Bin construction in seismic zones
May 1, 2010
Recent earthquakes in China, Haiti and Chile have reminded people who live in areas prone to seismic activity of the importance of constructing buildings and facilities that are designed to withstand the affects of the upheaval caused by these traumatic events. The global grain industry should also take these issues into account when constructing their facilities.
In a discussion with Tom Gettings, professional engineer for Assumption, Illinois-based GSI Group LLC, he noted that grain silos are highly loaded structures due to the grain content. In addition, silos are cantilevered structures with the material stacked up very high vertically. Because of this, a strong earthquake or seismic event can cause a great deal of force on the silo. Of course, the stronger the seismic event, the greater the load. Other structures at a grain facility, such as catwalk/conveyor support towers, are also affected by the seismic event.
Earthquake forces on the silos and other structures at a grain facility, and the foundations for these structures, are determined by applying seismic provisions of structure design codes. These design codes give engineers specific seismic or earthquake values to design to for a given location and provide a method to generate design forces for these structures. In flat-bottomed steel silos, this may result in increasing of some gauges of the silo stiffeners and sidewall. In hopper tanks, special strengthened support columns and bracing systems are often required, or wall thickness changes for cylinder supported hoppers. In addition, special anchorage (anchor bolts and the attachments to the foundation such as columns, sidewall stiffeners and anchor brackets) may be required. Concrete silos may have changes in reinforcement size, spacing, special placement details, and wall thickness increases. Similar considerations would exist with similarly loaded structures, such as tank support superstructures, etc.
“Silo manufacturers with the proper engineering staff and practical experience should be used as a supply source to ensure these forces are properly considered,” Gettings said.
He said the construction process and equipment used for construction of steel silos built in seismically active regions are similar in many ways to non-seismic regions. With flat bottomed silos, gauges may be different and anchorage methods may be somewhat special, depending on the situation and supplier. With hopper-bottomed silos, you may have more bracing between the support columns and larger braces. Anchorage may be modified as well. These changes would not normally be expected to greatly affect construction methods and equipment, although for hopper tanks time of construction may be increased slightly. Similarly, concrete silo construction techniques would be the same fundamental processes. Foundations may have special detailing requirements for seismic regions and the anchorage method may have special installation requirements.
Another issue to consider is that a seismic event can result in attachments to the silo from towers, catwalks and other equipment applying greater force or force in a direction that the attachment normally does not, as compared to normal operating conditions. Consideration of the forces that occur during the seismic event at such connections should be part of the site design process, with common standard practice revised as necessary.
Gettings noted that while each individual facility will have its own specific design criteria, a contractor can determine the general considerations on constructing steel grain silos and grain systems in seismically active regions. He said the following information is of a general nature and is not intended to address all situations or specific requirements of a system, customer, site location or national authority requirements.
- The grain handling facility owner should specify at the initial quote stages of a project the requirements for design of the silo or other grain system equipment for seismic conditions.
- The facility owner should specify to the contractors and equipment suppliers what building code or national standard the facility is required to comply with. Requirements for submittal documentation for building officials, national authority reviews or other regulatory authorities should be specified as well. These requirements would be based on state or local municipality building codes in the U.S. In the U.S., these local or state building codes are based on the International Building Code (IBC) “model” code. For U.S. locations, the latitude and longitude of the site of the grain system, or information by which the contractor and equipment supplier can determine these coordinates, should be provided. The IBC does not have provisions for locations outside the U.S.
- A geotechnical engineering firm should be retained to do a soil exploration and report for the site of the silos and grain system. This report will provide information that is not only important for the foundation design but for the storage silo design.
The foundation and silo anchorage system, as well as the foundations for other structures and equipment, must be designed for the seismic forces as well as the structures and equipment. A professional engineer should be retained for the design of the foundations. In most cases, the foundation will be designed by an engineering firm separate from the silo supplier and other equipment suppliers. Geometric requirements and loads should be obtained from the silo manufacturer and other suppliers so the foundation designer can combine the supplier’s information with the geotechnical data and applicable regulations to develop a suitable design. Anchorage requirements and foundation design will require coordination between the silo supplier, other equipment suppliers and the foundation engineer. In addition, it will be beneficial for the builder to share with its silo/bin supplier the type of foundation system that will be installed and planned actions to improve or strengthen the foundation soils.
- Seismic design should be considered across all structures of the grain system. Different suppliers will often be supplying different parts of the grain system. A design professional will need to review the total system and ensure that loads are not being applied to structures, such as the silo, without approval or knowledge of the supplier of the structure.
- Facility owners should consider carefully what type of storage silo is needed. At locations subject to a higher level of seismic activity and resulting higher forces, flat-bottomed steel silos are a more cost-efficient selection than hopper-bottomed steel silos (generally supported on steel columns). For applications where flat-bottomed silos will meet the customer’s requirements, they will be a preferred choice in locations where substantial seismic activity is a design criteria.
- Configuration of the storage silo is also important. The silo supplier may encourage the use of a larger diameter and shorter silo versus a smaller diameter and taller silo.
- Flush-floor aeration, where aeration ducts are installed in the foundation slab, versus full plenum floor should be considered if it will meet the customer requirements. If a full plenum floor is planned, the silo manufacturer should be informed. In concrete silos, elevated floor systems, such as are used for some unload systems affect the design of the silo.
Where hopper-bottomed silos are required, it is important to provide the silo/bin supplier details on any requirements for unloading or other equipment, which the silo supplier and designer must accommodate with their column/ bracing arrangement. It is important to determine such requirements as early in the design process as possible.
- Support of loading equipment via properly engineered towers and catwalks should be planned for versus use of wall support brackets.