dynamics of impact machines

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

1VIBRACONSULT, 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 ofindustrial vibrations. These foundations mostly transmit vertical dynamic loads on theground and generate ground vibrations which may harmfully affect surroundingbuildings. Dynamic effects range from serious disturbances of working conditions forsensitive devices and people to visible structural damage. Natural frequency of verticalmachine foundation vibrations and complete vibration records of ground and structurevibrations can be predicted prior to installation of machine foundations. Diverse measurescan be used to mitigate dynamic effects of impact machine foundations.

Introduction

Various machines with impact or shock loads are used for production processes at plantsand in industrial buildings. As a rule, such machines are installed on massive concretefoundations. Forge and drop hammers are most powerful machines producing impactloads.

Forge hammer production is usually accompanied with high vibration levels ofground vibrations because substantial dynamic loads are transmitted on hammerfoundations, and these vibrations may detrimentally affect adjacent and remote structures,sensitive equipment and people. It is likely that structure damage caused by vibrationsmay occur in close proximity of the dynamic sources. Nevertheless, unacceptablestructural vibrations may also be induced at long distances from the sources due to thedynamic effect of low-frequency ground vibrations. Therefore, it is important to predictground and structure vibrations before erection of foundations under machines withimpact loads and consider possible outcomes of vibration effects in the design stage ofmachine foundations.

Knowledge and experience in understanding the causes of vibration effects of impactmachine foundations can be helpful in prevention of detrimental structural vibrations.Each construction site is unique, and vibration mitigation measures before or afterconstruction of machine foundations should be correctly applied at a site because it ispossible that eliminating one dynamic excitation can trigger another one.

The paper is based on analysis and generalization of numerous case studies.

Geotechnical Earthquake Engineering and Soil Dynamics IV GSP 181 2008 ASCE

Copyright ASCE 2008 Geotechnical Earthquake and Engineering and Soil Dynamics IV Congress 2008 Geotechnical Earthquake Engineering and Soil Dynamics IV

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2Impact Machine Foundations as Sources of Vibrations

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

Impact Machine Foundations

There are various forge and drop hammers. In forge shops, two major types of hammersare used: a counterblow hammer (proper) and a short-stroke drop hammer. The formermachine provides free forging operations. The latter machine also called drop hammerfor die stamping is applied to the precision of blows required in forging. Besides, diversepunch-presses such as sizing presses, hydraulic presses and others are employed forproduction of machine parts at industrial plants.

Two other hammers put in practice to remake steel scrap heaps. Sizeable drophammers break scrap iron, and press-hammers are used to compress and pack lightweightsteel 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 blowhammers, footings under the frame are paced on the anvil foundation at both sides of theanvil with 2-3 cm layers of roofing felt between the frame footing and the anvilfoundation. Short-stroke drop hammers are usually installed on a single concrete block tosupport 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 foundationsunder the frames. Such separation can result in substantial settlements of the anvilfoundation.

Square timbers are used for pads under the anvil. The pad thickness of 0.1 1.2 mdepends 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 withupper 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 isabout 2 m.

Foundations under the press-hammers are relatively small and the anvils are usuallyinstalled 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 determinewhat major foundation vibrations are transmitted on the ground and how these vibrationsmay have effects upon adjacent and remote structures.



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3Most hammer foundations are undergone only vertical vibrations under appliedcentered impacts, but some machines with impact loads can cause vertical and rockingfoundation vibrations. An experimental study showed that rocking foundation oscillationsdo not affect soil vibration records with distance from the machine foundation (Figure 1).Identical vertical impact loads with different eccentricity produced vertical foundationvibrations with the frequency of 20 rad/s in one event and rocking foundation vibrationswith the frequency of 135 rad/s in another event. However, these impact loads producedsimilar 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 ofimpact machine foundations as sources of industrial vibrations.

The hammer foundation and the anvil are modeled as lumped-mass systems with oneor two degrees of freedom. In a reality, an anvil mass is substantially less than afoundation mass and stiffness of the anvil pad is much larger than soil stiffness under thehammer foundation. Therefore in most cases, the hammer foundations respond to impactloads generated by hammers as a SDOF system.

