Dynamics of Impact Machines

Geotechnical Earthquake Engineering and Soil Dynamics IV

GSP 181 © 2008 ASCE

Dynamic effects of impact machine foundations Mark R. Svinkin, Member, ASCE1 1

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VIBRACONSULT, 13821 Cedar Road, #205, Cleveland, OH 44118-2376; PH (216) 397-9625; FAX (216) 397-1175; [email protected]

Abstract Foundations for machines with impact loads are widespread powerful sources of industrial vibrations. These foundations mostly transmit vertical dynamic loads on the ground and generate ground vibrations which may harmfully affect surrounding buildings. Dynamic effects range from serious disturbances of working conditions for sensitive devices and people to visible structural damage. Natural frequency of vertical machine foundation vibrations and complete vibration records of ground and structure vibrations can be predicted prior to installation of machine foundations. Diverse measures can be used to mitigate dynamic effects of impact machine foundations.

Introduction Various machines with impact or shock loads are used for production processes at plants and in industrial buildings. As a rule, such machines are installed on massive concrete foundations. Forge and drop hammers are most powerful machines producing impact loads. Forge hammer production is usually accompanied with high vibration levels of ground vibrations because substantial dynamic loads are transmitted on hammer foundations, and these vibrations may detrimentally affect adjacent and remote structures, sensitive equipment and people. It is likely that structure damage caused by vibrations may occur in close proximity of the dynamic sources. Nevertheless, unacceptable structural vibrations may also be induced at long distances from the sources due to the dynamic effect of low-frequency ground vibrations. Therefore, it is important to predict ground and structure vibrations before erection of foundations under machines with impact loads and consider possible outcomes of vibration effects in the design stage of machine foundations. Knowledge and experience in understanding the causes of vibration effects of impact machine foundations can be helpful in prevention of detrimental structural vibrations. Each construction site is unique, and vibration mitigation measures before or after construction of machine foundations should be correctly applied at a site because it is possible that eliminating one dynamic excitation can trigger another one. The paper is based on analysis and generalization of numerous case studies.

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Impact Machine Foundations as Sources of Vibrations

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Impact machines generate intensive dynamic forces which comprise a single pulse of arbitrary form and relatively short duration. Such dynamic loads are of great importance in the design of impact machine foundations and assessment of vibration effects on adjacent and remote structures.

Impact Machine Foundations There are various forge and drop hammers. In forge shops, two major types of hammers are used: a counterblow hammer (proper) and a short-stroke drop hammer. The former machine provides free forging operations. The latter machine also called drop hammer for die stamping is applied to the precision of blows required in forging. Besides, diverse punch-presses such as sizing presses, hydraulic presses and others are employed for production of machine parts at industrial plants. Two other hammers put in practice to remake steel scrap heaps. Sizeable drop hammers break scrap iron, and press-hammers are used to compress and pack lightweight steel scrap. Research studies of hammer foundation dynamics have been accomplished by Rausch (1950), Barkan (1962), Novak (1987), Prakash and Puri (1988) and others. Each forge hammer has two major parts: an anvil and a frame. For counter blow hammers, footings under the frame are paced on the anvil foundation at both sides of the anvil with 2-3 cm layers of roofing felt between the frame footing and the anvil foundation. Short-stroke drop hammers are usually installed on a single concrete block to support the anvil and the frame. Such a design decreases stresses in hammer foundations. Also, it is possible to meet the old-designed anvil foundations separated from foundations under the frames. Such separation can result in substantial settlements of the anvil foundation. Square timbers are used for pads under the anvil. The pad thickness of 0.1 – 1.2 m depends on the weight of hammer dropping parts. Sizeable drop hammers are installed for breaking scrap iron and large iron blocks. These hammers generate the great energy during impacts and have large foundations with upper parts around the anvils for protection from flying iron pieces. Layers of timber, iron chips and steel plates are used for pads under the anvils and the pad thickness is about 2 m. Foundations under the press-hammers are relatively small and the anvils are usually installed directly on the foundations without pads. Punch-presses are installed directly on their foundations.

Dynamic Loads Transmitted on the Ground For impact machine foundations as the vibration sources, it is important to determine what major foundation vibrations are transmitted on the ground and how these vibrations may have effects upon adjacent and remote structures.

