Abstract
Lightweight steel framing is one of the modern construction technology systems. This system is mostly used for low to mid-rise buildings. The lightweight steel framing system has many advantages, including lightness, ease of installation, high execution speed, and being more cost-efficient. Since the manufacturers of cold formed steel frames use bricks in an unprincipled way to cover these structures and because of less laboratory research in this regard, in the present research to principled use of this structure, the effect of the middle stud was evaluated on seismic behavior of brick shear wall in cold formed steel frame with brick face. For this purpose, four cold formed steel frames were made in two different configurations (without middle stud and with middle stud) using cement sand mortar, wire mesh, and brick shear walls. Based on the results, the middle stud would cause weakness in the permissible deformation of the brick walls, and in shear walls without middle stud, deformations occur along with the acquisition of resistance to larger deformations. Accordingly, the presence of the middle stud increases the average shear strength by 30%, and this increase in resistance causes a decrease in the behavior factor and ductility of the walls, which practically indicates the seismic behavior of the frames with the middle stud.
1 Introduction
One of the modern construction technology systems is called the lightweight steel framing (LSF) system. This system has been used in developed countries and low-rise buildings. The LSF system is very similar to the construction method of wooden buildings. The skeleton of the LSF system is made of a combination of C- and U-shaped steel profiles prepared from galvanized sheets and made by the cold forming method, mostly connected with cold connections. The walls consist of several C-shaped vertical components (studs) connected to U-shaped horizontal components at the top and bottom. The execution speed in LSF structures is more than double compared to conventional structures. Due to the lightweight of LSF structures, the earthquake force created in them is less than in concrete and metal structures. On the other hand, there is no need for hot rolling of steel to prepare steel sections. Ease of repair and restoration of these instruments is another advantage which makes this construction system cost-effective compared to other methods. The disadvantages of this type of structural system include its low stability against lateral forces, especially those caused by earthquakes and wind [1]. In this type of structural system, there is a limit on the size of the opening and the number of floors [2, 3]. Shear walls are used to provide lateral stability of cold-formed steel frame (CFSF). The shear wall in buildings made of CFSF is mainly made of steel belts (belt braces) inside the frame or plate covers made of steel, wood, plaster, cement, etc., on the frame or a combination of them to deal with the horizontal forces caused by wind and earthquake.
Regarding the seismic performance of shear walls, a lot of research has been done, and in most of them, full-scale walls were built and subjected to cyclic load in the laboratory. Each wall's ultimate shear strength and behavior factor was calculated using the laboratory results. For instance, Ayatollahi et al. investigated the performance of CFSF shear wall panels with sheetrock covers under combined lateral and gravity loads. Their results showed that the panels made under a gravity load can increase shear strength, energy absorption, and stiffness but have less plasticity [4]. Mortazavi et al. investigated the seismic behavior of steel walls composed of cold-formed and heated-formed frames. They found out that to improve the seismic performance of these walls, connecting the panels and strengthening the joint is of particular importance [5]. Chen et al. investigated the seismic performance of several CFSF shear walls with steel and plaster coating. They found out that the ratio of the thickness of the frame sections to the thickness of the covering metal plates increases the shear strength and reduces the ductility [6]. Javaheri Tafti et al. conducted a laboratory study on CFSFs covered with steel sheets. This investigation was on a steel frame with CFSF, with the sheet covering in real scale under cyclic loading and different configurations in sections and screws. Finally, they recommended that the value of R given in AISI regulations can be increased to seven [7]. Yu and Shen investigated the shear walls with CFSF covered with metal sheets. The tests consisted of several CFSF walls with claddings, and in each wall, the details of studs and braces subjected to uniform and cyclic loading were changing. They concluded that thicker wall members can help to prevent failure within the stud if no belt brace is installed [8]. In another study, Yu et al. studied the condition of CFSF-like shear walls, focusing on bracing with metal plates. They concluded that covering with metal sheets can not greatly increase the shear strength of CFSF