Normalized responses of the hammer-foundation-soil systems are presented in Figure2 for four foundations under different machines producing impact loads: a press-hammerwith the ram mass MR=4 tonnes and the foundation base area AFB=12.3 m2, a short-strokedrop hammer with MR=7.25 tonnes and AFB=80 m2, a counterblow hammer with MR=6tonnes and AFB=58.8 m2 and a sizeable drop hammer with MR=15 tonnes and AFB=158m2. It can be seen that the responses of three hammer foundations are represented bySDOF transfer functions which almost coincide with the corresponding theoreticaltransfer functions for which parameters were determined from experiments. Only for thecounterblow hammer foundations, a transfer function represents the system with twodegrees of freedom but with the domination of the first shape. It is acceptable forpractical goals to consider the hammer-foundation-soil system as SDOF.



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4Variables of Machine Foundation Vibrations

Variables of hammer foundation vibrations presented in Table 1 for three groups ofhammer foundations were gathered on the basis of published data (Rausch 1950; Barkan1962; Scheglov 1960; Klattso 1965; Glazyrin and Martyshkin 1971) and studiesperformed by the writer (Svinkin 1980 and 1995).

The first and second groups represent hammer foundations installed on the groundwithout vibration isolation. These groups gathered foundations under hammers dependingon the mass of hammer rams: 5-25 tonnes range for the first group and below 5 tonnes forthe second group. Natural angular frequencies of these foundations are in limits of 40-90



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5rad/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 (Rausch1950), the impact loads on hammer foundations can be considered as instantaneousloading. The impact loads induce transient hammer foundation vibrations consisting of 1-2 cycles with large damping. Foundation vibrations are transferred onto the ground.

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

Values of the frequencies of free vertical foundation vibrations in certain degreedepend on the capacity of forge hammers. The masses of hammer rams and thefrequencies of free vertical foundation vibrations are shown in Figure 3. It can be seen thetrend that the larger mass of the hammer ram the lower frequency of free verticalfoundation vibrations. Foundations for powerful forge hammers have the lowestfrequencies, and the highest frequencies were found for foundations under forge hammerswith the ram mass of 1-3 tonnes.

This phenomenon has a reasonable explanation. It is obvious that the larger ram massrequires the larger foundation base area, and the larger volume of soil mass is involved inhammer foundation vibrations. An enlargement of the foundation base area increases thesoil stiffness under the foundation base area. However, an augmentation of the soil massis greater than that of the soil stiffness, and consequently it results in decreasing thefrequency 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 anddashpots are used. Records of free block vibrations from hammer impact are similar to

Variables of Hammer Foundation VibrationsGroupsof

FoundationsForge

Hammers Frequencyrad/s

Displacementmm

Velocitycm/s

Accelerationcm/s2

EnergyTransferred

onto SoilkJ

1 Largeforge

hammerswith ram

massbetween

5-25tonnes

40-60 0.4-1.0 2.0-6.0 120-420 0.8-5.9

2 Forgehammerswith rammass less

than5 tonnes

60-90 0.3-1.0 1.9-8.8 120-980 0.06-1.9

3 Vibrationisolated

forgehammers

19-38 0.1-0.7 0.4-1.6 14-17 0.01-2.8



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6low frequency damped sinusoid with small damping. The block vibrations are transmittedonto 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 threegroups of hammer foundations. The maximum energy transferred onto the ground andbig values of velocities and accelerations are observed for foundations under large forgehammers. The greatest velocity of 8.8 cm/s and acceleration of 980 cm/s2 were obtainedat foundations for relatively small forge hammers in group 2 because of comparativelyhigh natural frequencies of these foundations. The minimum values of vibration variablesare related to vibration isolated hammer foundations.

In addition to information about forge hammers and their foundations presented inTable 1 and Figure 3, sizeable drop hammers have somewhat different values of dynamicloads and variables of foundation vibrations. The maximum mass of dropping weight is15 tonnes and the maximum dropping height is 30 m. Frequencies of free verticalfoundation vibrations are in limits of 3-8 Hz. Maximum displacements of vertical androcking foundation vibrations are 3 and 6 mm respectively. Accelerations can reachvalues up to 600 cm/s2. Foundations under sizeable drop hammers can transfer muchenergy up to 35 kJ onto the ground. This considerable amount of energy is 6-14 timeshigher than energy transferred onto the ground from foundations under hammers with bigram masses.