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Most hammer foundations are undergone only vertical vibrations under applied centered impacts, but some machines with impact loads can cause vertical and rocking foundation vibrations. An experimental study showed that rocking foundation oscillations do not affect soil vibration records with distance from the machine foundation (Figure 1). Identical vertical impact loads with different eccentricity produced vertical foundation vibrations with the frequency of 20 rad/s in one event and rocking foundation vibrations with the frequency of 135 rad/s in another event. However, these impact loads produced similar ground vibrations at a distance of 43 m from the foundation for a drop hammer. Obviously, only vertical foundation vibrations have to be considered for analysis of impact machine foundations as sources of industrial vibrations. The hammer foundation and the anvil are modeled as lumped-mass systems with one or two degrees of freedom. In a reality, an anvil mass is substantially less than a foundation mass and stiffness of the anvil pad is much larger than soil stiffness under the hammer foundation. Therefore in most cases, the hammer foundations respond to impact loads generated by hammers as a SDOF system. Normalized responses of the hammer-foundation-soil systems are presented in Figure 2 for four foundations under different machines producing impact loads: a press-hammer with the ram mass MR=4 tonnes and the foundation base area AFB=12.3 m2, a short-stroke drop hammer with MR=7.25 tonnes and AFB=80 m2, a counterblow hammer with MR=6 tonnes and AFB=58.8 m2 and a sizeable drop hammer with MR=15 tonnes and AFB=158 m2. It can be seen that the responses of three hammer foundations are represented by SDOF transfer functions which almost coincide with the corresponding theoretical transfer functions for which parameters were determined from experiments. Only for the counterblow hammer foundations, a transfer function represents the system with two degrees of freedom but with the domination of the first shape. It is acceptable for practical goals to consider the hammer-foundation-soil system as SDOF. 3

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Variables of Machine Foundation Vibrations Variables of hammer foundation vibrations presented in Table 1 for three groups of hammer foundations were gathered on the basis of published data (Rausch 1950; Barkan 1962; Scheglov 1960; Klattso 1965; Glazyrin and Martyshkin 1971) and studies performed by the writer (Svinkin 1980 and 1995). The first and second groups represent hammer foundations installed on the ground without vibration isolation. These groups gathered foundations under hammers depending on the mass of hammer rams: 5-25 tonnes range for the first group and below 5 tonnes for the second group. Natural angular frequencies of these foundations are in limits of 40-90

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rad/s that correspond the periods of free foundation vibrations between 0.16 and 0.07 s. Because of the duration of ram impacts on the anvil is approximately 0.01 s (Rausch 1950), the impact loads on hammer foundations can be considered as instantaneous loading. The impact loads induce transient hammer foundation vibrations consisting of 12 cycles with large damping. Foundation vibrations are transferred onto the ground.

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Table 1. Variables of Hammer Foundation Vibrations (modified from Svinkin 1995) Groups of Foundations 1

2

3

Variables of Hammer Foundation Vibrations Forge Hammers Large forge hammers with ram mass between 5-25 tonnes Forge hammers with ram mass less than 5 tonnes Vibration isolated forge hammers

Frequency Displacement rad/s mm 40-60 0.4-1.0

Velocity cm/s 2.0-6.0

Acceleration cm/s2 120-420

Energy Transferred onto Soil kJ 0.8-5.9

60-90

0.3-1.0

1.9-8.8

120-980

0.06-1.9

19-38

0.1-0.7

0.4-1.6

14-17

0.01-2.8

Values of the frequencies of free vertical foundation vibrations in certain degree depend on the capacity of forge hammers. The masses of hammer rams and the frequencies of free vertical foundation vibrations are shown in Figure 3. It can be seen the trend that the larger mass of the hammer ram the lower frequency of free vertical foundation vibrations. Foundations for powerful forge hammers have the lowest frequencies, and the highest frequencies were found for foundations under forge hammers with the ram mass of 1-3 tonnes. This phenomenon has a reasonable explanation. It is obvious that the larger ram mass requires the larger foundation base area, and the larger volume of soil mass is involved in hammer foundation vibrations. An enlargement of the foundation base area increases the soil stiffness under the foundation base area. However, an augmentation of the soil mass is greater than that of the soil stiffness, and consequently it results in decreasing the frequency of free vertical foundation vibrations. The third group in Table 1 renders vibration isolated foundations for forge hammers. A concrete block with a hammer is mounted on vibroisolators for which springs and dashpots are used. Records of free block vibrations from hammer impact are similar to 5

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GSP 181 © 2008 ASCE

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low frequency damped sinusoid with small damping. The block vibrations are transmitted onto the foundations and induce elastic waves in the ground.

