Embed Size (px)
Citation preview
The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect the official position of ASAE or CSAE, and its printing and distribution does not constitute an endorsement of views which may be expressed. Technical presentations are not subject to the formal peer review process, therefore, they are not to be presented as refereed publications. Citation of this work should state that it is from an ASAE/CSAE meeting paper. EXAMPLE: Author's Last Name, Initials. 2004. Title of Presentation. ASAE/CSAE Meeting Paper No. 04xxxx. St. Joseph, Mich.: ASAE. For information about securing permission to reprint or reproduce a technical presentation, please contact ASAE at [email protected] or 269-429-0300 (2950 Niles Road, St. Joseph, MI 49085-9659 USA).
An ASAE/CSAE Meeting Presentation Paper Number: 044143
Design Considerations for the Construction and Operation of Feed Milling Facilities. Part I: Planning, Structural, and
Life Safety Considerations
Gregory D. Williams, Ph.D., P.E., S.E., Manager of Engineering FWS Design Builders Inc., 1600 E. Cliff Road, Burnsville, MN 55337, [email protected]
Kurt A. Rosentrater, Ph.D., Agricultural and Bioprocess Engineer USDA, ARS, Crop and Entomology Research Unit
2923 Medary Ave, Brookings, SD, 57006, [email protected]
Written for presentation at the 2004 ASAE/CSAE Annual International Meeting
Sponsored by ASAE/CSAE Fairmont Chateau Laurier, The Westin, Government Centre
Ottawa, Ontario, Canada 1 - 4 August 2004
Abstract. Feed mills represent an important segment of our food production system, supplying the nutritional need for animals in our meat supply system. Agri-industrial facilities such as feed mills have a number of unique design requirements that are relatively unknown. The purpose of this paper is to summarize state of the art design procedures for feed milling facilities constructed in North America. To this end, in Part I of this series, planning, life safety, and structural design criteria for these facilities are examined and relevant theory is presented. Additional resources are cited for further study of concepts. This paper should be of benefit to both academics who teach facility design and practitioners, alike.
Keywords. Agri-Industry, Concrete, Feed Mill, Foundations, Life-Safety, OSHA, Planning, Steel, Structural Design.
2
Introduction A key component in the production of meat in the US agri-industrial system is animal feed production. The US Census Bureau indicates that there are currently 1779 animal food manufacturing establishments in the United States (US Census Bureau, 2004). Although a number of contractors and engineers service this industry, that number has been diminishing in recent years. Moreover, the design and operation of feed milling facilities have unique requirements that have not been extensively documented to date. Thus, the purpose of this article is to summarize state of the art standards, practices, and procedures for the design and construction of feed milling facilities, to improve the current knowledge base of the industry.
Overview of a Feed Milling Facility Feed is produced for a number of animal types including (1) poultry, (2) swine, (3) ruminants, (4) pets, (5) fish, and (6) other small mammals. Feed mills may be single- or multiple-species production facilities. In the United States, feed mills tend to be single species feed mills. In Canada, however, it is more common to have multiple species feed mills. Feed mills can produce pellets, mash or liquid feeds, and these products are shipped in bulk or bagged form. Ingredient storage, conveyance, and proportioning are of utmost importance in a feed milling facility because dozens, if not more, raw ingredients are used in the production of most animal feeds.
In the United States, feed production facilities are regulated by the FDA and are considered true food production operations. The health and safety of our food supply is essential for the health and welfare of all North Americans. Because of this, good design, sanitation, and operational practices are essential for all feed mill operations.
Two major construction materials are used for feed mills: concrete and steel. Concrete feed mills are typically slip formed (a vertical continuous extrusion process), whereas steel feed mills are typically assembled from bolted bin construction, which are sold as modular units, but may occasionally be welded. Welded bins are usually shop fabricated due to the economies of shop techniques. Feed milling facilities typically involve the storage and handling of large quantities of grain, thus large grain storage units of concrete or steel adjacent to the mill are very common.
Major areas of a feed milling facility include the mill tower and associated grinding, mixing, pelleting, and cooling operations. Other areas include receiving, whole grain storage, and the warehouse for the stored products as well as boiler areas. These areas are shown in Figure 1, while Figure 2 depicts the relationship between these components via a general block flow diagram.
3
WarehouseMill Tower
Boiler Building
Receiving
Liquid Storage
Whole Grain Storage
Boot Pit
Reclaim tunnel
Tunnel
Figure 1. Plan view of a typical feed milling facility.
Grain Reciving
Grain Storage
Magnetic Seperator
Grain Cleaning
Milling
Surge Hopper
Mixing
Storage
Conditioning
Pelleting
Screen
Crumbler/Granulator
Pellet Cooler
Storage
Shipping
Storage
Weigh
Bulk Grain by
Storage
Truck or Rail
Other Ingredients
Meal/Mash
Steam
Pellets to Storage
Bagging
Figure 2. General feed mill block flow diagram.
4
Facility Planning Proper facility planning and design is dependent upon a number of long-range planning and financial decisions. Planning is critical to the long range success of an operating facility. Major items of consideration include: (1) long range planning, (2) expansion versus new construction, (3) site selection and design, (4) facility layout and design, and (5) economic considerations.
Long-Range Planning
In long range planning, the owner’s team must look at long term trends in demographics, current production capabilities, as well as consumption trends, to determine the justification for the construction of a new feed milling facility. The management team must also ask if the construction of a new feed milling facility fits into the longer-term strategic goals of the corporation. Finally, the team must examine if the local market can support the existence of a new feed milling facility. Once these decisions have been, made the owner’s team can proceed to the next level of planning, which is the planning of the actual facility.
Expansion vs. New Construction
Occasionally an owner may have an existing facility where the addition of extra storage, or the replacing or upgrading of various processing equipment, may be all that is needed to increase production capabilities to meet market demands. Items that must be considered in the decision to upgrade or improve an existing facility should include a thorough examination of the condition of the existing equipment, buildings, and storage. The local community must have the infrastructure present, or at least the desire, to support the expansion of an existing facility.