shear walls [9]. Zhao and Rogers studied the inelastic behavior of several CFSF shear walls with X-shape braces. They reported that the soft performance of CFSF walls, given with an R factor equal to four in some regulations is unreliable, and the number three is desirable [10]. Fülöp and Dubina put three walls with X cross belt braces on both sides with 2.4 × 3.6 m wall dimensions under uniform cyclic loading. The results showed that the shear strength of wall panels and corner reinforcement is significant in strength and load-bearing capacity and can effectively strengthen against side loads [11]. Gad et al. investigated the seismic performance of CFSF walls using experimental and numerical methods. This included testing with cyclic loading on walls with belt braces in two cases, one with plasterboards and the other without plasterboards. The results showed that in the wall without plasterboards, the failure started from one screw and continued with the other screws jumping out before the brace was handed over. Also, in all the tests, damage was observed in the connection of the harness to the wall or the tearing of sheetrock around the screws [12]. Serrette and Ogunfunmi studied the lateral performance of several full-scale belt-braced CFSF walls under uniform lateral loading. They concluded that the role and contribution of stud in shear strength is negligible compared to sheetrock plates, and braces' effect on the lateral stiffness of walls with belt braces and sheetrock plates is small. The same reaction and response were observed in samples with belt braces on both sides along with plaster plates [13]. Adham et al. investigated the performance of five full-scale CFSF shear walls with belt cross braces and back-to-back gypsum boards in studs at the end faces under cyclic loading. Experiments showed that with the increase of the cross brace's cross-sectional area, the panel's bearing capacity increases, and its displacement decreases [14].
Iran is under tectonic pressure from three sides, and every year a lot of financial and human damage is caused by earthquakes. The main problem of building damage is the heavy weight of materials, improper implementation and low plasticity of existing buildings. In order to solve the above disadvantages and achieve a safe economic system, one of the methods is the use of CFSF structures. Due to the possibility of filling the inner space between the steel frames with foam and similar, these structures have a good performance in terms of sound and heat transfer. The appearance of the coating used on the frames is mainly made of plaster and cement sheets, which can disturb the city view in urban areas, especially in areas where already there are buildings. Considering that the most of the facades of buildings are made of brick in Iran, it is necessary to investigate shear walls covering with brick in the CFSF. Currently, CFSF have been covered with bricks in an unprincipled way and there is no information about the seismic behavior of these structures. To localize and develop the use of this new construction method, and use them in a principled manner, seismic behavior of brick shear wall in CFSF should be examined in the laboratory. In brick shear walls with CFSF, gravity loads are carried out by studs, and middle studs must be used in opening of the buildings. So in this research, the effect of the middle stud on the characteristics of resistance and seismic behavior of brick shear walls with CFSF would be investigated in the laboratory.
2 Materials and methods
2.1 Materials
The steel sections used are CFSF of ST340H type based on ASTM A1003 standard [15]. The specifications of this type of steel are indicated in Table 1. Details of these sections are available in Table 2 and Fig. 1.
Specifications of the permitted steels
| Characteristic of steel | Yielding strength (MPa) | Ultimate tensile strength (MPa) | Increase in length at 50 mm |
| ST230H | 230 | 310 | %10 |
Size of the consumed sections in steel frames
| Type of section | Web depth (mm) | Flange width (mm) | Edge length (mm) | Thickness (mm) |
| Stud (C) | 90 | 35 | 10 | 0.9 |
| Track (U) | 90 | 35 | – | 0.9 |
The sections in steel frames (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00842
2.1.1 Styrofoam
Styrofoam plates are used to fill the space between cement and sand mortar and sheetrock inside the frame and adjust the thickness of cement and sand mortar. These plates are also used as sound and heat insulation [7].
2.1.2 Wire mesh
Wire mesh with a hole diameter of 19 mm, a wire diameter of 0.4 mm, and a width of 1,200 mm is used to strengthen the tensile strength of cement and sand mortar [4].
2.1.3 Cement and sand mortar
Cement and sand mortar with normal concentration was prepared with cement, sand, and water and then used to install bricks on shear walls. The specifications of the mortar used are given in Table 3. The cement used was type 1 cement. The sand used was washed sand according to the unified classification system in accordance with the standard ASTM-D-2487 [16] of fine-grained sand (SW).
