Hall-2006_ProblemsEncounteredFromUseRayleighDamping.pdf

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EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICSEarthquake Engng Struct. Dyn. 2006; 35:525545

Published online 30 September 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/eqe.541

Problems encountered from the use (or misuse) ofRayleigh damping

John F. Hall;

Department of Civil Engineering; Caltech; Pasadena CA; U.S.A.

SUMMARY

Rayleigh damping is commonly used to provide a source of energy dissipation in analyses of structuresresponding to dynamic loads such as earthquake ground motions. In a nite element model, the Rayleighdamping matrix consists of a mass-proportional part and a stiness-proportional part; the latter typicallyuses the initial linear stiness matrix of the structure. Under certain conditions, for example, a non-linear analysis with softening non-linearity, the damping forces generated by such a matrix can becomeunrealistically large compared to the restoring forces, resulting in an analysis being unconservative.Potential problems are demonstrated in this paper through a series of examples. A remedy to theseproblems is proposed in which bounds are imposed on the damping forces. Copyright ? 2005 JohnWiley & Sons, Ltd.

KEY WORDS: Rayleigh damping; non-linear dynamic analysis; earthquake ground motion

INTRODUCTION

Numerical models of vibrating structures account for three sources of energy dissipationthrough non-linear restoring forces, energy radiation, and damping in the structure. In most ap-

plications, energy dissipation is desirable since it reduces the level of response. Since too muchenergy dissipation can be unconservative, accurate representation in a model is important.

Knowledge of energy dissipation comes mainly from laboratory testing of structuralcomponents and materials, system identication using time history records of structural re-sponse, and eld testing with portable shaking machines. Laboratory test data tend to be veryspecic and can be translated directly into non-linear restoring force models, such as elastic

plastic hysteretic behaviour. System identication results are generally much less specic inthat all three sources of energy dissipation may be present, and seldom can characteristics of

the individual sources be separated out. Field vibration testing is usually always performed

Correspondence to: John F. Hall, Department of Civil Engineering, Mail Code 104-44, California Institute ofTechnology, Pasadena, CA 91125, U.S.A.

E-mail: [email protected]

Received 5 April 2005

Revised 9 August 2005Copyright ? 2005 John Wiley & Sons, Ltd. Accepted 9 August 2005


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526 J. F. HALL

at low amplitude levels, and so the determined damping values would contain only contribu-tions from energy radiation and damping in the structure. These data are useful for calibratingnumerical models in the low-amplitude range, but again, the relative amounts of these twomechanisms of energy dissipation are not generally determined. The same can be said of those

system identication studies that examine low-amplitude records, including ambient data.Given the nature of the available data, a logical procedure to calibrate the energy dissipation

features of a numerical model would be to include appropriate energy radiation mechanismssuch as energy-absorbing foundation elements or transmitting boundaries, and then provideenough damping in the structure to match the low-amplitude eld test or system identicationdata. For higher levels of response, non-linear restoring force models based on laboratory testswould be added, which would act together with the previously dened energy radiation andstructural damping mechanisms. Sometimes, for simplicity, the energy radiation componentscould be omitted; then, the low-amplitude calibration would be done only with the dampingin the structure.

Numerical models almost always represent damping in the structure as the linear viscoustype. Although an approximation, this type of damping is mathematically convenient. In theequation of motion, linear viscous damping takes the form of a damping matrix of con-stant coecients multiplying a vector of velocities of the degrees of freedom. The calibrationexercise consists of choosing the coecients so that amounts of modal damping consistentwith the data are produced for modes of the linear system. However, such calibration isnot always sucient to guarantee that the eects of linear viscous damping will be rea-sonable. Of course, in a standard linear analysis, viscous damping must behave predictably

because of the modal nature of linear response and the association of the damping matrix withmodal damping values. In other cases, problems can occur. Typical users of commercial niteelement programs may not be aware of these potential problems because damping forces areseldom included in a programs output. Nor would eects on the results that are included ina programs output necessarily be evident.