Site Selection and Design
If an existing facility cannot be expanded then a new site must be selected and designed. Proper site location can have a major impact on the profitability of a new feed milling facility. Items that should be examined in the planning phase for a facility include transportation, utilities, labor availability, ingredient supplies, and soil conditions. Location of infrastructure such as rail lines and roads can have a major influence on the cost and availability of transportation. Owners may be able to obtain better ingredient pricing via high volume sales with readily available rail service. Locating a facility near a major highway will have the potential to reduce truck shipping costs. Availability of a ready labor supply will ensure that there are people available to operate and maintain the facility. Because mill structures tend to be heavily loaded, soil conditions must be sufficient to support these higher bearing requirements or soil improvements and/or deep foundation systems, such as piling, must be considered, which will increase the capital outlay for a facility.
Facility Layout and Design
Identifying the type of operation (applicable species; mixed feed, pelleted feed, or a combination thereof) can have a major effect on specific operations within the feed milling operation. Determining the desired production capacity needs will allow for optimal selection of plant, storage and equipment sizes. Specific processing issues are more extensively discussed in Rosentrater and Williams (2004).
5
Economic Considerations
Return on Investment (ROI) is the final but most important item in the assessing the potential to design, construct, and operate a new feed milling facility. The management team must determine the discounted cash flow streams in terms of expected revenues and expenses to determine if the decision to build a new facility is economically justifiable.
Life Safety Design Considerations Once the decision to build has occurred and a site is selected, the structural and life safety planning must begin. Federal and state building and safety codes dictate life safety planning issues. These codes are administered at state, local, and federal levels. Federal regulations, such as Occupational Safety and Health Administration standards from Title 29 of the Federal Code of Regulations (NARA, 2004), or state-adopted model codes, such as the International Building Code (ICC, 2000) dictate how facilities are planned and constructed. Highlights of these codes as they relate to feed milling facilities are discussed in the following sections.
International Building Code
The primary model building code in the United States is the International Building Code (IBC) (ICC, 2000), which is the model building code used in forty-eight of the fifty states. The IBC defines a number of life safety-related issues, including occupancy types, construction types, height and floor areas, egress, stairs, access, as well as a number of other major life safety topics. Some of the major items of consideration in the IBC are detailed in the following subsections and additional details are summarized in table 1.
Occupancy Requirements
Chapter 3 of the International Building Code specifies ten different use and occupancy types for structures. Categories range from residential to hazardous industrial. Common use and occupancy types typical for feed milling facilities include: • Group B - Business - The business or laboratory sections of a feed milling facility fall under
this occupancy category.
• Group H-2 - Hazardous – The grinding area of a feed milling facility generates an enormous quantity of dust and has a high risk of explosion hazard. Additionally, many stored agricultural commodities shed fine dust, and under the right conditions are also highly explosive. Portions of the facility with these characteristics are usually categorized as an H-2 occupancy.
• Group F - Factory and Industrial – Non-hazardous building processing operations would fall under this category. Further processing such as packaging, extrusion or pelleting would fall under this category.
• Group S – Storage – This classification is used for the noncombustible storage of agricultural commodities. If the dust in the storage portions of a feed milling operation were non-explosive, it would be classified as category S. Warehousing of packaged materials typically would be classified as a category S.
Construction Type
Construction type influences the overall height of the buildings in a feed milling facility. Five major construction types are defined in the IBC (ICC, 2000). They vary from a highly protected
6
Type I construction to the least protected Type IV construction. They are further divided into subcategories of A and B, which define additional levels of added fire protection. Generally, construction types I and II consist of masonry, steel, and concrete structures. Type III construction has noncombustible exterior walls with interior materials of any material. Type IV construction encompasses heavy timber, and is not commonly used in agri-industry. Type V construction is construction where any combustible or non-combustible material is used for construction that does not fall under Types I to IV.
Height and Floor Area
Allowable heights and floor areas are a function of occupancy type and construction type. The milling area of a Feed milling facility falls under classification H-2 and under the exception 415.7.1, are allowed to be unlimited in height when they are Type I or II construction. The grain storage portion of a feed mill is either classification H-2 or S, and is also allowed to be of unlimited height when construction Type I or II are used. Table 503 in the IBC defines allowable heights and areas for all occupancies. This table is subject to the aforementioned exceptions in chapter 4 of the IBC. For example, where Type I construction is used along with a H-2 occupancy the allowable floor is a maximum of 21,000 square feet per level, but the height restrictions of table 503 do not apply to the storage silos or grain elevators when they meet the criteria of the exception clause. Additionally, allowable heights and areas for business and administrative areas are also located in Table 503.
Location on Site
Location on the site influences the allowable area of the facility as well as the required fire rating of the exterior walls. Grain storage silos for a feed milling facility or other facility cannot be closer than 30 ft to either the edge of a property line or adjacent structures, except where the railroad right of way can run adjacent to the structure. The design engineer should be aware of required rail clearances and work closely with the railroad to define the necessary facility clearances. Other building fire separation distances are defined in Table 602 of the IBC (ICC, 2000) and are related to the fire rating of the exterior walls.
Special Requirements in the IBC
Once the occupancy and type of construction are determined, the design engineer will need to determine special occupancy requirements that relate to the special features of the facility. Chapter 4 of the International building code outlines a number of building code requirements relating to grain elevator and feed mill construction and design. These special requirements are outlined below: • IBC 415.7.1.5 – States that bulk grain storage needs to be 30 ft from lot line or adjacent
structures except at the railroad right-of-way. This requirement allows bulk grain storage to be adjacent to rail lines, which is a major mode of commodity transportation, but keeps the facility away from adjacent structures where the hazards of a dust explosion could cause an injury or property damage.