It can be seen from Table 1 that displacements are in the similar ranges for all three groups of hammer foundations. The maximum energy transferred onto the ground and big values of velocities and accelerations are observed for foundations under large forge hammers. The greatest velocity of 8.8 cm/s and acceleration of 980 cm/s2 were obtained at foundations for relatively small forge hammers in group 2 because of comparatively high natural frequencies of these foundations. The minimum values of vibration variables are related to vibration isolated hammer foundations. In addition to information about forge hammers and their foundations presented in Table 1 and Figure 3, sizeable drop hammers have somewhat different values of dynamic loads and variables of foundation vibrations. The maximum mass of dropping weight is 15 tonnes and the maximum dropping height is 30 m. Frequencies of free vertical foundation vibrations are in limits of 3-8 Hz. Maximum displacements of vertical and rocking foundation vibrations are 3 and 6 mm respectively. Accelerations can reach values up to 600 cm/s2. Foundations under sizeable drop hammers can transfer much energy up to 35 kJ onto the ground. This considerable amount of energy is 6-14 times higher than energy transferred onto the ground from foundations under hammers with big ram masses. Displacements of machine foundation vibrations can be calculated using known procedures available in Barkan (1962) for foundations under forge hammers and sizeable drop hammers, in Svinkin (1993) for press-hammer foundations, and in Svinkin (1982)

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for punch-press foundations. The permissible displacement values are 1.0-1.2 mm for forge hammer foundations and 0.25-0.50 mm for punch-press foundations.

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Vibration Effects on Structures Forge shop structures, adjacent and remote buildings may be affected by vibrations generated by operating impact machines. Vertical oscillations of impact machine foundations induce elastic waves in the soil medium which trigger vertical and horizontal ground vibrations.

Dynamic Settlements Vertical ground vibrations from impact machine foundations in sand soils can be the cause of non-uniform dynamic settlements of column footings in forge shops. Column footings are usually designed for static loads transferred on the ground without taking into account the dynamic loads from ground vibrations which increase the pressure on the ground. According to Table 1, accelerations of hammer foundations may reach the value of 980 cm/s2 or 1.0 g, and consequently the real pressure from column footings on the ground will be up 2 times higher than the static pressure. For foundations under impact machines, this effect is less important because the design of machine foundation provides a smaller static pressure on the ground in comparison with structure footings which support only static loads like column footings. Accelerations attenuate very fast with distance from the impact machine foundations. Because of attenuation of vertical ground vibrations, dynamic loads under column footings are diverse and that may provoke additional differential settlements of column footings. A similar dominant frequency can be observed at various distances from the source. Therefore, accelerations at most locations of measurements of ground vibrations are proportional to displacements of ground vibrations, and the settlements are proportional to the maximum displacements or the maximum accelerations of vertical ground vibrations. Barkan (1962) reported three case studies of damaging effects of structure footing settlements caused by ground vibrations from forge hammer foundations. The hammers had dropping weights of 4.5, 2.5 and 3 tonnes. The static pressure on the ground under wall footings was in the 1.75-2.5 kg/cm2 range. The soil deposits of fine-grained sands were in all three cases. The water tables were at depths of 4.0-8.5 m. In the first study, a three story auxiliary building attached to a forge shop was completely destructed. This brick building was located at a distance of 6 m from the hammer foundation and erected much later than the forge shop. In the second study, ground vibrations from a hammer destroyed the forge shop building which brick walls were supported by continuous footing. In the third study, differential settlements of the shop columns nearest the hammer foundation were observed. These settlements were the cause of crack formation in the reinforced-concrete frame structures of the shop and in the brick walls.

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To assess the effect of ground vibration intensity on dynamic settlements, Savinov (1979) suggested the threshold of 15 cm/s2 for buildings sensitive to differential settlements and 30 cm/s2 for insensitive buildings. Differential dynamic settlements of forge shop structures and abutted buildings are the major harmful results of foundation vibrations under forge hammers.

