1. Introduction
Chapter 16 deals with headframes for mine hoists. Headframes are built with timber, steel, concrete, and a combination of steel and concrete. Wood headframes are no longer built in industrialized countries, but they still have application in the developing world. The question as to whether a steel or concrete headframe will be best for a particular project is a problem often encountered; therefore, this chapter examines this dilemma in detail.
The height of a headframe, for purposes of engineering, is defined as the vertical distance from the collar elevation to the center-line of the highest head sheave when ground mounted hoists are employed. For tower mounts, it is the distance from collar to the centerline of the highest hoist wheel or drum.
As a general rule, steel headframes are employed for drum hoists and ground mounted friction hoists while concrete headframes are employed for tower mounted friction hoists, sometimes including small drum hoists mounted in the same tower. No standard designs exist for steel or concrete headframes – each one is custom built.
2. Rules of Thumb
Wood Headframe
• The maximum height of a wood headframe is 110 feet. The maximum rope size for a wood headframe is 1.25 inches diameter, which corresponds to an 8-foot or 100-inch diameter doubledrum hoist. Source: Jack de la Vergne Steel Headframe
• A headframe (for a ground mounted hoist) should be designed with the backlegs at an angle of 60 degrees from the horizontal and the rope flight from the hoist at an angle of 45 degrees. Source: Mine Plant Design, Staley, 1949
• It is better to design a headframe (for a ground mounted hoist) such that the resultant of forces from the overwound rope falls about 1/3 the distance from the backleg to the backpost. Source: Mine Plant Design, Staley, 1949
• No members in a steel headframe should have a thickness less than 5/16 of an inch. Main members should have a slenderness ratio (l/r) of not more than 120; secondary members not more than 200. Source: Mine Plant Design, Staley, 1949
• Main members of a modern steel headframe may have a slenderness ratio as high as 160 meeting relevant design codes and modern design practice. Source: Steve Boyd
Steel Headframe versus Concrete Headframe
• The cost of a steel headframe increases exponentially with its height while the cost of a concrete headframe is nearly a direct function of its height. As a result, a steel headframe is less expensive than a concrete headframe, when the height of the headframe is less than approximately 160 feet (at typical market costs for structural steel and ready-mix concrete).
Source: Jack de la Vergne
• At the hoist deck level of a tower mount headframe for Koepe hoisting, the maximum permissible lateral deflection (due to wind sway, foundation settlement, etc.) is 3 inches. (This may favor a concrete headframe.) Source: R. L. Puryear
• A concrete headframe will weigh up to ten times as much as the equivalent steel headframe. (This may favor the steel headframe when foundations are in overburden or the mine site is in a seismic zone.) Source: Steve Boyd Headframe Bins
• To determine the live load of a surface bin for a hard rock mine, the angle of repose may be assumed at 35 degrees from the horizontal (top of bin) and the angle of drawdown assumed at 60 degrees. Source: Al Fernie
• A bin for a hard rock mine will likely experience rat-holing (as opposed to mass flow) if the ore is damp, unless the dead bed at the bin bottom is covered or replaced with a smooth steel surface at an angle of approximately 60 degrees from the horizontal. Source: Jennike and Johanson
• The live-load capacity of the headframe ore bin at a small mine (where trucking of the ore is employed) may be designed equal to a day’s production. For a mine of medium size, it can be as little as one-third of a day’s production. For a high capacity skipping operation, the headframe should have a conveyor load-out, either direct to the mill or elevated to separate load-out bins remote from the headframe. A conveyor load-out requires a small surge bin at the headframe of live load capacity approximately equal to the payload of 20 skips. Various Sources
3. Tricks of the Trade
• For Koepe (friction) hoist installations of modest capacity and shaft depth, a steel headframe tower can be less expensive than concrete. Major installations usually employ a concrete tower, but each project is unique and should be considered on its own merits. Source: V. B. (Jim) Cook
• A steel headframe that is custom-designed to provide a high clearance at the collar entry requires a rigid frame design for the large opening. This is only accomplished with a increase in design hours as well as increasing the weight (and hence the fabrication cost) of a steel headframe. Source: Bill Reid
• A steel headframe with backlegs built from a pipe section is aesthetically pleasing and simple to fabricate, but the top connection is a significant problem that requires an in-depth structural analysis and great difficulty in fabrication. Source: Laddie Malkiewicz
• A steel headframe with backlegs built to the shape of a box girder in the style that is popular in Germany is aesthetically superior, but when built to North American structural design codes, it becomes an expensive alternative. Source: Laddie Malkiewicz
• For a steel headframe less than 100 feet in height, it may be economical to build the backlegs from a wide flange or girder sections. A higher headframe is most economically served with backlegs built to a truss design. Source: Steve Boyd.