The next section gives background on Rayleigh damping, which is the particular formof linear viscous damping considered in this paper. The main body of the paper presentsexamples where Rayleigh damping is shown to generate unrealistically high damping forces.Each example is accompanied by a discussion of what goes wrong, and sometimes possibleremedies are suggested. Then, a capped viscous damping formulation is introduced that canovercome some of the problems described.

RAYLEIGH DAMPING

Of interest in this paper are those cases where for various reasons, such as the presenceof non-linear restoring forces, the matrix equation of motion of a structure must be solved

directly, rather than the uncoupled modal equations. The matrix equation of motion takesthe form [1]

[M]{x(t)}+ [C]{ x(t)}+{Rs(t)}={Fstat}+{Fdyn(t)} (1)where{x(t)} contains the displacement degrees of freedom (translations and rotations) as afunction of time t, an over dot denotes a time derivative, [M] is the mass matrix, [C] isthe damping matrix,{Rs(t)} is the vector of restoring forces (forces and moments),{Fstat} is

Copyright ? 2005 John Wiley & Sons, Ltd. Earthquake Engng Struct. Dyn.2006; 35:525545


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PROBLEMS ENCOUNTERED FROM THE USE (OR MISUSE) OF RAYLEIGH DAMPING 527

the static load vector, and{Fdyn(t)} is the dynamic load vector. Linear viscous damping inthe structure is assumed, and [C] could contain energy radiation terms. However, this paperfocuses on Rayleigh damping, in which case [C] is expressed as

[C] =aM[M] +aK[K] (2)

where aM and aK are constants with units of s1 and s, respectively, and [K] is the linear

stiness matrix of the structure constructed with initial tangent stinesses. Thus, [C] consists ofa mass-proportional term and a stiness-proportional term. This choice of [K] in the stiness-

proportional damping term may be the only option oered by a commercial nite elementprogram.

The procedure for determining aM andaK involves imparting appropriate values of damping,to the degree possible, to the modes of the linear system, which is represented by Equation(1) except that the restoring forces are given by

{Rs(t)}= [K]{x(t)} (3)Damping of mode i is quantied by the damping ratio i, the ratio of the modes damping tothe critical value [2]. If aM and aK are known, i can be found from

i = 1

2!iaM+

!i2aK (4)

where !i is the natural frequency (rad=s) of mode i. Thus, aM and aK can be set to give anydamping ratio to any two modes. Other modes will receive default amounts of damping thatcan be computed from Equation (4).

The following procedure is convenient for determining aM and aK. Select a desired amountof damping and a frequency range from ! to R! covering those modes important to theresponse, where R1. Compute from

=1 +R2R1 +R+ 2

R

(5)

where determines bounds on the damping ratios that are imparted to those modes within thespecied frequency range. Any such mode will have a damping ratio bounded by max= +and min= . If these bounds are satisfactorily narrow, the constants aM and aK are thenfound from

aM= 2! 2R

1 +R+ 2R

(6a)

aK= 2

1

!

2

1 +R+ 2R (6b)

and can be used to compute an actual damping value i for mode ifrom Equation (4) if!iis known. Figure 1 shows that i= max if!i= ! or!i=R!, and that i= min if!i=

R!.

If!i is outside the range ! to R!, then i max. Above R!, i increases with !i, approach-ing a linear relation as the last term in Equation (4) dominates.



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528 J. F. HALL

Figure 1. Actual damping ratio i of mode ias a function of frequency!i of mode iwhen [C] is given by Equation (2).

Having a small (relative to ) and a large R are desirable but, as is evident from Equa-tion (5), competing goals. If for a given R the value of from Equation (5) is unacceptablylarge, then R must be reduced. Some compromise will usually have to be made. Consideran example to illustrate what can be achieved. Suppose the fundamental frequency !1 of a

building is expected to shorten to 23!1 due to non-linear softening. Then, ! can be set to

23!1.