• IBC 415.7.1 – States that Type I and II construction for feed mills, grain elevators, and similar structures is unlimited. Type I and II construction includes all metal and concrete construction, which thus effectively includes all types of feed mills. For other portions of the facility the design engineer should follow Table 503in Chapter 5 of the IBC.
• IBC 415.7.1.2 – Requires that grinding rooms of 3000 ft2 or less are designed with a 2-hour fire rating. A 4-hour fire rating is required if the grinding room is greater than 3000 ft2.
7
Grinding rooms, which are common in grain and feed facilities must be isolated from other areas of the facility with a fire wall. Often it is necessary to have explosion panels on the exterior of the walls of the facility. Specific requirements are detailed in NFPA 68.
• IBC 415.7.1.4 – Requires dust-tight spouting and conveyor covers. This requirement reflects the inherent explosion hazard in feed milling facilities.
• IBC 415.1.7. – States that feed mill design must follow NFPA 61, 65, 85, 120, 651, 655, and 664, where applicable. The primary standard that is essential to agricultural process facilities is NFPA 61, which relates to the dust handling hazards in agricultural and food process facilities.
Guard Rails
Guard rails are an important feature for roofs, elevated platforms, and mezzanines. Section 1003.2.12 of the IBC (ICC, 2000) states that guard rails are required for open-sided walking surfaces such as platforms, mezzanines, and equipment access platforms. In most industrial situations the guards must be arranged such that a 21-inch diameter sphere cannot pass through the rails. This usually requires the use of a mid rail. The requirements for guards and stair handrails are different, however, and should be reviewed by the design engineer before proceeding with the project.
Stairs
According to IBC (ICC, 2000) section 1003.3.3, a 44-inch wide stairway is required. If the occupant load is less than 50 people, then a 36-inch wide stairway is required. Occupancy loads are defined in section 1003 and table 1003.2.2.2. Stair slopes in the IBC are more restrictive than older building codes and OSHA standards, with a typical 7-inch rise as the maximum rise and with an 11 inch run. For steeper stair slopes, a variance must be obtained. The design engineer should coordinate with the building code officials as early as possible during the project-planning phase to help with this important issue. Additionally, the design engineer should be aware that stair landings are required every 12 vertical feet. This has a substantial impact on the overall height of a structure and the size of the stairwell. Finally, section 1003.3.11 of the IBC gives the handrail requirements for stairs. These requirements vary significantly from guardrails, which were discussed in the previous section.
Exiting and Egress
Exiting and egress are covered in section 1005 in the International Building Code (ICC, 2000). Generally speaking, with occupant loads under 500 people, two exits are required from each floor or area, unless the occupant loads are below those described in Table 1004.2.1. Exit discharge is described in section 1006 and occupancy load is defined in table 1003.2.2.2 of the IBC. Two areas of concern for facility engineers and operators is the egress from the roof of the structure, and from tunnels. Usually in a feed milling facility, a man lift with either an exterior ladder system or an internal stair system provide the two methods of egress from each floor level. Egress ceiling heights should not be less than 7 feet.
Mezzanines
Section 505.1 of the IBC (ICC, 2000) discusses mezzanine design and construction. In general, a mezzanine should not be counted as part of the floor below, and shall not cover more than 1/3 of the floor area of the room it occupies, for purposes of building classification. Similar to other areas of the building, mezzanines are required to have two independent means of egress.
8
Equipment Access Platforms
Equipment access platforms are a special form of mezzanine and are discussed in section 505.5 of the IBC (ICC, 2000). In general, the total of all equipment platforms should occupy less than two-thirds of the building area, and for purposes are occupancy classification, shall not add to the floor area of the building. Moreover, equipment access such as stairs, ladders, walkways, etc., shall not serve as part of the egress system for that building level.
Building Envelope
Feed mill towers and related boiler areas produce an enormous amount of waste heat, thus meet the energy requirements of the building code. Similar to many other process facilities, this waste heat must be discharged to maintain an optimum environment. This heat may at times be recoverable for heating office or warehouse areas of the facility.
Fire Protection
The IBC gives details for additional fire protection, such as sprinklers, for feed mills, but they are impractical and expensive for these facilities, so designs are determined without such provisions. The IBC allows for such trade offs. Other than those requirements given in the referenced NFPA documents, there is little done for additional fire protection.
OSHA OSHA standards are described in Title 29 of the Federal Code of Regulations (NARA, 2004). These standards set workplace safety requirements, and are considered a minimum that must be met for non-public operational areas of facilities. They cover a number of construction-related issues such as access, exits, fixed ladder construction, stairs, ship ladders, guardrails, equipment access, and tunnel construction. Most items relating to constructed facilities are in Section 1910 of Title.29. OSHA standards are also of significant importance to facility operators as they influence a number of operational items related to worker safety. OSHA standards are only enforced if they are more stringent than the controlling building code, however. These issues are outlined in Table 1.
FDA Regulations
Title 21 of the Federal Code of Regulations (NARA, 2004) defines requirements for food-grade facilities. As BSE (i.e., “mad cow disease”) becomes a more serious issue, food-grade feed milling facilities will become the norm. Title 21 dictates wall and floor finishes, construction types, and operating practices for these types of facilities. Additionally, good housekeeping practices and dust accumulation prevention will limit the hazard for dust explosions and improve sanitation. Key to this issue is adding dust sheds to roof beams. Using closed structural steel shapes should be considered in the construction of floors, platforms, and machinery access to prevent dirt from accumulating and infestation from occurring. Additionally, dust control systems should be provided at major ingredient receiving points. Thus, the design engineer plays a crucial role in providing the owner with the ability to accomplish good housekeeping.