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Resonant Vibrations of Adjacent and Remote Structures A coincidence of the frequency of ground vibrations to one of natural building oscillations may generate the condition of resonance in the building. Even the proximity of those frequencies may strongly increase building vibrations. Oscillations of impact machine foundations also generate horizontal ground vibrations which have only one-two cycles in relatively small spaces of forge shops. Such horizontal ground vibrations cannot trigger horizontal resonant vibrations of forge shop structures. Displacements of these structural vibrations are usually similar to displacements of horizontal ground vibrations near footings under exterior forge shop structures. In various soils, waves propagate in all directions from impact machine foundations forming a series of quasi-harmonic waves with the predominant frequency equal or close to the frequency of the source. This phenomenon is particularly well observed in saturated sands. A coincidence of ground and structure frequencies may trigger resonant structural vibrations. Rausch (1950) described a case history where intolerable vibrations were observed in an administrative building located 200 m from the foundation of a hammer with a ram mass of 1.5 tonnes. Probably wave paths had low attenuation at that site. Svinkin (1993) reported resonant horizontal vibrations of one part of a five story apartment building located at approximately 500 m from the foundation under a vibroisolated block for a forge hammer with a ram mass of 16 tonnes.

Direct Vibration Effects on Forge Shop Structures Damage to masonry of the exterior walls is observed in various forge shops. Such damage can be produced by ground vibrations from impact machine foundations when frequencies of ground vibrations do not match natural frequencies of structures. The experimental studies of ten forge shops were performed because of visible damage in shop exterior structures at sites with diverse soil conditions, Svinkin (1995). The investigated forge shops had similar structural set-up: one story braced steel frames and exterior walls supported by spread footings or foundation beams installed on column footings. The brick walls were connected to the columns. There were various soil conditions at sites: fine and middle sands with natural moisture, moist and very moist loams, and clays. Cracks and other damage of exterior walls were found at the time of investigation. The most typical cracks were found in brick walls along the axes of steel columns. A length of cracks changed from 1 to 7 m and a crack width was in the 2-30 mm limits. Oblique cracks were detected at wall corners. The holes from fallen bricks were revealed

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at both sides of some parts of exterior walls. Considerable deformations of the masonry were found in the walls of auxiliary buildings abutted to the forge shops. Horizontal and vertical vibrations were measured on the column footings and brick walls during the operation of 24 forge hammers with a ram mass from 1 to 20 tonnes. Besides, brick wall vibrations from operating bridge cranes were recorded, particularly at the time of motion and braking of bridge cranes and crab motors. Vertical vibrations of the column footings in the proximity of the hammer foundations had shapes similar to vibrations of the hammer foundations, but their displacements decreased 2-5 times dependently on a distance from the source and soil conditions. Vibrations of the column footings attenuated quickly with distance from the hammer foundations. Records of horizontal structural vibrations showed that the maximum transverse displacement of 0.7 mm was measured at the upper parts of brick walls in the shop spans against the hammer foundations. Forces vibrations of brick walls had the dominant frequencies between 50-58 rad/s which coincided with the frequencies of free hammer foundation vibrations. At the rest of shop spans, free wall vibrations had frequencies in the 19-34 rad/s range and much smaller displacements. Horizontal displacements in the wall plane were 5-10 times less than the maximum transverse displacements at the same points. Dynamic loads from bridge cranes induced forced brick wall vibrations with the dominant frequencies in the 17-34 rad/s range and the maximum horizontal wall displacements between columns of the same order like those from operating hammers. Brick wall transverse vibrations had certain features at the locations of wall abutting to the columns. On the wall section located against the hammer foundations, vibrations of the brick wall on both sides of the column had the same phase and close displacements. A phase of these vibrations changed and differences between their displacements increased with moving away from the span with the hammer foundation. This phenomenon was pronounced during operations of bridge cranes. The performed experimental studies of ten forge shops revealed the causes of crack formation, minor and major masonry damage in shop exterior walls. It is common to consider differential column footing settlements induced by the static pressure and vibrations as the basic cause of cracks and damage in the exterior walls. It is correct for sites with sand deposits at close distances from the hammer foundations. Nevertheless, in numerous cases the masonry damage of the forge shop walls was observed at sites with other soil deposits than sands. Deformations of exterior structures due to non-uniform column footing settlements were not visible at the observed forge shops even built on cohesionless soils. The analysis of the obtained results showed that cracks and damage of the brick walls bordered with steel columns were caused by to the effects of wall vibrations relatively to the columns. While a part of the brick wall on one side of a steel column moved, a similar part of the wall on other side of the column stayed immovable because a phase of vibrations changed. These vibrations were induced mostly by dynamic loads generated by operating bridge cranes. Simple calculations confirmed that tension stresses in the masonry were greater than the allowable limits. It is necessary to point out that cracks found at the upper part of exterior walls were not dangerous for the masonry in the good condition. However, for the masonry with insufficient quality, vibrations developed