If the building behaves like a shear beam in its linear range, then its second-mode frequency

!2 equals 3!1. To cover the frequency range from 2

3!1 to 3!1, R is set to 4.5. This gives from Equation (5) as 0.129 . Thus, if= 0:05, then max= 0:056 and min= 0:044, whichare reasonably narrow bounds. If mode 3 has frequency !3 equal to 5!1, its damping would

be higher at 3= 0:083, as computed from Equations (4) and (6).The above procedure aims to achieve a near-constant value of damping for all modes whose

frequencies !i fall in the range from ! to R!. This is consistent with much eld data, whichindicates modal damping ratios are fairly constant for a given structure. In general, civilengineering structures are lightly damped, with modal damping ratios seldom exceeding 0.10and sometimes as low as 0.01 or 0.02.

Other formulations of the linear viscous damping matrix are available that oer greatercontrol over the damping ratios imparted to the modes [2]. However, these [C] matrices arefull; they lack the sparse and banded features of [K] and [M]. Thus, their presence greatly

increases the computational eort required to integrate the matrix equation of motion. Forthis reason, they are seldom used. With Rayleigh damping, sparseness and bandedness areimparted to [C] since it is the scaled sum of [M] and [K].

Another feature of Rayleigh damping is that [C] has a simple physical interpretation [3].If [M] is a diagonal matrix (often the case or can be made so with good approximation),then the diagonal coecients ofaM[M] are mass-proportional damping constants for a set oflinear viscous dampers that connect each degree of freedom to a xed support. The coecients



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ofaK[K] are stiness-proportional damping constants for a set of linear viscous dampers thatinterconnect the degrees of freedom in an arrangement parallel to the structural stiness.

The terms aM[M] and aK[K] can be viewed as originating from rate-dependent additions tothe equations of elasticity that provide damping. The momentum and constitutive relations [4]

are augmented as follows:

@ij@xi

+Xi= ui+aMui (7)

ij=Kijklkl+aKKijklkl (8)

where , and Kare stress, strain and elasticity tensors, respectively, x is spatial co-ordinate,X is bodyforce, is mass density, u is displacement, and i; j, etc. denote directions in arectangular co-ordinate system. Upon application of nite element discretization procedures,the aMui term ends up as aM[M]{ x(t)}, and the aKKijklkl term ends up as aK[K]{ x(t)}.

The augmented constitutive relation (8) represents a form of viscoelasticity known as a

Voigt solid [4] and is a valid type of material model. However, the augmented momentumequation is invalid because the aMui term represents the eects of a viscous, penetratingether in which the structure is immersed. Even though this mass-proportional part of Rayleighdamping cannot exist for an actual structure, it is still widely used in nite element practice

because of the additional control it provides over the modal damping ratios.The simple physical interpretation of the Rayleigh damping matrix serves as a general tool

that can be used to assess the eects of the damping and the size of the damping forces.For a linear structure, modal concepts are also available for this purpose using the modaldamping ratios. However, modal damping ratios may not always be a suitable tool, such aswhen Rayleigh damping is being applied in a non-standard way or when the restoring forcesare non-linear. In such cases, the question arises as to whether the damping forces generated

by the aM[M] and aK[K] terms are reasonable. The following section presents some examples

from earthquake engineering to illustrate problems that can arise.

EXAMPLES

Earthquake formulation

This rst example has to do with the way in which earthquake excitation is included in theequation of motion of a structure. For simplicity, the interaction between the structure and itsfoundation or supporting medium is neglected; the earthquake ground motion is assumed to

be spatially uniform; and only one horizontal component is considered. This simple case issucient to describe how Rayleigh damping can have dierent eects depending on how theearthquake excitation is formulated.

One method to apply the earthquake loading is to specify the earthquake motions at thebase of the structure. A convenient way to do this, which can be accommodated by mostcommercial computer programs, is to attach a large articial mass mb to each node at the

base of the structure and apply to each of the horizontal degrees of freedom in the directionof the ground motion a horizontal force

Feq(t) =mb xeq(t) (9)



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530 J. F. HALL

Figure 2. Two formulations to implement earthquake excitation: (a) structural motions{x(t)} relative to xed frame are computed; and (b) structural motions {xrel(t)}

relative to moving frame are computed.

where xeq(t) is the ground acceleration time history. The vector{Fdyn(t)} in Equation (1)would be comprised of such forces. Typically, the total articial mass added to the

base would be at least 103104 times the total mass of the structure. The formulation isdepicted in Figure 2(a) which also shows the mass-proportional and stiness-proportionaldamping mechanisms.