Displacements of machine foundation vibrations can be calculated using knownprocedures available in Barkan (1962) for foundations under forge hammers and sizeabledrop hammers, in Svinkin (1993) for press-hammer foundations, and in Svinkin (1982)



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7for punch-press foundations. The permissible displacement values are 1.0-1.2 mm forforge hammer foundations and 0.25-0.50 mm for punch-press foundations.

Vibration Effects on Structures

Forge shop structures, adjacent and remote buildings may be affected by vibrationsgenerated by operating impact machines. Vertical oscillations of impact machinefoundations induce elastic waves in the soil medium which trigger vertical and horizontalground vibrations.

Dynamic Settlements

Vertical ground vibrations from impact machine foundations in sand soils can be thecause of non-uniform dynamic settlements of column footings in forge shops. Columnfootings are usually designed for static loads transferred on the ground without takinginto account the dynamic loads from ground vibrations which increase the pressure on theground. According to Table 1, accelerations of hammer foundations may reach the valueof 980 cm/s2 or 1.0 g, and consequently the real pressure from column footings on theground will be up 2 times higher than the static pressure. For foundations under impactmachines, this effect is less important because the design of machine foundation providesa smaller static pressure on the ground in comparison with structure footings whichsupport 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 columnfootings are diverse and that may provoke additional differential settlements of columnfootings. A similar dominant frequency can be observed at various distances from thesource. Therefore, accelerations at most locations of measurements of ground vibrationsare proportional to displacements of ground vibrations, and the settlements areproportional to the maximum displacements or the maximum accelerations of verticalground vibrations.

Barkan (1962) reported three case studies of damaging effects of structure footingsettlements caused by ground vibrations from forge hammer foundations. The hammershad dropping weights of 4.5, 2.5 and 3 tonnes. The static pressure on the ground underwall footings was in the 1.75-2.5 kg/cm2 range. The soil deposits of fine-grained sandswere in all three cases. The water tables were at depths of 4.0-8.5 m. In the first study, athree story auxiliary building attached to a forge shop was completely destructed. Thisbrick building was located at a distance of 6 m from the hammer foundation and erectedmuch later than the forge shop. In the second study, ground vibrations from a hammerdestroyed the forge shop building which brick walls were supported by continuousfooting. In the third study, differential settlements of the shop columns nearest thehammer foundation were observed. These settlements were the cause of crack formationin the reinforced-concrete frame structures of the shop and in the brick walls.



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8To assess the effect of ground vibration intensity on dynamic settlements, Savinov(1979) suggested the threshold of 15 cm/s2 for buildings sensitive to differentialsettlements and 30 cm/s2 for insensitive buildings.

Differential dynamic settlements of forge shop structures and abutted buildings arethe major harmful results of foundation vibrations under forge hammers.

Resonant Vibrations of Adjacent and Remote Structures

A coincidence of the frequency of ground vibrations to one of natural buildingoscillations may generate the condition of resonance in the building. Even the proximityof those frequencies may strongly increase building vibrations.

Oscillations of impact machine foundations also generate horizontal groundvibrations which have only one-two cycles in relatively small spaces of forge shops. Suchhorizontal ground vibrations cannot trigger horizontal resonant vibrations of forge shopstructures. Displacements of these structural vibrations are usually similar todisplacements of horizontal ground vibrations near footings under exterior forge shopstructures.

In various soils, waves propagate in all directions from impact machine foundationsforming a series of quasi-harmonic waves with the predominant frequency equal or closeto the frequency of the source. This phenomenon is particularly well observed insaturated sands. A coincidence of ground and structure frequencies may trigger resonantstructural vibrations. Rausch (1950) described a case history where intolerable vibrationswere observed in an administrative building located 200 m from the foundation of ahammer with a ram mass of 1.5 tonnes. Probably wave paths had low attenuation at thatsite. Svinkin (1993) reported resonant horizontal vibrations of one part of a five storyapartment building located at approximately 500 m from the foundation under avibroisolated 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. Suchdamage can be produced by ground vibrations from impact machine foundations whenfrequencies of ground vibrations do not match natural frequencies of structures. Theexperimental studies of ten forge shops were performed because of visible damage inshop exterior structures at sites with diverse soil conditions, Svinkin (1995).