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cracks which length reached several meters with a width of 2-3 cm. Such cracks are unacceptable because they split the brick wall into separate parts. The appearance of extensive masonry damage at locations where the shop brick walls were attached to auxiliary buildings can be explained by inadequate quality of expansion joints between buildings and unequal settlements of the attached buildings.

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Disturbance of People Vibrations from impact machines shake working places near machine foundations, disturb people at forge shops and other places at plants, and may be annoying for residents of adjacent and sometimes remote buildings. According to ANSI S3.29-1983, vibrations from impact machines with respect to human response can be divided into impulsive and intermittent vibrations. An impulsive vibration is a transient isolated event with the duration less than two seconds. Such dynamic loads are generated by most of hammers in forge shops and hammers for remaking steel scrap heaps. Vibration values at workshop areas with such industrial process are specified in ANSI S3.18-1979, but ANSI S3.29-1983 should apply for assessment of impulsive vibration magnitudes in offices and adjacent building. Intermittent vibration is a string of vibration incidents with short duration less than two seconds separated with intervals of much lower vibration amplitudes or without vibration at all. Such dynamic loads are generated by various punch presses and some old small forge hammers. ANSI S3.29-1983 should apply for assessment of impulsive vibration magnitudes in offices and adjacent building.

Predicting Vibrations from Impact Machine Foundations Predicting soil and structure vibrations from impact machine foundations is important for proper assessment of vibration effects on structures, sensitive devices and people. The ground under machine foundations plays the major role in forming the machine foundation response and the ground responses to dynamic loads generated by impact machines.

Natural Frequency of Vertical Foundation Vibrations Dynamic loads on the ground induce elastic waves in the medium of soil. The spectra of ground vibrations caused by impact loads have few maximums which are the natural frequencies of the soil layers. The experimental study (Svinkin 1996) revealed that values of these frequencies are practically independent of the condition at the contact area where impacts are made directly on the ground. It has been found that the natural damped frequency of vertical foundation vibrations coincides with the dominant natural frequency of the soil profile, Svinkin (1997a, 2001). This finding is the basis of the method for predicting the natural frequency of vertical foundation vibrations.

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According to the method, impact loads are applied onto the ground within an area for installation of the machine foundation. Output locations are also at this area but beyond the zone of plastic deformations of the ground caused by impact forces. The dominant frequency of the spectra of ground vibrations at the location for installation of machine foundation is the predicted natural frequency of vertical damped vibrations of the machine foundation for the specified impact machine. The result of predicting is shown for the foundation under a press-hammer with the ram mass of 4 tonnes and the foundation base area of 12.3 m2 installed at the site with mostly a fine sand deposit (Figure 4). There is a good coincidence of predicted and measured results.

Ground and Structure Vibrations An IRFP method can be used to predict complete time-domain records of ground and structure vibrations from impact machines, Svinkin (1997b, 2002). This method is founded on the utilization of the impulse response function technique that eliminates the need to use mathematical models of soil profiles, foundations and structures in practical application. The method takes into consideration the variety of soil and structural