Ideally, the role of damping in a structure under earthquake excitation should be to dampthe vibrational motion of the structure relative to its base motion. As seen in Figure 2(a), themass-proportional dampers, being connected to xed supports, damp the total motion of thestructure. A better arrangement would be for these dampers to connect to a frame that moveswith the base of the structure. To accomplish this in the equation of motion, express the totalmotion of the structure

{x(t)

} as the sum of a rigid part moving with the base and a part

relative to the base:

{x(t)}={r}xeq(t) +{xrel(t)} (10)where{r} is a vector of 1s in positions corresponding to horizontal degrees of freedom inthe direction of the ground motion and 0s elsewhere. Substitution into Equation (1) gives

[M]{xrel(t)}+ [C]{ xrel(t)}+{Rs(t)}={Fstat} [M]{r}xeq(t) (11)where a term [C]{r} xeq(t) has been discarded. Omitting this term is equivalent to recon-necting the mass-proportional dampers to a frame moving with the base. (Note that [C]{r}equals aM[M]{r} since{r} is a rigid body motion.) See Figure 2(b).

Equation (11) is a standard earthquake loading formulation [2] and is implemented in

some commercial nite element computer programs. It is an alternative to Equations (1)and (9) and is preferred because it excludes the extraneous damping forces aM[M]{r} xeq(t).To demonstrate the potential signicance of these forces, take !=(1400m=s)=H, = 0:10,and R= 4:5 as for the concrete gravity dam example presented later, where H is the heightof the dam and is the desired damping ratio. With H= 100 m, aM is computed as 2:6 s

1

from Equation (6a). The peak for the ground velocity xeq(t) is chosen as 100 cm=s, which ison the high side but not an upper bound. With these values, the extraneous damping forces



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are evaluated as [M]{r} 260 cm=s2, which would not likely be negligible. However, theextraneous damping forces would be smaller for more lightly damped structures or for oneswith a lower fundamental frequency !1 (giving a lower value of !).

Another issue with the mass-proportional damping term arises with the formulation of

Equations (1) and (9). The large base masses mb should not be included in the massmatrix [M] when it is used to compute the aM[M] part of the damping matrix [C].Otherwise, the forces Feq(t) in Equation (9) need to be augmented as

Feq(t) =mb xeq(t) +aMmb xeq(t) (12)

to obtain the correct base motion. To demonstrate the potential importance of the last termin Equation (12), again take aM as 2:6 s

1 and the peak for xeq(t) as 100 cm=s. With thesevalues, the last term in Equation (12) is evaluated as mb260 cm=s2, which indicates thatthe imposed base acceleration of the structure would be signicantly in error if the last termwere omitted. This error would be smaller for lighter damping or a lower!1.

Examples in the following sections use the earthquake formulation in terms of relativemotion, i.e. Equation (11). This avoids the potential problems discussed above.

Ten-storey building

In this example, a 10-storey building (Figure 3) is subjected to an actual ground motion record(Figure 4). The buildings stiness is modelled with a bilinear shear spring in each storey, theratio of post-yield stiness to initial stiness being 0.03. Initial stinesses kj of the springsare proportional to the storey values shown in Figure 3 and are scaled to give the buildinga fundamental period T1 of 1.4 s, all oor masses being taken as equal. The yield strengthsRsy; j of the springs are also proportional to the same storey values and are scaled so that therst-storey spring has a yield strength of 0.12 times the weight Wof the building; W= 10mg,

where m= oor mass and g = gravitational acceleration. This is a reasonable strength for Zone4 seismic design and includes a signicant margin above typical code requirements. The shearspring in each storey has the same yield displacement of 1.1 cm.

Rayleigh damping is employed using the following parameters: != 23

(2=T1) to accountfor some drop in !1 due to non-linear softening, = 0:05, and R= 4:5. The aK[K] termis computed using the initial spring stinesses, which, as mentioned earlier, is how manycommercial computer programs operate.