9
Table 1. Design matrix for feed mill life safety considerations. Code Item Description of Application Code
Section/Requirements IBC
Occupancy Determined based on the use of the structure. E.g. hazardous, manufacturing, business office. Methods of calculation for mixed occupancy
Chapter 3 outlines occupancy types. Grain elevators are typically group H-2. Further processing may be group F. Business office may be group B. Group S occupancy if dust is non explosive
Detailed Requirements based on Use and Occupancy
Special requirements for the construction of hazardous facilities such as grain elevators or feed mills
Chapter 4 Section 415.7
Types of Construction Based on the materials used and the fire resistance of the components. Most grain elevators are type I and II construction
Chapter 6.
Location on Property Location of the structure on the site. Influences the fire rating. Special lot line distances are defined in section 415.7 for grain storage
Chapter 6, table 602
Floor Area Maximum floor area is a function of construction type and occupancy
Chapter 5, table 503
Height and Number of Stories
Influenced by floor area, construction type and fire protection. See provision 415.7 for height requirements
Chapter 5, table 503
Fire-Resistance-Rated Construction.
Code prescribed requirements for materials and assemblies used to separate adjacent areas and prevent the spread of fire and smoke
Chapter 7
Stairs Design of stair stringers, rise and run, and stair construction details
Chapter 10, section 1003.3.3
10
Table 1 cont. Design matrix for feed mill life safety considerations Code Item Description of Application Code
Section/Requirements Guard rails Design of guard rails,
construction requirements for walkways, stairs, and openings
Chapter 10, section 1003.2.12
Mezzanines Special requirements for mezzanine construction
Chapter 5, sections 505.1 and 505.5
Egress Egress fire ratings, size, occupant loads, arrangement
Chapter 10
OSHA Egress Egress fire ratings, size,
occupant loads, arrangement
Section 1910.37
Tunnels Noted in the egress section. Two methods of exit required.
Section1910.37
Guard rails Structural Design of guard rails, construction requirements for walkways, stairs, and openings
Section 1910.23
Man lifts and Powered Platforms
Section1910.66
Fixed Industrial Stairs Structural design of stair stringers, rise and run, and stair construction details
Section 1910.24
Fixed Ladders Features, clearance, hatches, cages, offsets, and landings
Section 1910.27
NFPA 61 – Dust and Fire Control in Agricultural Facilities
A major design requirement for equipment such as bucket elevators, conveyors, and such.
NA
68 – Guide for venting of Deflagrations
A guide for the design and construction on explosion forces and explosion panels
NA
69 - Standard on explosion prevention systems
A guide for the design of explosion prevention systems
NA
85 - Boiler and Combustion Systems Hazards Code
A guide for the design of boiler system operation and design
NA
11
Supplemental Codes and Standards The International Building Code specifies that specialty facilities must incorporate a number of additional special design provisions. Chief among these is the Nation Fire Protection Association (NFPA) documents outlined below. • NFPA 68 – Guide for Venting of Deflagrations (NFPA, 1998a). This standard relates to the
design and installation of devices and systems to relieve pressures and gasses that result from explosions. This document includes information on the calculation of explosion pressures. This standard relates to a number of industries, but is commonly used in the feed and grain industry to determine bucket elevator venting as well as venting for silos that contain powder-like agricultural substances such as flour. Specifically, for feed mills, this standard would be used for grinding room venting calculations.
• NFPA 69 – Standard on explosion prevention systems (NFPA, 1998b). This standard details the design, construction, operation, maintenance and testing of systems for the prevention of deflagration explosions. This document covers the prevention of explosions by the following methods: (a) control of oxidant concentrations, (b) control of combustible concentrations, (c) explosion suppression, (d) deflagration pressure containment, and (e) spark extinguishing systems.
• NFPA 61 – Standard for the prevention of fires and dust explosions in agricultural and food products facilities (NFPA, 1999). As the name implies, NFPA 61 is a standard that relates to dust explosion safety in agri-industrial process facilities that handle bulk materials. This document covers construction requirements such as egress, interior wall construction, building fire protection, and equipment, including dryers, venting, heat transfer operations, dust control, pneumatic conveying, and building fire protection.
• NFPA 85 - Boiler and combustion systems hazards code (NFPA, 2004). This document applies to the design, installation, operation, training, and maintenance (as related to safety issues) of combustion systems, including single burner boilers, multiple burner boilers, and atmospheric fluidized bed boilers.
12
Occupancy
Construction Type
Floor Area
Height
Egress
Location on Property
Grain Elevator, H or S
Type IType II
Business Office, B
30 ft From Lot Line
Ulimited Height
DetailsStairs 7/11 Unless VarianceMezzaninesGaurds
NFPA OSHA
Table 503
For Type I and II
Two Methods of Egress From Each Area
LaddersGaurd rails
NFPA 61NFPA 68NFPA 69 Ladders
Manlifts
Rail Right of Way, Table 603
Further Processing, F
NFPA 85
Table 503
Occupancy Load
Figure 3. Flow chart of life safety design considerations.
13
Structural Design Considerations Feed mills consist of square and rectangular ingredient bins suspended above the process operations of the mill. These rectangular bins hold the ingredients that are to be mixed and processed, and are generally square or rectangular units 4 to 12 ft in width, and can be of concrete slip-formed construction or modular steel structures. Larger, circular storage silos are used to store whole grain, and are nearly identical to those used in the design and construction of grain elevator facilities. Similar to grain elevators, they can be of corrugated steel bin construction, smooth wall steel bin or of slip-formed concrete construction. Buildings designed to house secondary packaging, processing, and warehousing are closer to conventional industrial construction, and can be of slip-formed concrete, conventional steel stick frame, or precast concrete construction, and are subject to the construction types described in the aforementioned building codes.
Loads
Loads on feed mills arise from a variety of sources, including ingredients, roof and floor live loads, equipment, dead loads, and lateral loads such as wind and seismic forces. Feed milling facilities utilize a large number of ingredients. Therefore the proper definition of their physical properties is critical for proper structural design of the facility. Material handling characteristics and flow properties for common ingredients are provided in Table 2.