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properties and reflects real behavior of soil and structures without investigation of soil and structure properties. The parameters of a machine foundation system can be determined using the existing theories, e.g. Rausch (1950), Barkan (1962), Lysmer and Richart (1966), Richart et al. (1970), Wong and Luco (1976), Arya et al. (1979), Roesset (1980), Dobry et al. (1986), Novak (1987), Prakash and Puri (1988), Gazetas (1991), Veletsos (1993), Wolf (1994), and others. The reason for the agreement of predicted results with the use of all of these theories is that ground vibration responses are negligibly dependent on the parameters of the foundation-soil system. The procedure for predicting soil and structures vibrations prior to installation of foundation for impact machines includes experimental and computational parts. In an experimental part, the place for a machine foundation should be chosen at an industrial site. At the place of installation of the machine foundation, impact forces of known magnitude are applied on the ground. The impact can be created using a rigid steel sphere or pear-shaped weight falling from a bridge or mobile crane. At the moment of impact on the ground, oscillations are recorded at the points of interest, for example, at the locations of instruments and devices sensitive to vibrations. These oscillations are the impulse response functions of the considered system which automatically take into account complicated soil conditions. In a computational part, after preliminary calculation of the frequencies and the damping constant of soil, a convolution integral is used to compute predicting soil and structure vibrations, Svinkin (2002). The following example demonstrates the application of the IRFP method for predicting ground surface oscillations excited by vibrations of the foundation under a vibroisolated block for the large forge hammer with a ram mass of 16 tonnes. The foundation base area was 116.4 m². The soil deposit consisted of about 1.5 m of earth fill followed by about 5 m of gray and brown moist soft sandy clay underlain by about 7 m of yellow moist dense sand deposited on green moist soft clay. The water table was not encountered during soil boring at the site. Measured and predicted soil vibrations at distances 7.7 m and 28.8 m from the hammer foundation are shown in Figure 5. The predicted soil vibrations demonstrate a close fit to the measured data. The IRFP method predicts complete three-dimensional wave forms, vertical and two horizontal, with reasonable accuracy. A comparison of computed and measured records confirms the acceptability of the IRFP method for prediction vibrations in target points prior to installation of foundations for impact machines.

Mitigation of Vibration Outcomes There are a limited number of means which can mitigate vibrations generated by impact machines. Some ways can be used at a design stage; other ways can be employed before and after construction of impact machine foundations. It is possible that a measure for decreasing one type of structural vibrations will trigger another vibration excitation. A proper analysis is needed for each site.

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Prevention of Differential Settlements Footings under columns in forge hammer shops are designed for a constant static pressure transferred from structures onto the ground. An additional dynamic pressure from ground vibrations generated by the impact machine foundations depends on a distance from the source. Because the settlements are proportional to the accelerations of vertical ground vibrations, column footings have differential settlements.

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To diminish detrimental vibrations effects on building footings, Barkan (1962) proposed to assign the permissible static pressures from column footings on the ground depending on the displacements or the accelerations of ground vibrations at the locations of column footings. The IRFP method (Svinkin 2002) can be used for predicting ground displacements before construction of a hammer foundation.

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Resonant Building Vibrations Low-frequency transient vibrations appear at some distances from machine foundations and may trigger resonant structure vibrations. It occurs seldom and there are no readily apparent means for reducing resonant vibrations. However, resonance problem can be detected in advance with the IRFP method.

Wave Barriers There are numerous studies of the application of wave barriers for diminishing ground vibrations from different dynamic sources, for example Woods (1968), Haupt (1995), Naggar and Chehab (2005) and others. However, there is no example of the successful application of the wave barriers technique to mitigate ground vibrations from foundations under machines with impact loads, Woods (2007).

Active Vibration Isolation Vibrating isolation of forge hammers is used at industrial plants in order to diminish harmful vibration effects on adjacent and remote buildings, technological processes, sensitive devices and people. A forge hammer is installed on an isolated concrete block which is supported by steel springs and rubber dashpots. Natural frequencies of vertical block vibrations usually are in 3-6 Hz range. Low frequencies are typical for sizeable hammers. It is necessary to point out that the first mode of multi-story buildings has frequencies between 2-5 Hz, while for row-rise buildings these limits are 4-10 Hz. The proximity of source frequencies to ones of natural building oscillations may generate resonant building vibrations. The following is an example of unacceptable building vibrations induced by the foundation under a vibroisolated concrete block and a sizeable forge hammer with a ram mass of 16 tonnes, Svinkin (1993). A five story apartment building was located at a distance about 500 m from the hammer foundation. The building had two perpendicular parts. A major part of the building was oriented in a radial direction from the hammer foundation and had large stiffness in this direction. There were no vibration problems with this building part. In the same direction, a minor part of the building had low stiffness and harmful vibrations particularly sensitive at night. Vibrations of the hammer foundation induced horizontal transversal structural vibrations with the frequency of 3.1 Hz. This frequency was certainly closed to the natural frequency of horizontal building vibrations. A change of the frequency of vertical

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vibrations of the vibroisolated block with the hammer is the simplest and economical way for diminishing building vibrations. To rich this goal, it is necessary to decrease stiffness of vibroisolators by eliminating part of them. A number of steel springs cannot be reduced because they are chosen in accordance with the condition of strength to support the concrete block. Therefore, it is necessary to decrease the quantity of dashpots. Buildings usually have a narrow resonant zone. In the described case history, the natural frequency of vibroisolated concrete block with hammer was decreased from 3.1 to 2.9 Hz due to elimination of a few dashpots. It was sufficient to diminish structural vibrations to the acceptable limits.