Shown in Figures 5 and 6 are comparisons of time histories of the shear-spring force inthe rst storey and the total damping force on the building. These are consistent quantitiesto be compared since the shear-spring force in the rst storey represents the total restoringforce acting on the building. The total damping force is found by adding the damping forcein the rst storey, computed as aKk1 x1; rel(t), to the external damping force on the building,

computed by summing the aM[M]{ xrel(t)} forces.For the results in Figure 5, the ground acceleration history is scaled down to a peak

acceleration of 0.15g so that the response of the building just remains in the linear range. Onaverage, the ratio of peaks in the total damping force to peaks in the rst-storey spring forceis about 9%. This is as expected since, in the present case, the building vibrates primarilyin its fundamental mode and frequency. Theoretically, for such vibration the ratio of theseforces is 21, which is 0.092 for this 10-storey building.



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532 J. F. HALL

Figure 3. Ten-storey building shown with the forcedisplacement relation for a typicalstorey: xj; rel(t) = displacement of oor j; m= oor mass;kj= initial shear stiness of storeyj; Rs; j= restoring shear force in storey j; Rsy; j= yield value of restoring shear force;and j=displacement of storey j (dierence of oor displacements above and below).

The storey values are scaled to compute the kj and Rsy; j values.



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-40.0

0.0

40.0

Disp.

(cm)

-150.0

0.0

150.0

Vel.(cm/sec)

0 2 4 6 8 10 12-1.0

0.0

1.0

Time (sec)

Accel.(g)

0.0 0.5 1.00.0

1.0

2.0

3.0

Period (sec)

Pseudoacc.

(g)

2.0 3.0 4.0 0.0

50.0

100.0

150.0

Displacement(cm)

Figure 4. Top: displacement, velocity and acceleration time historiesrecorded at the Olive View Hospital free-eld site during the 1994Northridge earthquake. Bottom: pseudo-acceleration (light grey) and

displacement (dark grey) response spectra at 5% damping.

When the full-scale ground acceleration is applied (Figure 6), signicant yielding takesplace, and the ratio of the peak total damping force to the peak rst-storey spring force is46%. In terms of the buildings weight, the peak total damping force is 0.072W, whichequals 60% of the yield strength of the building (0.12W). No conceivable mechanism in an

actual building exists that could produce a damping force this high relative to the strength ofthe building. Such large damping forces mean the results of the analysis are unconservative.

The situation still exists if mass-proportional damping only is used (aK= 0, aM= 2!) orif stiness-proportional damping only is used (aM= 0, aK= 2=!). For these two cases, the

peak total damping force reaches 29 and 102%, respectively, of the yield strength ofthe building. The source of the problem is that a yielding mechanism is provided to limit thespring forces, but no such mechanism is used for the damping forces. The linearity assumption



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534 J. F. HALL

2 4 6 8 10-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Time (sec)

Normalizedforc

e

Figure 5. Time histories of the shear-spring force in the rst storey (solid) and the total

damping force on the building (dashed) for the Olive View ground motion scaled by 0.15(linear response). The forces are normalized by the yield strength of the rst storey.

2 4 6 8 10-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Time (sec)

Normalizedforce

Figure 6. Time histories of the shear-spring force in the rst storey (solid) and thetotal damping force on the building (dashed) for the full-scale Olive View ground motion(non-linear response). The forces are normalized by the yield strength of the rst storey.

on the damping forces means they are always proportional to the velocities, no matter howlarge they become.

Finally, if the building were modelled as a frame of beam and column elements that arecapable of forming plastic hinges to represent non-linear bending eects, a similar situationto that described above for the building comprised of non-linear shear springs would beencountered. The plastic hinges would limit the moments that could develop at the ends ofthe beams and columns, which in turn would limit the shear forces in these elements and tosome extent the axial forces. The damping forces and moments, on the other hand, withouttheir own limiting mechanism, could reach unrealistically high values.