Table 2. Properties of common feed ingredients (ACI 313, 1997, Rotter 2002, & Material Storage Systems Inc, 2004).
Description Bulk Density (lb/ft3)
Angle of Repose
(degrees)
Coefficient of Friction for Steel
Coefficient of Friction for Concrete
ALFALFA PRODUCTS Alfalfa meal, dehydrated, 13% 16-18 45 0.6 -
Alfalfa meal, dehydrated, 17% 18-22 45 0.6 -
Alfalfa meal, sun cured, 13% 14 45 0.7 -
Alfalfa leaf meal 14.5 45 0.6 -
Alfalfa stem meal 12 45 0.9 -
Alfalfa pellets, 13% 41-43 30 0.46 -
Alfalfa pellets, 17% 41-43 30 0.46 -
Alfalfa seed 48 21 0.5 -
ANIMAL PRODUCTS Blood meal 38.5 45 1.7 -
Blood flour 30 - 0.6 -
Meat meal 37 45 0.9 -
Tankage (digest) 49 45 0.9 -
Poultry by-prod. meal 34-37 35 0.6 -
Bone meal 50-60 40 1.7 -
14
Table 2 cont. Properties of common feed ingredients (ACI 313, 1997, Rotter 2002, & Material Storage Systems Inc, 2004).
Description Bulk Density (lb/ft3)
Angle of Repose
(degrees)
Coefficient of Friction for Steel
Coefficient of Friction for Concrete
BARLEY PRODUCTS Barley, rolled 21-24 45 0.58 -
Barley, scoured 41 30 0.4 -
Barley, whole 38-43 - .47 to .56 0.58 to 0.7
Barley, ground 24-26 40 0.4 -
Barley, malt 30-31 - - -
BREWERS PRODUCTS Brewers dried grains 14-15 - - -
Brewers grains, spent, dry 25-30 45 0.44 -
Brewers grains, spent, wet 55-60 50 0.6 -
Hops, spent, dry 35 - - -
Malt, wet 60-65 45 0.4 -
Malt, dry 27-32 35 0.4 0.4 to 0.5
Malt meal 36-40 85 0.4 -
Malt sprouts 13-16 - 0.5 -
CITRUS PRODUCTS Dried citrus pulp 20.5 45 1 -
CORN PRODUCTS Corn, whole, shelled 45 30 0.36 to 0.57 0.46 to 0.62
Corn, ground 34-36 35 0.8 -
Corn meal 38-40 85 0.4 -
Corn bran 13 80-45 0.5 -
Corn feed meal 33.5 30 0.7 -
Corn chops (fine) 36-39 - - -
Corn chops (medium) 38-42 - - -
Corn chops (coarse) 40-44 - - -
Corn grits (fine) 40-43 - - -
Corn gluten feed 26-33 - 0.67 -
Corn gluten meal 32-43 30-40 0.6 -
Corn oil meal 33-36 - - -
Corn germ meal 35 - 0.7 -
Hominy feed 25-28 - 0.4 -
Kibbled corn 20-22 - - -
Ear corn chops 35 - - -
15
Table 2 cont. Properties of common feed ingredients (ACI 313, 1997, Rotter 2002, & Material Storage Systems Inc, 2004). Description
Bulk Density (lb/ft3)
Angle of Repose
(degrees)
Coefficient of Friction for Steel
Coefficient of Friction for Concrete
CORN PRODUCTS
Ear corn 28 - - -
Ground cobs 17 - - -
Pop corn, ear 0.28 - - -
Pop corn, shelled 44-45 28 0.374 -
OAT PRODUCTS Oats, crushed 22 - - -
Oats, crimped 19-25 35 0.5 -
Oats, whole 25-35 - - -
Oats, rolled 19-24 - - -
Feed oatmeal 16-32 40 0.4 -
PEANUT PRODUCTS Peanuts, unshelled 17-24 40 0.6 -
Peanuts, shelled 15-19 30 0.4 -
Peanut meal 29 - - -
RICE PRODUCTS Rice bran 20-21 45 0.4 -
Rice hulls 20-21 45 0.4 -
Rice, hulled 45-49 35 0.364 -
Rice, grits 42-45 40 0.4 -
Rice, rough 32-36 40 0.4 -
Rice, polished 30 30 0.35 -
RYE PRODUCTS Rye bran 15-20 30-45 0.8 -
Rye feed 33 - - -
Rye middlings 42 28 0.5 -
Rye shorts 32-33 - - -
Rye, whole 43-45 28 0.4 -
SCREENINGS Grain screenings 30 30 0.4 -
Chaff and dust 20 40 0.8 -
Dust 11 35 0.5 -
16
Table 2 cont. Properties of common feed ingredients (ACI 313, 1997, Rotter 2002, & Material Storage Systems Inc, 2004). Description
Bulk
Density lb/ft3
Angle of Repose
(degrees)
Coefficient of Friction for Steel
Coefficient of Friction for Concrete
SOYBEAN PRODUCTS Soybeans, whole 46-48 - 0.2 to 0.32 0.2 to 0.36
Soybeans, ground 25-34 - - -
Soybean millfeed 25-27 40 0.5 -
Soybean hulls, unground 7-Jun 45 0.9 -
Soybean hulls, ground 20 40 0.8 -
Soybean millfeed pellets 37-38 35 0.6 - Sol. ext. soybean oil meal, 41% 34-36 35 0.5 - Sol. ext. soybean oil meal, 44% 35-38 35 0.5 - Sol. ext. soybean oil meal, 50% 41-42 32-37 0.5 -
Expeller soybean oil meal 36-40 35 0.5 -
VITAMINS Riboflavin 37 - - -
Vitamin A, dry 48 - - -
While several methods currently exist for the determination of the vertical and lateral loads for bulk solids storage structures, the most commonly used equation for loads in North America is Janssen’s equation. This equation is dependent on the coefficient of friction of the material, the bulk density of the material, the lateral-to-vertical pressure coefficient, and the depth of the material. Janssen’s equation represents the at-rest, or static, pressure of the grain in the bin or silo. It is necessary to adjust the pressures for the dynamic effects of filling or discharging of the grain, however. In practice, this is commonly accomplished by using an over-pressure facture. Recommendations for over-pressures vary by country, standard, and material. When the height to diameter ratio is less than 1 it is common for the bin or silo to be classified as shallow, and experience among design engineers has shown that the over-pressure factors can be ignored. It should be noted that shallow bins/bunkers can experience impact; therefore use of impact factors should be considered. Some common over pressure values are shown in Table 3. For reference, Jansen’s equation is given below:
)1( ''
kYR
eq kR µ
µγ −= (1)
Where:
q = vertical pressure (lb/ft2)
R = hydraulic radius (ft)
µ= material coefficient of friction
k = lateral-to-vertical coefficient
γ = material bulk density (lb/ft3)
17
Y = depth of material in bin (ft)
And the lateral wall pressure (psf) due to the bulk solids is given by:
kqp = (2)
Finally, the following equation gives resulting wall friction (lb/ft) due to the bulk solids:
RqYV )( −= γ (3)
Table 3. Recommended over-pressure factors.