Mitigation of Vibration Effects on People Impact machine foundations generate high levels of ground vibrations which are considerably bigger than the threshold of human exposure to vibrations in buildings. Therefore, offices have to be located at relatively large distance from the dynamic sources. Sometimes there is the need for mitigating vibration effects at the existing offices located at distances with perceptible vibrations. It is a complicated problem. For the remedial work at those places, it is reasonable to use passive vibration isolation in the offices. Composite high damping panels for flooring and walls should to be used to decrease vibrations.

Conclusions Impact machines generate intensive dynamic forces which induce machine foundation and ground vibrations. Forge and drop hammers are most powerful machines producing impact loads. In most cases, the hammer foundations respond to impact loads generated by hammers as a SDOF system, and only vertical foundation vibrations have to be considered for analysis of impact machine foundations as sources of industrial vibrations. There is a trend of decreasing the natural frequency of vertical foundation vibrations with increasing the ram mass and the foundation base area. A real pressure under column footing in forge shops can be up two times higher than the static pressure due to vibrations from hammer foundations. Accelerations attenuate very fast with distance from the impact machine foundations. Therefore, dynamic loads under column footings are diverse and that may provoke additional settlements of column footings. Differential dynamic settlements are the major cause of damage to exterior walls in forge shops at sites with a sand deposit. Horizontal vibrations of exterior forge shop structures triggered by ground vibrations from impact machine foundations are not dangerous for integrity of these structures. However, low-frequency ground vibrations can trigger resonant building vibrations at relatively large distances from the hammer foundations. The analysis of the obtained results showed that cracks and damage of the brick walls bordered with steel columns in forge shops had occurred due the effect of wall vibrations

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relatively to the columns. These vibrations were induced mostly by dynamic loads from operating bridge cranes. The natural frequency of vertical machine foundation vibrations can be predicted before installation of a machine foundation. An IRFP method can be used to predict complete time-domain records of ground and structure vibrations from impact machines at the time of design of the machine foundation. There are a limited number of means which can mitigate vibrations generated by impact machines. Some measures can be used at a design stage; others can be employed before and after construction of impact machine foundations. Mitigation measures should be correctly applied because it is possible that eliminating one dynamic excitation can trigger another one. It is better to mitigate vibration effects on peoples in offices at the time of a design of forge shops and surrounding areas than decrease unacceptable vibrations after construction.

References ANSI S3.18 (1979). AMERICAN NATIONAL STANDARD, Guide to the Evaluation of Human Exposure to Whole Body Vibrations. ANSI S3.29 (1983). AMERICAN NATIONAL STANDARD, Guide to the Evaluation of Human Exposure to Vibration in Buildings. Arya, S.C., O'Neill, N.M., and Pincus, G. (1979). Design of structures and foundations for vibrating machines. Gulf Publishing Company, Houston, Texas. Barkan, D.D. (1964). Dynamics of Bases and Foundations, McGraw Hill Co., New York. Dobry, R., Gazetas, G, and Stokoe, K.H., II (1986). "Dynamic response of arbitrarily shaped foundations: Experimental verification." Journal of Geotechnical Engineering, ASCE, 112, No. 2, 136-149. Gazetas, G.. (1994). "Foundation vibrations." Foundation Engineering Handbook, 2nd Ed., H.Y.Fang, ed, Van Nostrand Reinhold, New York, 553-593. Glazyrin, V.S. and Martyshkin, B.C. (1971). “Investigation of vibrations of vibration isolated foundations under forge hammers.” Bases, Foundations and Soil Mechanics, Stropiizdat, 3 (in Russian). Haupt, W.A. (1995). “Wave propagation in the ground and isolation measures.” Proceedings of the Third International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics, St. Louis, Missouri, V.II, 985-1016. Lysmer, J. and Richart, F.E., Jr. (1966). "Dynamic response of footings to vertical loading." Journal of Soil Mechanics and Foundation Division, ASCE, 92(1), 65-91. Klattso, M.M. (1965). “Parameters of ground vibrations induced by forge hammers.” Industrial Construction, Stropiizdat (in Russian). Naggar, M.H.E. and Chehab, A.G. (2005). “Vibration barriers for shock-producing equipment.” Canadian Geotechnical Journal, 42, 297-306. Novak, M. (1987). "State of the art in analysis and design of machine foundations." SoilStructure Interaction, Elsevier/CML Publisher, New York, 171-192. Prakash, S. and Puri, V.K. (1988). Foundations for machines: analysis and design, John Wiley and Sons, Inc., New York. Rausch, E. (1950). Maschinen Fundamente, Verlag, Dusseldorf, Germany (in German).