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Base-isolated building

Consider a base-isolated building (Figure 7) with fundamental period of the superstructuredenoted by TS (period if base were xed) and with design period of the isolated building

denoted by TI. Suppose [C] is constructed using only the superstructure properties since allenergy dissipation associated with the isolators is to be modelled explicitly in the isolatorelements. Let denote the desired damping in the superstructure. The frequency ! is chosenas 2=TS since the superstructure is expected to remain linear. The ratio ofTS to TI is takenas 1 to 5, which is reasonable for a 3- or 4-storey superstructure and a typical value of 2.5 sforTI.

During vibration from an earthquake, the mass-proportional damping forces aM[M]{ xrel(t)}on the superstructure produce a total damping force of

Rmpd (t) =aMMSvS; rel(t) (13)

where MS is the mass of the superstructure and vS; rel(t) is the velocity of the superstructure

relative to the ground. Equation (13) is a good approximation because, with TS= TI=5, theentire superstructure can be assumed to move as a rigid body on the exible isolators, i.e. asa single degree of freedom. Note that the stiness-proportional part of [C] does not contributeto the total damping force on the superstructure. From Equation (13) the damping value CIin a single-degree-of-freedom equation of motion for the isolated building is aMMS, whichcan also be expressed in the usual way as 2I!IMS, where I is the resulting damping ratiofor the single-degree-of-freedom structure (i.e. the isolated building) and !I= 2=TI. Settingthe two expressions forCI equal to each other and solving for I gives

I = 1

2aM=!I (14)

Substitution for aM

from Equation (6a) and replacing frequencies by periods leads to

I = 2R

1 +R+ 2R

TITS

(15)

With = 0:05 for the superstructure, R= 4:5, and TS=TI=5 as stated above, the resultingdamping ratio I for the isolated building equals 0.23. This unintentionally high value iscompletely unrealistic and does not even include any energy dissipation associated with theisolation system. In this case the mass-proportional damping term is the source of the prob-lem, along with the non-standard application of Rayleigh damping as it is used only for therelatively high-frequency superstructure. Unlike the example 10-storey building in the previoussection, no non-linearity in the restoring forces is required for the unrealistic damping eect

to occur in the base-isolated building.A possible solution to the problem is to use only stiness-proportional damping. Suppose

this is done, but also the contribution from the isolator stiness is included when computingthe stiness-proportional damping term aK[K]. Bilinear springs are to be employed for theisolators (Figure 8), where the initial stiness kL is times the secant stiness kI used indesign to achieve the period TI for the isolated building (idealized as a single-degree-of-freedom structure). It is kL that is used in [K].



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536 J. F. HALL

Figure 7. Base-isolated building.

Figure 8. Forcedisplacement relation for the isolation system (all isolatorstogether):RI= restoring shear force of the isolation system;RIy= yield valueof the restoring shear force; DD=isolator design displacement; kI= secantshear stiness of the isolation system when displaced at the design

displacement; and kL= initial shear stiness of the isolation system.

During earthquake vibration, the stiness-proportional damping exerts a force on the super-structure at the location of the isolators given by

Rspd (t) =aKkLvS; rel(t) (16)

where, again, the superstructure is assumed to move as a rigid body and vS; rel(t) is its velocityrelative to the ground. For stiness-proportional damping only, aK= 2=!, where is thedesired damping for the isolated building and != 2=TI. The term aKkL in Equation (16)is the damping value CI in a single-degree-of-freedom equation of motion for the isolated



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building, which also equals 2I!IMS as before. Equating the two expressions for CI andsolving for I gives

I= 1

2aKkL

1

!I

1

MS(17)

Substituting for aK and replacing !I by (kI=MS)1=2 leads to I= where = kL=kI.

Thus, the actual damping ratio I for the isolated building, idealized as a single-degree-of-freedom structure, is a factor larger than intended. This does not include additionalenergy dissipation from hysteresis in the bilinear isolator model (Figure 8). Since could

be a factor of three or more [5], the eect is signicant. The problem is basically similarto that encountered in the previous section with the 10-storey building, although a slightlydierent interpretation can be oered here. When the isolated building vibrates from thedesign earthquake, the equivalent linear stiness of the isolators is the secant stiness kI, yetthe stiness-proportional damping term aK[K] is computed with the higher initial stiness kL.This leads to a damping shear force across the isolators that is too large by the factor

relative to the intended damping force. Use of kI in the aK[K] damping term would be abetter choice.