Material/Construction Type Minimum Over-Pressure Factor, Cd
Source
Concrete 1.5, 1.4 ACI 313 (1997), ASAE (2001)
Smooth Wall Steel 1.35 min, 1.4 Rotter (2002), ASAE (2001)
Corrugated 1.5, 1.4 Rotter (2002), ASAE (2001) Floor loadings are dependent on the use of each respective area of the feed milling operation. Key areas for consideration include the roof, bin deck, head house, bin bottom floor, mixing and grinding areas, and warehouse floors. Representative floor live loads are provided in Table 4.
Table 4. Typical floor live loadings for feed mill floors.
Area Uniform Load (lb/ft2) Point Load (lbs) Comments
Mill Roof 50 Varies. See comments
Use equipment point loads from equipment, towers and other major point load sources
Mill Head House and Bin Deck
200 concrete
50 steel
Equipment supports for distributors, spouting and other equipment
Access Platforms 50 Varies. Equipment should be considered separately.
Access platforms for mixers, conveyors, and grinding equipment
Warehouse Floor 500 1000 Point loads from pallets and forklifts should be considered
Annex Bin Roof 50 Equipment point loads Temperature cables and equipment towers
Floors 50 to 250 Equipment point loads should be added.
Concrete or steel construction
Stairs 100 1000 OSHA
18
Lateral loads from wind or seismic forces, on bulk storage and feed milling facilities can be significant. They are determined using the provisions in ASCE 7-98 “Minimum Design Loads for Buildings and Other Structures” (ASCE, 1998). Tall slender towers, when subjected to wind, can be highly susceptible to overturning, especially when the tower is empty. Seismic forces can also be a major issue for tall towers if the resisting width and weight is not large enough. Anchorage to prevent uplift or sliding is often needed when lateral forces are high. Similar to buildings, lateral loads will be concentrated at the floor diaphragms.
Foundations
Foundations for mill processing towers generally consist of combined or isolated spread footings or mat foundations. The design of spread footings is well documented and the reader is referred to one of the many introductory concrete texts for design procedures. Foundations for feed mills may be soil-supported or pile-supported. Occasionally, a composite soil/pile foundation system may be used. Common pilings types include: (1) auger cast piling, (2) steel H or pipe piles, (3) caissons, and (4) precast piling. Piling design procedures are discussed in ASCE 20-97 “Standard Guidelines for the Design and Installation of Pile Foundations”. The soil engineering requirements for most feed mills are demanding, and most geotechnical engineers are not familiar with feed milling facility designs or the high pressures and settlements that accompany these rigid structures. The structural engineer designing the facility must be aware of the capabilities of the geotechnical engineers prior to beginning the project. Furthermore, extensive soil preparation may be required for soil-supported mat and spread footings for feed milling facilities.
HP Pipe Auger CastCaisson
Precast
Figure 4. Typical piling types used for foundations.
The soil-supported mat slab analysis uses either the rigid slab method or the finite element analysis. As the name implies, the rigid method involves treating the mat slab as an infinitely rigid body with respect to the distribution of the load for purposes of analysis, thus the applied load is uniform. The analysis is performed using elementary mechanics or plate theory analysis (Timoshenko, 1959) combined with superposition theory. Before an analysis is conducted, however, the design engineer should consider that concrete slip walls are very rigid and tend to drive isolated footings down equally. In the same light, the design engineer should be aware of the possibility of differential settlement for footings supporting steel superstructures. Concrete feed mills typically are taller and cover a large area, thus requiring a large mat foundation. Smaller steel feed mills may require a mat or individual spread footings.
The finite element analysis of a mat foundation involves the use of soil springs and plate elements to determine the distribution of loads. The first step is the determination of the sub grade modulus. Typically the soils engineer will determine the subgrade modulus, but sometimes the structural engineer must estimate the value. Figure 4 shows the commonly
19
accepted relationship between soil bearing strength and spring stiffness. Other important considerations in mat foundation design are the relative density of the finite element mesh, shell element formulation, mat structure stiffness, and a cracked or uncracked section. Each of these variables can influence the distribution of forces through the mat, thus the design engineer must be aware these effects (Ulrich, 1995). Finally, the foundation must checked at critical sections for the effects of flexure as well as one way and punching shear as defined in ACI 318 (ACI 1999). ACI 336 (ACI, 1993) provides recommended analysis and design procedures for combined footings and mats.