The investigated forge shops had similar structural set-up: one story braced steelframes and exterior walls supported by spread footings or foundation beams installed oncolumn footings. The brick walls were connected to the columns. There were various soilconditions at sites: fine and middle sands with natural moisture, moist and very moistloams, 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. Alength 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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9at both sides of some parts of exterior walls. Considerable deformations of the masonrywere found in the walls of auxiliary buildings abutted to the forge shops.

Horizontal and vertical vibrations were measured on the column footings and brickwalls 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 atthe time of motion and braking of bridge cranes and crab motors.

Vertical vibrations of the column footings in the proximity of the hammerfoundations had shapes similar to vibrations of the hammer foundations, but theirdisplacements decreased 2-5 times dependently on a distance from the source and soilconditions. Vibrations of the column footings attenuated quickly with distance from thehammer foundations.

Records of horizontal structural vibrations showed that the maximum transversedisplacement of 0.7 mm was measured at the upper parts of brick walls in the shop spansagainst the hammer foundations. Forces vibrations of brick walls had the dominantfrequencies between 50-58 rad/s which coincided with the frequencies of free hammerfoundation vibrations. At the rest of shop spans, free wall vibrations had frequencies inthe 19-34 rad/s range and much smaller displacements. Horizontal displacements in thewall plane were 5-10 times less than the maximum transverse displacements at the samepoints.

Dynamic loads from bridge cranes induced forced brick wall vibrations with thedominant frequencies in the 17-34 rad/s range and the maximum horizontal walldisplacements between columns of the same order like those from operating hammers.Brick wall transverse vibrations had certain features at the locations of wall abutting tothe columns. On the wall section located against the hammer foundations, vibrations ofthe brick wall on both sides of the column had the same phase and close displacements. Aphase of these vibrations changed and differences between their displacements increasedwith moving away from the span with the hammer foundation. This phenomenon waspronounced during operations of bridge cranes.

The performed experimental studies of ten forge shops revealed the causes of crackformation, minor and major masonry damage in shop exterior walls. It is common toconsider differential column footing settlements induced by the static pressure andvibrations as the basic cause of cracks and damage in the exterior walls. It is correct forsites with sand deposits at close distances from the hammer foundations. Nevertheless, innumerous cases the masonry damage of the forge shop walls was observed at sites withother soil deposits than sands. Deformations of exterior structures due to non-uniformcolumn footing settlements were not visible at the observed forge shops even built oncohesionless soils.

The analysis of the obtained results showed that cracks and damage of the brick wallsbordered with steel columns were caused by to the effects of wall vibrations relatively tothe columns. While a part of the brick wall on one side of a steel column moved, a similarpart of the wall on other side of the column stayed immovable because a phase ofvibrations changed. These vibrations were induced mostly by dynamic loads generated byoperating bridge cranes. Simple calculations confirmed that tension stresses in themasonry were greater than the allowable limits. It is necessary to point out that cracksfound at the upper part of exterior walls were not dangerous for the masonry in the goodcondition. 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 areunacceptable because they split the brick wall into separate parts. The appearance ofextensive masonry damage at locations where the shop brick walls were attached toauxiliary buildings can be explained by inadequate quality of expansion joints betweenbuildings and unequal settlements of the attached buildings.

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 forresidents of adjacent and sometimes remote buildings.

According to ANSI S3.29-1983, vibrations from impact machines with respect tohuman response can be divided into impulsive and intermittent vibrations.

An impulsive vibration is a transient isolated event with the duration less than twoseconds. Such dynamic loads are generated by most of hammers in forge shops andhammers for remaking steel scrap heaps. Vibration values at workshop areas with suchindustrial process are specified in ANSI S3.18-1979, but ANSI S3.29-1983 should applyfor assessment of impulsive vibration magnitudes in offices and adjacent building.

Intermittent vibration is a string of vibration incidents with short duration less thantwo seconds separated with intervals of much lower vibration amplitudes or withoutvibration at all. Such dynamic loads are generated by various punch presses and some oldsmall forge hammers. ANSI S3.29-1983 should apply for assessment of impulsivevibration magnitudes in offices and adjacent building.

Predicting Vibrations from Impact Machine Foundations

Predicting soil and structure vibrations from impact machine foundations is important forproper assessment of vibration effects on structures, sensitive devices and people. Theground under machine foundations plays the major role in forming the machinefoundation response and the ground responses to dynamic loads generated by impactmachines.