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Richart, F.E., Hall, J.R. and Woods, R.D. (1970). Vibrations of soils and foundations, Prentice-Hall, Inc., Englewood Cliffs, NJ. Roesset, J.M. (1980). "The use of simple models in soil-structure interaction." Civil Engineering and Nuclear Power, ASCE, No. 1/3, 1-25. Savinov, O.A. (1979). Modern construction of machine foundations and their calculations. Second Edition, Stroiizdat, Leningrad (in Rissian). Scheglov, V.F. (1960). Soil vibrations from forge hammers with various degree of vibration isolation.” Forge-Punching Production, Mashinostroenie, 8 (in Russian). Svinkin, M.R. (1980). “Determination of dynamic loads transmitted to a hammer foundation.” Soil Mechanics and Foundation Engineering, Publishing Corporation, New York, 17(5), 200-201, a translation of Osnovaniya, Fundamenty i Mekhanika Gruntov in Russian. Svinkin, M.R. (1982). “Determining the vibration amplitude of foundations supporting crankshaft presses.” Soil Mechanics and Foundation Engineering, Publishing Corporation, New York, 19(3), 99-102, a translation of Osnovaniya, Fundamenty i Mekhanika Gruntov in Russian. Svinkin, M.R. (1993). “Analyzing man-made vibrations, diagnostics and monitoring”. Proceedings of the Third International Conference on Case Histories in Geotechnical Engineering, St. Louis, Missouri, June 1-4, 663-670. Svinkin, M.R. (1995). “Vibrations of impact machine foundations and footing settlements.” Proceedings of the Third International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics, St. Louis, Missouri, V. II, 797-802. Svinkin, M.R. (1996). Discussion of ‘Impact of weight falling onto the ground’ by Roesset et al." Journal of the Geotechnical and Geoenvironmental Engineering, ASCE, 120(8), 414-415. Svinkin, M.R. (1997a). A method for estimating frequencies of machine foundations. U.S. Patent No. 5,610,336 issued March 11, 1997. Svinkin, M.R. (1997b). "Numerical methods with experimental soil response in predicting vibrations from dynamic sources." Proceedings of the Ninth International Conference of International Association for Computer Methods and Advances in Geomechanics, A.A. Balkema, Rotterdam, 3, 2263-2268. Svinkin, M.R. (2001). “Natural frequency of vertical foundation vibrations evaluated from in-situ impact test.” Proceedings of the Fourth International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics, San Diego, California, March 26-31, Paper No. 2.33, CD-ROM. Svinkin, M.R. (2002). "Predicting soil and structure vibrations from impact machines." Journal of the Geotechnical and Geoenvironmental Engineering, 128(7), 602-612. Veletsos, A.S. (1993). "Design concepts for dynamics of soil-structure interaction." Developments in Dynamic Soil-Structure Interaction, Kluwer Academic Publishers. Wong, H.L. and Luco, J.E. (1976). "Dynamic response of rigid foundations of arbitrary shape." Earthquake Engineering and Structural Dynamics, 4, 579-587. Wolf, J.P. (1994). Foundation vibration analysis using simple physical models, PTR Prentice Hall, Englewood Cliffs, NJ. Woods, R.D. (1968). “Screening of surface waves in soils.” Journal of the Soil Mechanics and Foundation Division, ASCE, 94(SM4), 951-979. Woods, R.D. (2007). Personal communications.

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