• When selecting a friction hoist for a tower mount in a steel headframe, it is important to have an over-hung direct drive for the hoist. In this way, the hoist is suspended at two points rather than three. Source: Gerald Tiley
• It is a practical certainty that the cladding on a steel headframe will sustain local damage at some time during the life of the mine – and cladding can be difficult to match several years down the road. When purchasing the cladding, be sure to order some extra for dedicated inventory. Source: Eric Seraphim
• A concrete headframe is better designed with a widened corridor on the load-out side to facilitate skip removal. Source: Leo Roininen
• A concrete headframe designed to a circular rather than rectangular cross-section will save on concrete quantity and reduce the wind load by 35%. Source: Harry Braun
• The aesthetics of a concrete headframe are improved with the provision of vertical flutes in the concrete. It is a simple matter to incorporate flutes into the slip forms, but the price paid is extra concrete equal in thickness to the depth of the flute. Source: Harry Braun
• A new concrete headframe should be designed to include plastic pipe inserts in the poured concrete to facilitate the use of explosives for ultimate demolition when the mine site is eventually reclaimed. Source: Peter R. Jones
• Underground headframes are often best served by friction hoists, the components of which are smaller and easier to transport underground. Source: Jack de la Vergne
• Where an underground headframe is to be lined with concrete and must be enlarged in crosssection from a circular concrete shaft, the simplest means for construction is to put a dutchman in the shaft forms to increase the diameter and use these forms to build the headframe. Source: Chris Hickey
• The height of a headframe designed for skip hoisting is often later found too short for shaft sinking, where it is planned to dump shaft muck from sinking buckets into the permanent bin. Source: Jim Tucker
• The sheaves on a headframe designed for ground mounted drum hoists should be oriented such that the rope flights aim at the center of the drum face. Source: Largo Albert
• Where the flight of rope between the hoist and headframe is of such a length and trajectory that intermediate idler sheaves are necessary to support the hoist rope, the suspended distances of hoist rope should be made unequal to dampen the effects of vibration (rope whip). Source: William Staley
• Anyone involved with the installation of a friction hoist in a headframe tower would be wise to consider the early installation of the headframe passenger elevator of paramount importance. Source: Roy Lonsdale
4. Steel Headframe versus Concrete Headframe
Following are the advantages of a steel headframe.
• A steel headframe is usually less expensive than a concrete headframe.
• A steel headframe is more adaptable to modifications.
• A steel headframe is considerably lighter and, therefore, requires less substantial foundations.
• A steel headframe is simpler to design (and design errors are less likely) using off-the-shelf design programs.
• Construction errors and blunders are more readily remedied on a steel headframe than on a concrete headframe.
• A steel headframe is more readily adapted to design against high seismic loads in earthquake zones.
• Steel erection is convenient to interrupt for statutory holidays or stormy weather while slip forming of concrete is not.
• Quality assurance is simpler with a steel headframe whose components were milled and fabricated under “shop conditions” as opposed to a concrete headframe where quality control is more concerned with “field conditions.”
• A steel headframe is simpler to re-plumb if differential settlement occurs at the foundations, particularly if the backleg base connections are designed to be adjustable (as is the case for frozen shafts).
• At mine closure, a steel headframe is simpler to demolish and its components may have scrap value.
Following are the advantages of a concrete headframe.
• A concrete headframe requires less maintenance and is less susceptible to corrosion.
