A Comparasion of Rack Supprted Warehouse vs. Conventional Steel Frame Buildings - Research Paper Example

?Introduction Warehouse structures are essential building types that serve the commercial purpose of storage of goods and merchandise for exporters, importers, industrialists, etc. These are usually large structures covering vast spans to provide the capacity required. Warehouse construction is a flourishing industry and thus requires sophisticated methods of construction to fulfill the requirement. Loading and unloading of goods is required that are stored on pallets which are in turn placed on pallet racks. Cranes and forklifts are used to transport these goods to and from the pallets racks to the transportation vehicles. Conventional steel frame structures have been used to build the warehouses, but with the advancement of technology, rack supported warehousing is also in practice. But the two structural designs need to be analyzed and compared in terms of their usage, economic viability, technical feasibility and storage capacity. The two structural systems will be explained briefly before proceeding with the comparison. Steel frame structure A skeleton frame is used in the conventional steel frame which is based on vertical steel columns and horizontal I-beams or trusses. The roofs and walls are attached to the frame and are supported by constructing the building in a rectangular grid. Material storage is provided through freestanding racks (Kotecki and McHugh, 2009). Hot rolled steel structures and cold form steel structures are the two types of steel frame structures. A wide choice is available in the cross sections of the single storied hot rolled steel industrial building, but economical viability is usually granted through limited number of shapes. A 250Mpa of strength is yielded by the cross section used in the hot rolled steel industrial building (Satpute and Varghese, 2012). The cold form steel building is a pre-engineered structure that has been tested through time for its aesthetic and structural quality. Components of cold form steel Main frame, which is based on I-sections with columns and rafters made of hot rolled sections. Secondary frame, which is provided through Z or C sections. Purlins, eave struts and side girts are used. Wind bracing is provided through circular cross section rods. Exterior cladding is provided through trapezoidal sheeting. Various options are provided through the steel frame construction in terms of width, height and roof slope to suit the operational requirement of the main building structure that will house the pallet racks. In case of a warehouse the following options are commonly used: Clear span, straight column steel building frame: no interior supporting columns are needed, which provides a column free space for steel storage racks (Armstrong Steel, 2012). Width =16’ to 50’ and over, Height =8’ to 60’ and over, Standard roof slope = 1/2:12 to 1:12. Multiple span, straight column steel building frame: interior columns are provided to distribute the load and reduce the cost (Armstrong Steel, 2012). Width =10’ to 600’ and over, Height of=20’ to 60’ and over, standard roof slope =1/2:12. Multiple spans, tapered column steel building frame: tapered columns and interior straight columns are provided in this structure (Armstrong Steel, 2012). Width = 10’ to 600’ and over, Height =20’ to 60’ and over, standard roof slope =1/2:12. Multiple spans, single slope steel building frame: a sloped roof is provided by adding walls of different height on both the sides creating a slope (Armstrong Steel, 2012). Width =10’ to 600’ and over, Height =20’ to 60’ and over, Standard roof slope =1/2:12. The above options may provide the main frame, but free standing pallet racks may be required separately to store the goods. A teardrop style is used to manufacture these racks. The pallets rest on the horizontal load beams that are held in position by mounting clips. The shelves can be adjusted to various heights as the clips on the teardrop configuration can be quickly moved. Shelves can be adjusted according to the goods. Rack Supported Structures Steel storage racks are used in this form of construction to build structures, which are also referred to as the rack clad buildings. The cladding of the building is supported by the peripheral frame. This implies that the racking system acts as the basic structural support for the building’s roof and walls (Kotecki and McHugh, 2009). The roof and walls are the skin of the building. This is very similar to the selective pallet rack system, but in this case bolts are used to attach the horizontal beams to the uprights. This increases the weight bearing capacity of the structure so that the upright columns can support the weight of the roof as well. This replaces the need of supporting columns. The main components may include; structural beams formed from structural C shapes and connecting clips at either end; upright frames, designed according to the load requirements with modified C shape and returns (holes are punched at standard intervals up and down the column for mounting the structural beams), diagonal braces and horizontal braces and pallet supports (Haque, 2012). Figure 1. Geometry of the base-isolated rack structure with its two outer supporting structures (Taken from: Kilar et al., 2011) COMPARATIVE ANALYSIS Construction process: The first step of construction for a steel frame is the pouring of foundation, after which the structural steel is erected. The roof deck and siding are added afterwards. The slab is poured after the completion of the underground works. The racks are erected one frame at a time after the slab has cured (Kotecki and McHugh, 2009). While in the rack supported building, the slab is poured first, after which the racks are erected. The multiple frames are assembled in fixtures while lying on their sides. The modules are then stood up on the slab. The conventional method of erecting the steel framed buildings gives a limited headroom and accessibility. The final bolt up and framing has to be done in the air which adds to the complications of the process. On the contrary, in the rack supported construction, the multiple frames are assembled while lying on their sides and it is at the ground level that the bolting and assembling of the support steel is carried out, which allows space for construction, accessibility and convenience. More accuracy in the plumbing is attained in the rack supported structures within less time as compared to the conventional