Natural Frequency of Vertical Foundation Vibrations

Dynamic loads on the ground induce elastic waves in the medium of soil. The spectra ofground vibrations caused by impact loads have few maximums which are the naturalfrequencies of the soil layers. The experimental study (Svinkin 1996) revealed that valuesof these frequencies are practically independent of the condition at the contact area whereimpacts are made directly on the ground.

It has been found that the natural damped frequency of vertical foundation vibrationscoincides 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 verticalfoundation vibrations.



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According to the method, impact loads are applied onto the ground within an area forinstallation of the machine foundation. Output locations are also at this area but beyondthe zone of plastic deformations of the ground caused by impact forces. The dominantfrequency of the spectra of ground vibrations at the location for installation of machinefoundation is the predicted natural frequency of vertical damped vibrations of themachine foundation for the specified impact machine.

The result of predicting is shown for the foundation under a press-hammer with theram mass of 4 tonnes and the foundation base area of 12.3 m2 installed at the site withmostly a fine sand deposit (Figure 4). There is a good coincidence of predicted andmeasured results.

Ground and Structure Vibrations

An IRFP method can be used to predict complete time-domain records of ground andstructure vibrations from impact machines, Svinkin (1997b, 2002). This method isfounded on the utilization of the impulse response function technique that eliminates theneed to use mathematical models of soil profiles, foundations and structures in practicalapplication. 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 soiland structure properties.

The parameters of a machine foundation system can be determined using the existingtheories, 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 thesetheories is that ground vibration responses are negligibly dependent on the parameters ofthe foundation-soil system.

The procedure for predicting soil and structures vibrations prior to installation offoundation for impact machines includes experimental and computational parts.

In an experimental part, the place for a machine foundation should be chosen at anindustrial site. At the place of installation of the machine foundation, impact forces ofknown magnitude are applied on the ground. The impact can be created using a rigid steelsphere or pear-shaped weight falling from a bridge or mobile crane. At the moment ofimpact on the ground, oscillations are recorded at the points of interest, for example, at thelocations of instruments and devices sensitive to vibrations. These oscillations are theimpulse response functions of the considered system which automatically take into accountcomplicated soil conditions.

In a computational part, after preliminary calculation of the frequencies and the dampingconstant of soil, a convolution integral is used to compute predicting soil and structurevibrations, Svinkin (2002).

The following example demonstrates the application of the IRFP method for predictingground surface oscillations excited by vibrations of the foundation under a vibroisolatedblock for the large forge hammer with a ram mass of 16 tonnes. The foundation base areawas 116.4 m. The soil deposit consisted of about 1.5 m of earth fill followed by about 5 mof gray and brown moist soft sandy clay underlain by about 7 m of yellow moist dense sanddeposited on green moist soft clay. The water table was not encountered during soil boringat the site. Measured and predicted soil vibrations at distances 7.7 m and 28.8 m from thehammer foundation are shown in Figure 5. The predicted soil vibrations demonstrate aclose fit to the measured data.

The IRFP method predicts complete three-dimensional wave forms, vertical and twohorizontal, with reasonable accuracy. A comparison of computed and measured recordsconfirms the acceptability of the IRFP method for prediction vibrations in target points priorto installation of foundations for impact machines.

Mitigation of Vibration Outcomes

There are a limited number of means which can mitigate vibrations generated by impactmachines. Some ways can be used at a design stage; other ways can be employed beforeand after construction of impact machine foundations. It is possible that a measure fordecreasing one type of structural vibrations will trigger another vibration excitation. Aproper 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 staticpressure transferred from structures onto the ground. An additional dynamic pressurefrom ground vibrations generated by the impact machine foundations depends on adistance from the source. Because the settlements are proportional to the accelerations ofvertical 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 grounddepending on the displacements or the accelerations of ground vibrations at the locationsof column footings. The IRFP method (Svinkin 2002) can be used for predicting grounddisplacements before construction of a hammer foundation.

Resonant Building Vibrations

Low-frequency transient vibrations appear at some distances from machine foundationsand may trigger resonant structure vibrations. It occurs seldom and there are no readilyapparent means for reducing resonant vibrations. However, resonance problem can bedetected in advance with the IRFP method.