Specifications of cement sand mortar
| Sand to cement ratio of 1–5 | ||||
| Density (MPa) | Average tensile strength (MPa) | Average compressive strength (MPa) | Sample Dimensions (mm) | Number of samples |
| 226 | 6.3 | 51.7 | 50 × 50 × 50 | 6 |
2.1.4 Clay brick
Brick covers the exterior of the shear walls. Brick has dimensions of 200 × 55 × 20 mm. Gypsum plates covered with paper called leaf gypsum with a thickness of 10 mm have been used for sheathing the interior of the walls.
2.1.5 Screw
The used screw was an automatic screw with a washer. These screws connect steel parts and install wire mesh on the steel frame, and flat head screws install plasterboards to the steel frame at intervals of 20 mm. These screws are under the ASTM C1513 standard [17]. For easier installation of bricks on the wall and to increase its durability and beauty, ceramic was installed at a height of 300 mm along the wall.
2.2 Samples characteristics and test methods
In this research, to investigate the effect of middle studs on the performance of steel shear walls made in CFSF with brick faces, 4 shear walls with 2 different configurations were built.
2.2.1 B1 and B2 shear walls
To make the samples, firstly, with the help of CFSF sections and automatic screws, we made a steel frame with dimensions of 240 × 120 cm (height × length). Inside the steel frame, foam with a certain thickness (240 × 120 × 5 cm) was installed. On the other side, the wire mesh was installed with the help of screws. Then, for the construction of shear walls, ceramic was installed with sand-cement mortar at the bottom of the frame. Sections are shown in Fig. 2.
Cross-section of B1 and B2 (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00842
2.2.2 D1 and D2 shear wall
In samples D1 and D2, similar to samples B1 and B2, steel frames were prepared and installed on one side of the wire mesh, and on the other side of gypsum boards and inside of which there was foam, with the difference that an intermediate stud was installed in the middle of these frames. By installing ceramics at the foot of the wall and bricks at the top of the ceramics, the A1 and A2 type shear wall was completed. The existence of the middle stud in the CFSF, in addition to increasing the resistance of the structure against gravity loads, causes the shear resistance of the frames [7]. Thus in this research, the behavior of the frames with the middle stud covered with brick was investigated in the laboratory and the results in the design of the structure used by designers. The sections are shown in Fig. 3. The general view of the walls is shown in Fig. 4. The construction and installation steps of B1 and B2 shear wall are shown in Fig. 5. The construction and installation steps of type A1 and A2 shear wall are shown in Fig. 6.
Cross-section of D1 and D2 wall (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00842
The general view of the walls (Left: B1 and B2, and right: D1 and D2) (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00842
Steps of construction and installation of B1 and B2 shear wall (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00842
Construction steps of D1 and D2 shear wall (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00842
2.3 Tools and equipment
The seismic loading device for shear wall testing with its peripheral equipment is shown in Fig. 7. The walls are properly connected to the upper and lower beams at the four corners. Moving the lower beam introduces the lateral load to the walls by a two-way hydraulic jack. A transducer for measuring horizontal displacements (Horizontal drift transducer) and a load cell for measuring force have been used. The forces and displacements made during the test are transferred to the LabVIEW SignalExpress software, and the load-displacement curve is drawn for each sample [7].
The testing rig (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00842
2.4 Loading regime
The loading regime used in this research is based on method B in the ASTM standard, originally developed for the ISO 16670 standard (International Organization for Standardization). The loading includes one complete cycle at 2.5, 5, 10, 15, and 20%, and three complete cycles at 40, 80, and 1,200% of the maximum displacement of the specimen. The test is stopped with early failure or a significant reduction in strength in Fig. 8 [18].
Cyclic loading protocol (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00842
2.5 The methods used in bilinearization of the pushover curve
Uang's method was used in this research to bilinearize the bearing curve obtained from seismic loading results. This method was stated by Uang and is currently used by FEMA publications [19].
In Uang's method, the wear curve of the samples is bilinear, and then by equalizing the area under the bilinear diagram with the area under the elastic curve diagram, the necessary parameters for calculating R (Behavior Factor) can be obtained.