steel frames, where the plumbing and alignment is a difficult and a costly process. As the height and the width of the racks increases in the conventional steel frames, the process becomes even more complicated (Kotecki and McHugh, 2009). Flexibility in Usage: As evident from the structural variations available in the steel frame and the method of construction, the structural steel frame buildings offer multiple usage as the storage can be removed later on. Building configuration can be changed to suit the changing business requirements, which may not be the case in the rack supported structures, as its primary structure is based on pallet racks. The rack supported warehouses are only serving the purpose of storage, while conventional steel frame buildings can combine the purpose of storage with other processes such as manufacturing. Rack supported warehouses are only specified for storage but provide an optimized storage density as compared to the steel frame structures, as complete height of the structure is used in storage through rack supports. The teardrop connectors allow flexibility in adjusting storey heights according to requirement (Beattie, 2006). Technical feasibility: The steel framed structures may use bolted or welded connections which gives them a high resistance against the forces of moment thus reducing rotation as compared to the RCB structures. As demonstrated by (Bernuzzi and Castiglioni, 2001) the welded and bolted connections will show a low rate of degradation of reloading stiffness under cyclic loading as compared to the RCB structures. Thin walled perforated steel sections are used in the rack supported structures where the beam column joint use semi-rigid boltless connectors to form moment resisting frames. Under cyclic loading, these structures may suffer from high strength degradation. But these structures have been observed to perform efficiently under gravity loads as compared to steel structures and carry 20 to 50 times higher loads in addition to their self weight (Haque, 2012). RCB structures similar to the multi-storey buildings can be constructed to considerable heights (Kilar et al., 2011). Large lateral loads are experienced by these structures in highly seismic zones, which threaten the workers due to risk of shedding merchandise (Haque, 2012). Thin walled cold formed steel sections form these structures, where the columns are usually open sections made from perforated steel plates with one axis of symmetry. Closed box section beams are used (Filiatrault et al., 2008). Connecting columns and beams are used to create the frame. The structural elements of the RBC structure are pre-fabricated with cold formed steel sections. Similarly, steel framed structures are also offering pre-fabricated solutions using both cold formed and hot rolled sections, although pallet racking will need to be added separately to the structure. Pallet racking can be automated for both the RBC and steel framed, but according to the statistics provided by the Reidsteel manufacturers below in some cases of the steel structure it might not be economically viable. The following table gives the economic feasibility of warehouse steel structures with respect to spanning width, seismic resistance and automated storage suitability. Table 1. Economic feasibility of warehouse steel structures with respect to spanning width, seismic resistance and automated storage suitability (Reid Steel, 2013) Typical economic sustainability Portal type structure Propped type High bay Wide span Monoslope Multispan Up to 60m wide Up to 120m wide Up to 200m wide Up to 40m wide Seismic/hurricane Automated storage and retrieval On the other hand, the RCB structures are best suited for automated technology as RCB has the potential to expand to greater heights for racking. Expansion of conventional steel frame structures in terms of height may prove costly. In the steel framed buildings, the main structure is acting as an enclosure for the free standing racks and has to protect these racks from wind or earthquake loads. The primary force resisting system of the main warehouse structure is used against the forces which limits the response of the racks under seismic loading (Castiglioni et al., 2009). On the contrary, the environmental loads calculated for the design of RCB such as, wind or earthquake loading can be significant, as the RBC frames are much larger and self sustaining. These loads are not considered in the case of free standing racks. Braces have to be applied for the RBC structures in their down-isle direction to grant more stability (Haque, 2012). RCB structures require a lighter foundation as compared to the steel framed structures. The rack supports eliminate the need for trusses in these structures, which add to the cost in steel framed structures. The RCB structures require less civil works as compared to steel framed structures. Design considerations have to be made for both the structural types according to the wind load in an area, types of rack and material handling system and area that needs to be covered. Risks: The failure of the primary load carrying member can lead to fatalities in a RBC structure, as a large number of workers are working inside these structures. As demonstrated by (Affolter et al., 2009) in an accident investigated by him, that overloading of columns beyond their capacity can lead to a collapse of that structure. Higher safety factor should be applied by the design guidelines. Seismic feasibility: RCB structures are suited for regions with high winds and seismic activity, as the forces will be distributed among the structural frames (Kotecki and McHugh, 2009). Yet RCB structures need to be properly designed against lateral forces as they pose higher risk towards public safety compared to conventional steel storage racks. Although the National Building Code of Canada (NBCC 2005) recognizes the seismic risk of rack storage systems and recommends that seismic provisions be provided while designing these types of structures, it has no guideline for designing RCB structures against seismic loading (Haque, 2012). Installation time and convenience: The RBC structures are manufactured and installed with entire components and the clients do not need to do anything during installation. The client can reuse the structure at any other location again, as the structure can be easily disassembled and transported to another destination. Clients would usually prefer these structures, as the semi rigid boltless connectors used by the columns and the