Allowable Soil Bearing, psf5000 10000
100
200
300
15000
Figure 5. Relationship between soil bearing capacity and soil spring constant (PCI Handbook,
2003)
Wall Construction
There are four major types of bins used for the storage of major and minor ingredients in a feed milling operation. They are (1) square metal bins, (2) round metal bins, (3) concrete silos, and (4) square concrete bins. For major ingredients, such as whole grain, round silos constructed from concrete or corrugated steel are used as storage bins (figure 1). For these, the primary design considerations are the hoop tension on the walls and the vertical wall compression. Eccentric discharge can also have a major influence on the design of the silo. For a complete discussion on circular silos the reader is referred to Williams and Rosentrator (2004).
Processing of ingredients occurs in the mill structure (figure 1). Almost all mill towers are rectangular. Concrete mill structures are fairly large and the walls typically need stiffening using vertical wall pilasters that extend the entire height of the structure. Bins are constructed integrally with the outer mill shell, and extend about 1/3 the height of the tower, centered approximately at mid-height. The basic wall thickness below the pressure zone is determined using the span-to-thickness ratios defined in chapter 14 of ACI 318 (ACI, 1999); bin walls span horizontally between the vertical pilasters. As most vertical mill towers are constructed using slip form construction techniques, the floors are not in place at the time of construction. Thus it is a common for the construction load case to control. Most design engineers consider the pilasters as the main vertical load carrying members and also use the pilasters to transfer lateral force in the floor diaphragms. Rectangular bins are designed for axial (tension), flexural, and shear forces using the provisions of ACI 318 (ACI 1999). Forces are determined using any frame analysis software assuming a horizontal load distribution for a unit height.
For the design and analysis of rectangular metal bins, there are two common design
20
procedures. The first approach is to use linear elastic design of the bin walls. This method is detailed in Gaylord and Gaylord (1984) and Troitwsky (1982), and is extremely conservative. The second method involves the use of large deflection theory for the design of the bin structure. Although structural efficiency is increased using this method, there will be noticeable deflections of the bins and hoppers. Details of these procedures are included in Gaylord and Gaylord (1984). Both methods of analysis and design are based on two-way load distribution. Regardless of the method of design, it is essential to include a vertical column or built-up member at the intersection of bin sheets to help transfer vertical wall friction forces to the base of the bin cluster. Metal bin clusters are supported on large steel framework support structures with columns placed under the intersection of every second or third bin cluster. Transfer beams support the bins between the columns.
Eccentric discharge is an issue that design engineers of both concrete and steel rectangular bins have to contend with. Several methods for computing eccentric effects have been proposed. The method the authors prefer is the imaginary bin method (Troitwky, 1982), whereby an increased pressure, pe , is added to the design pressure for the sides that are affected by the eccentric discharge:
pe = pi-ph (4)
Where:
pi = lateral static pressure at depth H in the imaginary bin (psf)
ph = lateral static pressure at depth at y equals h in real bin (psf)
And the design pressure ph,des is defined as:
Ph,des = Cdph+y/h(pe) (5)
Where:
Ph des = design lateral pressure (psf)
Cd = over pressure factor (Table 3)
Y = depth at point of concern (ft)
H = height of bin or silo (ft)
The imaginary bin properties are determined as shown in Figure 6.
21
a + 2ea
a/2 + ea a/2 + ea
Imaginary Bin size
b/2
b
b/2
ea
Original Bin
Figure 6. Bin schematic showing relationship between eccentricity and imaginary bin size. (Troitwky, 1982)
Pits and Tunnels
Boot pits and tunnels form the underground portion of the reclaim system for the whole grain storage and at the load out portion of a feed milling facility. Because they are either directly below or adjacent to the mat slab (mill or whole grain storage), there are high roof pressures and lateral surcharges on the walls from the grain pressure above. These pressures are defined by Janssen’s equation (ACI 1997) and can total into the thousands of pounds per square foot. Because of this, these elements must be robust. Reclaim tunnels are comprised of slabs and walls, and must meet the provisions of ACI 318 (ACI 1999) for walls and slabs. Figure 7 shows a typical reclaim tunnel section with surcharge.
Figure 8 shows a typical horizontal section of a boot pit. Depending on geometry, boot pit walls can span either horizontally or vertically. When loads are too high, the span can be segmented by using a vertical pilaster. Utilization of continuity for walls can be used to increase bending efficiency. Boot slabs are designed as two-way plate elements using two-way plate formulas (Timoshenko, 1959). Similar to tunnels, the elements of the boot pit must meet the design requirements of ACI 318 (ACI 1999) for slabs and walls.
22
Floor load, q*1.7
qk*1.7
(qk + γhk)1.7
qk*1.7
(qk + γhk)1.7
h q = floor pressure basedon jansens equation (ksf)
k = lateral-to-verticalcoefficient
γ = Soil density(ksf)
1.7 = load factor
Flexural Steel
Dowels
Temperature and shrinkage steel
Temperature andshrinkage steel
Figure 7. Typical reclaim tunnel section, based on ACI, 1999
Wall thickness 24" or less
Pilaster is used in high pressure regionsOr where spans are large
Soil pressure
q = floor pressure basedon jansens equation (ksf)
k = lateral-to-verticalcoefficient
γ = Soil density(ksf)
1.7 = load factor
(qk + γhk)1.7
Temperature andshrinakge steel
Postive flexural steel Negative flexural steel
Pilaster flexural steel
Soil and surchargepressure
Figure 8. Typical boot pit horizontal section.
23
Hoppers
In feed milling facilities, regardless of whether the structures are of concrete or steel construction, almost all hoppers are suspended hoppers. Because most bins are square or rectangular in cross-section, the hoppers are pyramidal in shape. Pyramidal hoppers are subject to bending forces and membrane forces acting simultaneously. ACI 313 (ACI 1997) and Gaylord and Gaylord (1984) provide specific details on the design and analysis of hoppers.