2.6 Calculation of behavior factor
3 Results
The results were used to evaluate the effect of the middle stud on the performance of brick shear walls in CFSF. These results, including failure modes, load-displacement curves, maximum strength of walls against shear force, calculation of behavior factor in the tested samples, and their comparison, are presented in the next sections.
3.1 Failure modes
3.1.1 Type B1 and B2 shear walls
In the sample B1 wall during seismic loading, nothing special happened except for hearing some sort of brick cracking sounds on top of the sample in the third cycle of more than 48 mm displacement. In sample B2, no failure was observed in different loading cycles except for hearing slight sounds caused by the crushing of the mortar behind the wall (Fig. 9).
B1 and B2 wall after seismic loading (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00842
3.1.2 Type D1 and D2 shear walls
In sample D1, nothing special happened until the change of position of the third cycle of ±48 mm, but in the first cycle of −60 mm, diagonal and vertical cracks were observed at the top and middle of the sample on the side of the brick face (Fig. 10).
Failure mode of D1 shear wall (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00842
In sample D2, nothing special happened until the change of position of the third cycle of ±24 mm, but in the first cycle of +48 mm, small diagonal and vertical cracks were observed at the top and middle of the sample on the side of the brick facade. The cause of this failure would possibly be the separation of the concrete plate cement-sand mortar in place of the middle stud. These cracks expanded +48 mm in the third cycle, and parts of the brick facade collapsed. In the first cycle, 60 mm of the wall was broken (Fig. 11).
Failure mode of D2 shear wall (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00842
3.2 Displacement – force plots
In the present research, a number of shear walls with different compositions were built in the laboratory. The bearing diagram of them (hysteresis curves) were shown in Fig. 12. For a better understanding of the behavior of shear walls against seismic loads, their comparison is shown in Fig. 13. This figure shows the effect of wall construction and the distance of the studs on the shear walls' behavior [7]. According to Fig. 13, it can be seen that the ductility and energy absorption in the samples B1, B2 are more than in the samples D1, D2 and the use of the midle stud increases the hardness of the wall and reduces the ductility. However, the use of middle stud can significantly increase the shear strength of shear walls (Fig. 13).
Hysteresis curves and push-over diagram for B1, B2, D1 and D2 specimens (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00842
Comparative diagram between B1, B2, D1 and D2 shear walls (Own source)
Citation: International Review of Applied Sciences and Engineering 16, 1; 10.1556/1848.2024.00842
3.3 Ultimate strength
Using the text file that shows the force-displacement values for each of the walls, it is possible to extract the shear force generated in positive and negative displacement in different cycles [4]. Thus the maximum shear strength of the walls and the corresponding displacement can be obtained. The ultimate strength of shear walls and their corresponding displacements are obtained in cycles of ± displacements extracted from the test results, and by averaging the maximum shear strength in ± cycles, the average shear strength of each wall is obtained. Ultimate strengths of shear walls and corresponding displacements are obtained in cyclic displacements extracted from the test results. Table 4 shows the results of determining ultimate strength values.
Maximum shear resistance values – Displacement corresponding to maximum shear resistance
| Wall | Maximum shear resistance at (+) displacement (kN) | Displacement corresponding to maximum strength (mm) | Maximum shear resistance at (−) displacement (kN) | Displacement corresponding to maximum strength (mm) | Average maximum shear resistance (kN) | Force average |
| B1 | 13 | 72.5 | 15.5 | 71.5 | 14.3 | 14.8 |
| B2 | 17 | 67.8 | 13.6 | 46.6 | 15.3 | |
| D1 | 18.5 | 60 | 20.9 | 60 | 19.7 | 19.20 |
| D2 | 18 | 48 | 19.1 | 48 | 18.6 |
The D1 and D2 shear wall showed good shear resistance (30% more than B1 and B2) (Table 4), which is due to the presence of the middle stud and the increase in shear hardness of these walls compared to the B1 and B2 wall.
3.4 Calculation of behavior factor
The results of calculations to determine the necessary factors by Uang's method to calculate R are given in Table 5. These calculations consider the target displacement equal to the maximum permissible displacement (60 mm).