beams gives flexibility of adjusting heights (Beattie, 2006, Temesist, 2011). The conventional steel structures may use bolted and welded structures both and may not offer flexibility in adjusting the storage capacity according to requirement. RCB structures are engineered modular steel building system that can be erected 40% faster than traditional systems (Zaharia, 2000). The steel framed structures can be adjusted according to usage and modifications can be induced in the structure to accommodate any change in the client’s business plans. It is more feasible to expand these structures horizontally by unbolting walls and adding another clear spanning frame in its place (Syed Firoz et al., 2012). Both the systems are however installed by companies that have expertise in the erection of these structures and as these structures are pre-engineered, the contractor is familiar with the technique. Cost effectiveness: It provides highest density storage solution within very low construction costs. Cost is further reduced as no additional warehouse building is needed to support the roof and walls. A good salvage value is offered by these structures as the structural elements can be salvaged without any damage when required. The warehouse industry is frequently using these structures, thus adding to its popularity. While rack supported structures have the capacity to cover more heights, conventional steel frames spread horizontally. The steel structures become extremely costly when built over heights of 45’. In terms of cost, steel frame will require an initial investment of $10, 400, 000 for 15,000 very narrow aisle rack locations as compared to $ 9,900,000 for rack supported structure with similar specifications. Steel frame structures undergo an annual depreciation of $415, 000, while the rack supported structures depreciate at $625, 000 annually. This is because the walls and roof of the rack supported structures are deemed as equipment enclosures, which means that the cost of the structure can be depreciated at a course of 15 years while normally a time period of 30 years is considered. The cost of utilities for the steel frame structures is $100,000, while for rack supported $40,000 is required. These statistics have been provided by the HKS systems and show that rack supported structures are cheaper to construct as compared to the steel frame structures and cause a 40% reduction in the construction cost as compared to the steel structures, as it eliminates the need for steel columns and long span roof trusses. The rack structure has columns at much closer centre which reduces the size of the supporting members, further reducing the cost. The dimension of the structure is considerably reduced as compared to the steel frame structures for the same number of storage units. In cases where land value is high, rack supported structures are an ideal solution as footprint can be minimized. This will allow cutting the additional costs needed for every acre that is added for construction, as specified by the building laws. Investments in land are reduced and more acreage is available for productive usage (HK Systems, 2013). The entire project for rack supported buildings are mostly turnkey projects that are executed by one party, which is preferred by the clients as it saves them botheration and also cuts down the costs. Multiple contractors may be involved for the conventional steel framed warehouses, where the structure and the pallet racks are two different elements. In case of conventional steel frame structures, the client may require separate contractors, thus implying more costs. In addition, the total cost of construction for the steel framed structures also includes design charges as well. Cost of the equipment is also added as it has to be purchased separately, while the rack supported structures have the equipment integrated into the construction package. Less money is amounted for insurance by the rack supported structures due to its low cost of construction while in the case of steel frame structures it is vice versa. Storage capacity: Rack supported structures will surpass the steel framed structures in terms of storage capacity. A relatively small increase in storage capacity is obtained when erecting a steel structure more than 45’, which becomes undesirable. Overall space utilization is reduced as the columns and aisles take up the space. The modern ASRS applications are best applied on rack supported warehouses, as they can go to heights of 110’. Steel frame construction offers 55% floor space utilization with maximum height of 40’, which renders 126,000 ft2 for storage and 8.3 ft2 per pallet. In case of rack supported structures, a 72% floor space utilization can be obtained as a height of 90’ can be managed, which implies that 36,000 ft2 is needed for storage with 2.4 ft2 per pallet (HK Systems, 2013). Time required for completion: Rack supported structures require less time as compared to the steel framed structures as the process of erection rack cladding and material handling equipment can take place simultaneously. As one contractor is handling all the processes, the process is speeded up even more than the steel frames structures. Less time is required in design adjustments of these pre-fabricated structures according to the site conditions and requirements. Therefore the business can almost start simultaneously within this building. The erection of the rack supported structures is also faster owing to its uniform symmetrical construction, which is not the case for the steel frame structures. CONCLUSION Rack supported and steel frame construction is both suitable options for constructing warehouses keeping in mind the operational requirements and site conditions. The client’s business priorities and future requirements will determine the structure best suited. These two structural systems are similar to each other in terms of solutions provided with respect to pre-engineered structures. The primary difference arises in terms of the main supporting structure. Rack supports acting as columns serves the dual purpose of storage and support for the structure that has increased its benefits by many folds for the warehouse industry in terms of cost, storage capacity and efficiency. REFERENCES AFFOLTER, C., PISKOTY, G., WULLSCHLEGER, L. & WEISSE, B. 2009. Collapse of a High Storage Rack. Engineering Failure Analysis, 16, 11. ARMSTRONG STEEL. 2012. Steel Frame Buildings [Online]. Armstrong Steel. 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