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The owner of a warehouse, Al-Maha Co. for Food Stuff, wanted to know whether it was possible to increase the height of three sheds each by 4 meters (13.1 feet) to increase capacity. The project under consideration was constructed in the mid-1990s . It consists of three typical structural steel warehouse sheds that are 9-meters high (29.5 feet), 30-meters (98.4 feet) wide, 150 meters (492 feet) long and with 6 meters (19.7 feet) typical bay size. The project is located about 20 kilometers (12.4 miles) southeast of Baghdad, Iraq. All sheds are fabricated with structural steel from European shape sections, some of which were non-standard (commercial). The shed roofs consist of trusses made up of double angle sections for the upper and lower chords, as well as the vertical and diagonal members. The sheds' steel column bases were designed as fixed-based connections, as reflected in custom detailing.
This challenge was extensively studied. Due to the lack of original construction design drawings, a comprehensive site survey also was conducted for all warehouse sheds. This included exposing structural components to determine their dimensions and non-destructive/destructive material testing. For instance, existing concrete core samples were extracted from the foundation for compressive strength values. In addition, scanning for steel reinforcement location and bar size within the foundation were conducted.
Based on the site survey, as-built drawings were prepared for the sheds. Gravity loads (dead and live) and wind load were calculated in accordance with the specifications of the American Society of Civil Engineers ASCE 7-22. Then, shed steel frames were modeled using the STAAD-Pro structural analysis software. AISC Load Resistance Factor Design method (LRFD) was adopted as the design code/method, and a static loading structural analysis was conducted. Before increasing the building height, analysis results and design checks showed that all members were sufficient for strength. The shed span deflection and drift (lateral displacement) were also checked against the limits recommended in standard practice and international specifications. Lateral drift was limited to H/65 to H/100 for such type of structures (warehouses), where “H” is defined as building full height.
The next step was to increase the height of the main columns from 9 to 13 meters (29.5 to 42.6 feet). This was accomplished by inserting steel extensions at the top of the columns, as shown in the AutoCAD BIM model used for construction (Figure 1). The structure was re-analyzed and as expected, the main columns and parts of the truss failed in strength since the increased structural height also increases the lateral wind projection area. As such, the main columns required shear, flexure, and torsion strengthening.
In addition, and as one of the biggest challenges of the project, the increased building height significantly increased the force demands to the foundation and the fixed-based column connections. Per owner direction, it was not possible nor practical to enlarge the existing foundation footprints, especially in plan dimension. The design solution was then directed toward changing column base connection from a fixed-base to a pinned-based (hinge) connection. The latter is particularly advantageous to use since a pinned-base connection theoretically does not transfer flexural moment into the foundation, which is the governing foundation design.
The design solution for the column base connection had to satisfy increased force demands to both reinforced concrete and steel elements and at the same time combine ease of application, economy, and aesthetic issues. In addition, construction and equipment could not occupy the interior space of the warehouses since material goods were still actively being stored there. As a result, structural details were developed to change the existing column fixed-base connection to a new hinged connection. This was accomplished through the following construction sequence and graphically shown in Figure 2.
- Provide temporary shoring and lateral support to the main steel columns.
- Disconnect the roof trusses from the steel columns and store the still-in-tact trusses on the ground. This releases a majority of the gravity loading to the columns.
- Partially cut the steel plate stiffeners and flange/web components at the column base (Fig. 2a).
- Cut the column end connection to the existing base plate in order to make a gap.
- Prepare and chamfer 4 – M30 mm (1.18 in.) threaded studs that are approximately 140 mm (5.5 in.) long, and weld them to old existed base plate with pre-defined spacing.
- Prepare a 2L150X5mm angles with cut end and drill 4-Ø35mm (1.38 in.) holes in to their horizontal leg with same spacing of studs. Insert them through prepared welded studs of step (5), and weld their vertical leg to column web as shown (Fig. 2b).
- Fill the gap between the existing and new base plates with non-shrink epoxy grout.
- Cut the outer remaining parts of the existing steel stiffener plate.
Aside from the column bases, the main column steel section required strengthening. This was accomplished by adding flange cover plates along the full height of the columns and with circular cut holes at vertical increments. These holes were introduced to weld plates to the column flanges to obtain greater cross-sectional properties (Fig. 3).
In conclusion, the column height extensions, column base connection modifications, and column strengthening are almost finished to date. However, additional project work continues for installation of other services per the owner request. ■
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