R values and their parameters using the push-over curve and bilinearization. by Uang's method
| Wall | Vs (kN) | Vy (kN) | Ve (kN) | Rd | R | Rave | |
| B1+ | 1.9 | 15.2 | 19.2 | 8 | 1.3 | 10.4 | |
| B1– | 1.5 | 11.4 | 16.7 | 7.6 | 1.7 | 11.1 | 10.8 |
| B2+ | 1.6 | 16.3 | 17.3 | 10.1 | 1 | 10.1 | |
| B2– | 1.7 | 11.8 | 20.3 | 6.9 | 1.7 | 11.8 | |
| D1+ | 2.6 | 21 | 19.4 | 8.1 | 0.9 | 7.5 | |
| D1– | 4.7 | 20 | 45.1 | 4.2 | 2.2 | 9.5 | 8.3 |
| D2+ | 3.3 | 17.3 | 23.6 | 5.2 | 1.4 | 7.1 | |
| D2– | 3.7 | 18.5 | 34.7 | 5 | 1.9 | 9.4 |
By calculating the behavior factor of shear walls and using the results of Table 5, it is possible to reach this important and predictable result that the behavior factor in the walls of B1and B2 is 30% higher than 7in the samples of D1 and D2, and that is due to the lower hardness of the walls of B1, B2 and their greater plasticity. In some laboratory studies, the behavior factor showed values between 3 and 8 [7–13], but the behavior factor values on shear wall with CFSF coating and brick in the present research was between 8 and 10. For example, Ayatollahi et al. [4] obtained a behavior factor between 5 and 8 for a wall covered with ordinary gypsum boards. In another study Zhao and Rogers [10] proposed a value of behavior factor of 3 for a wall with cross bracing. In a separate study by Javaheri-Tafti et al. [7] the R value of the cold-formed steel walls sheathed by thin steel plates varied in between 6.85 and 8.23 with the majority havig been above 7. In the current study the behavior factor values on the shear wall with CFSF and brick cover were between 8 and 10.
4 Conclusion
Using CFSF structural system has many advantages, but due to some limitations, this system has not yet become popular in different countries. One of the most important causes of less public popularity of this structural system would possibly be the lack of coordination of the exterior of this structure with the existing structures. Therefore, the most important step to expand and promote the use of this kind of useful and relatively low-cost system by the general public is to produce it on a local level. In this regard, four shear walls in two types, including B and D were made in a CFSF, after processing, the samples were subjected to cyclic loading based on the ASTM-2126B loading protocol. By using the outputs obtained from loading and calculations and analyzing the results, regarding B and D type shear walls, the following results were obtained:
According to the results diagonal and vertical cracks in the place of the middle stud were observed. The presence of the middle stud is considered a major defect in this type of shear walls (D1 and D2), while such cracks are not observed in the walls without the middle stud (B1 and B2 wall). The middle stud causes the mortar behind the bricks to split into two parts, which can cause weakness in the permissible deformation of these walls, so that the walls will suffer shear failure if the deformation is less than 60 mm (permissible deformation). But in shear walls without middle stud, deformations occur along with the acquisition of resistance to larger deformations (D1 and D2 is 30% more than B1 and B2).
The middle stud in shear walls increases the shear strength, because the presence of the middle stud strengthens the steel frames, and finally, the set of shear walls with the middle stud can show more resistance against the created shear forces. The middle stud increases the shear strength, but this increase would increase the wall stiffness and decrease the ductility of the shear wall, besides, the seismic performance that arises from the wall's ductility decreases (A1 and A2 is 30% less than B1 and B2), so the absence of the middle stud increases the walls' malleability. In order to use as much CFSFs as possible in the skeleton of different buildings, it is recommended to carry out laboratory studies on the seismic behavior of shear walls with CFSF skeletons with stone and ceramic coating or a combination of brick, stone and ceramic.
References
- [1]↑
P. Sharafi, M. Mortazavi, N. Usefi, K. Kildashti, H. Ronagh, and B. Samali, “Lateral force resisting systems in lightweight steel frames: recent research advances,” Thin-Walled Struct., vol. 130, pp. 231–253, 2018.