Other factors that must be accounted for in the design of hoppers include switching forces at the hopper and wall interface if the discharge is mass flow in nature. ACI 313-97 (ACI, 1997) and Gaylord and Gaylord (1984) contain extensive discussions on mass flow forces. Furthermore, the design engineer will also need to consider how to suspend and support elevated hoppers.
Floors
Elevated floors in feed milling facilities primarily support equipment for conveying, grinding, mixing, chilling, extruding, and other processing operations. These floors therefore experience moderate distributed and point loads. The design engineer should also be aware of the need for proper equipment clearances, as well as the possibility of severe vibratory loads. Table 4 shows a number of recommended minimum loads for a number of use areas. Roofs experience equipment, snow, and live loads. On steel feed mills the roofs are constructed using structural steel and for slip formed concrete feed mills concrete roof beams and slabs are formed using the slip forms as the roof beam and slab form.
Scales Scales are commonly used for incoming and outgoing products at a feed milling facility. Typically, there are two major types of scale construction. These are pit less, and pit-type scales. Even with full trucks with loads at or above 80 kips, the pressures under scale pit foundations rarely rise above 2 ksf. Often times pit walls may not be supported at the top and will instead be cantilevered. Coordination with the scale vendor to determine the reactions and geometric requirements for the scale are critical for proper design of these structures.
Towers
Heights of equipment support towers on feed mills are dictated by the minimum flow angles of the materials being distributed by the spouting to the storage bins. Structural towers support a variety of equipment such as bucket elevators, scalpers, screeners, bulk scales, distributors, and spouting. In addition there are ladders, ladder cages, worker access platforms, and drive torque arm forces that must be resolved into the structure.
Most structural towers are of braced-frame construction, with “X”, “K”, and “V” forms most commonly used, but other admissible forms of structural bracing can also be used. The design engineer is reminded to review the requirements of the building related to bracing in seismic zones, as some bracing configurations are not allowed in the more restrictive seismic design categories. Generally the wind load for the tower must be checked for the two orthogonal directions, plus the two diagonal directions, and the wind load pressure is determined for the projected face. Tower anchorage is often challenging because the towers are tall, but the anchorage often occurs on narrow concrete slip formed walls which will limit the breakout cone
24
of the concrete. One way to work around this issue is to ensure that there is special supplemental steel that intersects the plane of rupture on the wall.
Summary This paper summarizes design considerations and procedures related to the planning, construction, and operation of feed milling facilities. In particular, strategic planning, life safety, and structural provisions were discussed. Design engineers and educators should find this paper useful.
References ACI. 1997. Standard Practice for the Design and Construction of Concrete Silos and Stacking
Tubes for Storing Granular Materials. ACI 313-97. Detroit, MI.: American Concrete Institute
ACI. 1999. Building Code Requirements for Structural Concrete. ACI Publication 318-99. Detroit MI.: American Concrete Institute.
ACI 336 1993. Suggested Analysis and Design Procedures for Combined Footings and Mats. ACI Publication336-93. Detroit MI.: American Concrete Institute
ASAE. 2001. Loads Exerted by Free-Flowing Grain on Bins. ANSI/ASAE EP433. St. Joseph, MI.: American Society of Agricultural Engineers
ASCE. 1997. Standard Guidelines for the Design and Installation of Pile Foundations. ASCE 20-97. Reston, VA.: American Society of Civil Engineers.
ASCE. 1998. Minimum Design Loads for Buildings and Other Structures. ASCE 7-02. Reston, VA.: American Society of Civil Engineers.
Gaylord, E. H. and C. N. Gaylord. 1984 Design and Storage of Steel Bins for Storage of Bulk Solids. Englewood Cliffs, NJ: Prentice Hall, Inc.
ICC. 2000. International Building Code. Falls Church, VA.: International Code Council. Material Storage Systems Inc. 2004. Apparent Densities of Dry Feed Ingredients. Available at:
http://www.mssistorage.com/html/densities.html NARA. 2004. Code of Federal Regulations. United States National Archives and Records
Administration. Available at: http://www.access.gpo.gov/nara/cfr/cfr-table-search.html NFPA. 1998a. Guide for Venting of Deflagrations. NFPA 68. Quincy, MA.: National Fire
Protection Association. NFPA. 1998b. Standard on Explosion Prevention Systems, NFPA 69, National Fire Protection
Association, Quincy, MA. NFPA. 1999. Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food
Products Facilities. NFPA 61. Quincy MA: National Fire Protection Association. NFPA. 2003. Boiler and Combustion Systems Hazard Code. NFPA 85. Quincy, MA: National
Fire Protection Association. Precast/Prestressed Concrete Institute. 1999. PCI Design Handbook. Chicago Illinois Rotter, J.M 2002. Guide for the Economic Design of Circular Metal Silos. New York, NY. Spon Press. Rosentrater, K. A. and G. D. Williams. 2004. Design Considerations for the Construction and
Operation of Feed Milling Facilities. Part II: Process Engineering Considerations. ASAE Paper No. 044144. St. Joseph, MI.: ASAE.
Timoshenko, S and S Woinowsky-Krieger. 1959. Theory of Plates and Shells. McGraw-Hill.
25
Troitwsky, M. S. 1990. Tubular Steel Structures – Theory and Design. James :Lincoln Arc Welding Foundation
Ulrich, E. J; ed., 1995. Design and Performance of Mat Foundations – State-of-the-Art Review. ACI. SP-152.
Williams, G.D and K. A. Rosentrater. 2004. Design Considerations for the Construction and Operation of Feed Milling Facilities. Part I: Planning, Structural, and Life Safety Considerations. ASAE Paper No. 044145. St. Joseph, MI.: ASAE.
Williams, G. D. and D. R. Bohnhoff. 2004. An Overview of North American Agri-Industry. ASAE Paper No. 044173. St. Joseph, MI.: ASAE.
