  for beams.

= the applied load at beam.

= the beam length to the applied load point.

= the effective depth of the beam

= full depth of column.

= distance between column supports.

• The FEM meshing of the models (Fig. 9) has no regularity of reinforcement. How was the contact between concrete and reinforcement modeled?

Reinforcement and concrete were embedded in the same degree of freedom to ensure a perfect bond between concrete and reinforcement.

• It is not clear to me how the Ductility Index was calculated, especially how over the yield deformation was determined?

The displacement ductility index by Park and Ang is measured by dividing the maximum deformation of members or structures over the yield deformation. The yield deformation is approximately taken from the envelope load-displacement curves of the joint specimens shown in Figures 17 and 18 at the approximate yielding point.

7) I do not think that Figure 9 shows any kind of cracki pattern (page 18). Maybe zone, where cracking can be expected.

Figure 9 caption and the text are modified to crack zone instead of crack pattern as suggested.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript is interesting and analyses the effect of different concrete mixes on the cyclic behaviour of beam/column nodes.

However, there are some flaws that need to be corrected. For example, details on the materials and load displacement graphs of the materials used are missing. This aspect makes the experiments non-reproducible.

I attach an annotated pdf with my comments.

Comments for author File: Comments.pdf

Author Response

All comments attached in the file of reviewer-2 are implemented and highlighted in the revision.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Dear Authors. Thank you for all the corrections made.

We did not fully understand each other about the method of simulating material changes under temperature. The degree of degradation of concrete depends on the temperature altitude. It is different at 720C and different at 400C. Under the conditions of the ASTM-E119 temperature curve, a small sample (cylinder, cube) and a large model will heat up differently. An 8-hour heating cycle may not be sufficient to heat large model uniformly. It is likely that its interior will be cooler (and thus less degraded). I asked if the conditions for uniform heating were controlled somehow?

Author Response

The author would like to thank the reviewers for their time and efforts in reviewing the manuscript.

The furnace was designed to ensure uniform heating, and the following ways were adopted to enhance temperature uniformity.

• The furnace pressure was balanced where the relationship of the pressure inside an oven to the outside (ambient) conditions is referred to as the pressure balance. To operate efficiently and achieve good uniformity, the pressure inside the oven must be neutral or slightly positive. The proper balance prevents cold air from being drawn in around gaskets, under the door, or elsewhere and impairs uniformity.
• The oven has sufficient recirculation airflow. The recirculated air is what delivers the heat from the source to the work chamber. An oven with insufficient air will not achieve proper uniformity. Generally, the greater the volume of air circulated, the tighter the uniformity will be. Reputable oven vendors will be able to calculate the expected tolerance.
• A Sufficient Insulation was used where a portion of the heat generated inside an oven is constantly lost through the insulated walls. To ensure the temperature is uniform throughout the heating chamber, this loss must be minimized.
• Air was delivered along the full length of both sides of the work chamber via supply ducts.

 

Reviewer 2 Report

I recommend the publication of the manuscript in its present form

Author Response

Many thanks. I appreciate your time and efforts in reviewing my manuscript.