- [4]↑
S. R. Ayatollahi, N. Usefi, H. Ronagh, M. Izadinia, and M. R. Javaheri, “Performance of gypsum sheathed CFS panels under combined lateral and gravity loading,” J. Constr. Steel Res., vol. 170, 2020.
- [5]↑
M. Mortazavi, P. Sharafi, H. Ronagh, B. Samali, and K. Kildashti, “Lateral behaviour of hybrid cold-formed and hot-rolled steel wall systems: experimental investigation,” J. Constr. Steel Res., vol. 147, pp. 422–432, 2018.
- [6]↑
Z. Chen, H. Sun, and B. Cao, “Experimental study on seismic behavior of cold-formed steel shear walls with reinforced plastered straw-bale sheathing,” Thin-Walled Struct., vol. 169, 2021.
- [7]↑
M. R. Javaheri-Tafti, H. R. Ronagh, F. Behnamfar, and P. Memarzadeh, “An experimental investigation on the seismic behavior of cold-formed steel walls sheathed by thin steel plates,” Thin-Walled Struct., vol. 80, pp. 66–79, 2014.
- [8]↑
C. Yu and Y. Chen, “Detailing recommendations for 1.83 m wide cold-formed steel shear walls with steel sheathing,” J. Constr. Steel Res., vol. 67, no. 1, pp. 93–103, 2011.
- [9]↑
C. Yu, “Shear resistance of cold-formed steel framed shear wall assemblies,” in 19th International Specialty Conference on Cold-Formed Steel Structures. Missouri University of Science and Technology, 2008, pp. 441–455.
- [10]↑
Y. Zhao and C. A. Rogers, “Preliminary R-values for seismic design of steel stud shear walls,”, in 16th International Specialty Conference on Cold-Formed Steel, 2002, pp. 468–682.
- [11]↑
L. Fülöp and D. Dubina, “Performance of wall-stud cold-formed shear panels under monotonic and cyclic loading: Part I: experimental research,” Thin-Walled Struct., vol. 42, no. 2, pp. 321–338, 2004.
- [12]↑
E. Gad, A. Chandler, C. Duffield, and G. Hutchinson, “Earthquake ductility and overstrength in residential structures,” Struct. Eng. Mech. J. Int. J., vol. 8, no. 4, pp. 361–382, 1999.
- [13]↑
R. Serrette and K. Ogunfunmi, “Shear resistance of gypsum-sheathed light-gauge steel stud walls,” J. Struct. Eng., vol. 122, no. 4, pp. 383–389, 1996.
- [14]↑
S. Adham, V. Avanessian, G. Hart, R. Anderson, J. Elmlinger, and J. Gregory, “Shear wall resistance of lightgage steel stud wall systems,” Earthq. Spectra., vol. 6, no. 1, pp. 1–14, 1990.
- [15]↑
A. Committee and A. International, Specification for Steel Plate, Carbon, Metallic- and Nonmetallic-Coated for Cold-Formed Framing Members. West Conshohocken, PA: ASTM International, 2009.
- [16]↑
ASTM, Manual of Aggregate and Concrete Testing, Amerrican Socity for Testing and Materials. West Conshohocken, Pennsylvania, 2000.
- [17]↑
T. American Society for, Materials, ASTM standards related to cold-formed steel raming. West Conshohocken, PA: ASTM International, 2007.
- [18]↑
Standard Test Methods for Cyclic (Reversed) Load Test for Shear Resistance of Walls for Buildings. ASTM International. 2007.
- [19]↑
R. Engineers, “FEMA 356: prestandard and commentary for the seismic rehabilitation of buildings,” Report No, FEMA, 2000.
- [21]↑
A. S. Whittaker, C.-M. Uang, and V. V. Bertero, Earthquake Simulation Tests and Associated Studies of a 0.3-scale Model of a Six-Story Eccentrically Braced Steel Structure, Earthquake Engineering Research Center, College of Engineering, University, 1987.
- [22]↑
O. International Conference of Building, Uniform building code (UBC), International Conference of Building Officials, Whittier, Calif., 1994.