• Little ready-mix concrete is wasted during construction of a concrete headframe as opposed to a steel headframe, where there is waste of steel from (1) scrap ends during fabrication, and (2) upgrading the size of substituted members sizes not immediately available at the time of fabrication.
• Reinforced concrete is not subject to residual stresses from manufacture or fabrication.
• A concrete headframe provides an enclosure upon construction while a steel headframe requires insulated cladding to be weather tight.
• A concrete headframe provides better opportunity for architectural aesthetics in the opinion of most designers.
• A concrete headframe is less susceptible to vibration.
• A concrete headframe is less susceptible to sway during high winds.
• Except for remote locations, ready mix concrete and rebar is almost always readily available on short notice at a predictable cost. Structural steel is sometimes in short supply and the price of fabricated steel is volatile.
• A concrete headframe designed to accommodate a ground-mounted hoist is most often designed without backlegs, saving desirable real estate.
• A concrete headframe is less susceptible to damage from run-away vehicles, wayward mobile crane booms, etc.
5. Weight of a Steel Headframe
The weight of a steel headframe must be known to estimate its cost. The weight is dependent primarily on the height. Height is discussed in Chapter 16.6. The weight of a steel headframe can be estimated from the following formulas:
For a single production hoist1 W1 =0.12H3(D/100)2
For a single production hoist2 W1 = 1.75H2.45
For a separate skip and service hoist1 W2 =1.20 W1
For a separate skip and service hoist2 W2 =1.55 W1
Where W is in Lbs., H is the height of headframe in feet, and D is the drum diameter of the larger hoist in inches. Where the headframe is designed to support stage sheaves for sinking a circular concrete shaft, its weight may approach that required for a separate skip and cage hoist even though only one hoist is planned for the production phase.
Example
Estimate the weight of a steel headframe equipped with both one and two ground mounted double drum hoists.
Facts:
1. The headframe is 128 feet high
2. The headframe is first designed for a single ten-foot diameter double drum hoist
3. The headframe is then designed with the addition of a cage hoist
Solution:
Single hoist
Weight = 181 tons by the first formula and 127 tons by the second formula
Two Hoists
Weight = 217 tons by the first formula and 197 tons by the second formula
Note
The wide variation in results obtained from the two formulas confirms that there is no simple means to accurately estimate the weight of a steel headframe in advance. In the absence of better information, it is suggested that the results of the two formulas be averaged to obtain an approximate value.
1. O’Hara and Suboleski, Costs and Cost Estimation, SME Mining Engineering Handbook, Ed II.
2. Jack de la Vergne
6. Height of a Steel Headframe
Since the weight of a steel headframe is directly related to the height, it is required to first estimate the height of the headframe, which in this case is defined by the distance from the collar to the centerline of the highest sheave.
The height may be calculated from the following published formulas.
Headframe height3 = 0.25D + 5.5(D/100)3 + 6.3T1/3
Headframe height4 = 8.0T0.3 + 1.2V1/2
Where
D = drum diameter (inches)
T = daily production (short tons)
V = hoist rope speed (fpm
Example
Determine the headframe height in the following case.
Facts:
1. Hoist drum diameter of 10 feet
2. Production 3,000 short tpd
3. Rope speed 2,400 fpm.
Solution: Headframe height = 128 feet by the first formula and 147 feet by the second one!
As can be seen in Table 16-1, the headframe height can vary significantly for a particular application depending on the degree of overwind clearance desired, bin capacity, etc. The variance provides good evidence that headframe height should be determined on a case-by-case basis (with due consideration to the exponential cost increase of a higher headframe). It also explains why the quick estimate of headframe weight performed in the previous section could not be accomplished with accuracy.
3. O’Hara, Allan, Quick Guides to the Evaluation of Orebodies, CIM Bulletin, February, 1980
4. O’Hara and Suboleski, Costs and Cost Estimation, SME Mining Engineering Handbook, Ed II.
Table 16-1 Height of Steel Headframe for a 10-foot (3m) Double Drum Production Hoist