Wave Barriers

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

Active Vibration Isolation

Vibrating isolation of forge hammers is used at industrial plants in order to diminishharmful vibration effects on adjacent and remote buildings, technological processes,sensitive devices and people. A forge hammer is installed on an isolated concrete blockwhich is supported by steel springs and rubber dashpots.

Natural frequencies of vertical block vibrations usually are in 3-6 Hz range. Lowfrequencies are typical for sizeable hammers. It is necessary to point out that the firstmode of multi-story buildings has frequencies between 2-5 Hz, while for row-risebuildings these limits are 4-10 Hz. The proximity of source frequencies to ones of naturalbuilding oscillations may generate resonant building vibrations.

The following is an example of unacceptable building vibrations induced by thefoundation under a vibroisolated concrete block and a sizeable forge hammer with a rammass of 16 tonnes, Svinkin (1993). A five story apartment building was located at adistance about 500 m from the hammer foundation. The building had two perpendicularparts. A major part of the building was oriented in a radial direction from the hammerfoundation and had large stiffness in this direction. There were no vibration problems withthis building part. In the same direction, a minor part of the building had low stiffness andharmful vibrations particularly sensitive at night.

Vibrations of the hammer foundation induced horizontal transversal structuralvibrations with the frequency of 3.1 Hz. This frequency was certainly closed to the naturalfrequency 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 wayfor diminishing building vibrations. To rich this goal, it is necessary to decrease stiffness ofvibroisolators by eliminating part of them. A number of steel springs cannot be reducedbecause they are chosen in accordance with the condition of strength to support theconcrete block. Therefore, it is necessary to decrease the quantity of dashpots.

Buildings usually have a narrow resonant zone. In the described case history, thenatural frequency of vibroisolated concrete block with hammer was decreased from 3.1 to2.9 Hz due to elimination of a few dashpots. It was sufficient to diminish structuralvibrations to the acceptable limits.

Mitigation of Vibration Effects on People

Impact machine foundations generate high levels of ground vibrations which areconsiderably bigger than the threshold of human exposure to vibrations in buildings.Therefore, offices have to be located at relatively large distance from the dynamicsources.

Sometimes there is the need for mitigating vibration effects at the existing officeslocated at distances with perceptible vibrations. It is a complicated problem. For theremedial work at those places, it is reasonable to use passive vibration isolation in theoffices. Composite high damping panels for flooring and walls should to be used todecrease vibrations.

Conclusions

Impact machines generate intensive dynamic forces which induce machine foundationand ground vibrations. Forge and drop hammers are most powerful machines producingimpact loads.

In most cases, the hammer foundations respond to impact loads generated byhammers as a SDOF system, and only vertical foundation vibrations have to beconsidered for analysis of impact machine foundations as sources of industrial vibrations.

There is a trend of decreasing the natural frequency of vertical foundation vibrationswith increasing the ram mass and the foundation base area.

A real pressure under column footing in forge shops can be up two times higher thanthe static pressure due to vibrations from hammer foundations. Accelerations attenuatevery fast with distance from the impact machine foundations. Therefore, dynamic loadsunder column footings are diverse and that may provoke additional settlements of columnfootings. Differential dynamic settlements are the major cause of damage to exteriorwalls in forge shops at sites with a sand deposit.

Horizontal vibrations of exterior forge shop structures triggered by ground vibrationsfrom impact machine foundations are not dangerous for integrity of these structures.However, low-frequency ground vibrations can trigger resonant building vibrations atrelatively large distances from the hammer foundations.

The analysis of the obtained results showed that cracks and damage of the brick wallsbordered 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 fromoperating bridge cranes.

The natural frequency of vertical machine foundation vibrations can be predictedbefore installation of a machine foundation. An IRFP method can be used to predictcomplete time-domain records of ground and structure vibrations from impact machinesat the time of design of the machine foundation.

There are a limited number of means which can mitigate vibrations generated byimpact machines. Some measures can be used at a design stage; others can be employedbefore and after construction of impact machine foundations. Mitigation measures shouldbe correctly applied because it is possible that eliminating one dynamic excitation cantrigger another one. It is better to mitigate vibration effects on peoples in offices at thetime of a design of forge shops and surrounding areas than decrease unacceptablevibrations after construction.

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