Gravity dam undergoing sliding

Consider a concrete gravity dam of approximately triangular cross-section with height H andbase length B= 0:8H (Figure 9). Water is represented as added mass that augments the massof the dam, i.e. [M] = [Mdam] + [Mwat]. A pre-existing horizontal crack, along which the damcan slide, runs through the bottom row of nite elements in the dam mesh. If sliding is nottaking place, dxy=Gcdxy, and during sliding, xy= y, where xy and xy are the shearstress and strain associated with the sliding plane, Gc is the shear modulus of concrete, yis the vertical stress, and is the coecient of friction. These relations are enforced at the

integration points of the elements, a technique known as the smeared crack method.Rayleigh damping is employed with both mass and stiness-proportional damping parts.The desired level of damping is = 0:10, and R is chosen as 4.5. The frequency ! is setto 2

3(2=T1) to account for non-linear softening. The fundamental period T1 of the linear dam

water system is expressed as (0.003 s=m) H, consistent with approximate period formulas [6].A possible value for the sliding velocity vsld along the base crack during an earthquakeis 50 cm=s based on previous studies [7]; this should not be considered as an upper bound.

The magnitude of the damping force exerted on the dam by the mass-proportional damp-ing term during sliding can be estimated by assuming the dam slides as a rigid body atvelocity vsld. Thus,

Rmpd =aMMvsld (18)

where M is the combined mass of the dam and water taken approximately as

M =Mdam32

= 1

2HB

cg 3

2 (19)

where the 32

approximately accounts for the water contribution to the mass, c is theunit weight of concrete, and g is gravitational acceleration. Substitution of Equations (6a)



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538 J. F. HALL

Figure 9. Concrete gravity dam that slides along a pre-existing crackthrough the bottom layer of nite elements.

and (19) into Equation (18) leads to

Rmpd = 3!

2R

1 +R+ 2R

vsldg Wdam (20)

where Wdam is the weight of the dam. Further substitution of the suggested values of !, T1,R and vsld gives

Rmpd =

20m

H Wdam (21)

Thus, for a 100 m high dam, the damping force would reach 20% of the dams weight for asliding velocity of 50 cm=s, acting in a direction to oppose the sliding. This can be comparedto a frictional force of 70% of the dams weight if, say, the friction coecient were 0.70. So,the additional resistance to sliding provided by the mass-proportional damping is moderatelysignicant. Of course, no such additional resistance should be present since, as pointed outearlier, mass-proportional damping cannot exist in a real structure.

The damping force of interest from the stiness-proportional damping term acts along thecrack as a shear force during sliding. The magnitude of this force is given by

Rsp

d =d; xyB (22)

where the damping shear stress is

d; xy =aKGcxy (23)

This assumes the stiness-proportional damping is computed using the initial linear [K] ofthe dam, i.e. the presence of the base crack is not represented. Substitution of Equation (6b)



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and noting that xy= vsld=h during sliding, where h is the height of the bottom row of niteelements in the dam mesh, gives

R

sp

d = 2

1

!

2

1 +R+ 2RGcvsld

h B (24)

Some further modication results in

Rspd =

1

350s=m

2

1 +R+ 2R

Gcc

vsldh Wdam (25)

where the previously mentioned expressions for ! and T1 have been used, and B has beeneliminated using Equation (19). Finally, taking Gc=c= 400 000 m, setting h to 5 m, andsubstitution of the other suggested values for , R and vsld gives R

spd = 2:3Wdam. How-

ever, such a huge damping force probably would never develop because the sliding velocitywould stay small. Regardless, the conclusion is that stiness-proportional damping can greatly

inhibit sliding.This problem with stiness-proportional damping is similar to that seen in the previousexamples with the 10-storey building and the base-isolated building. However, the situationwith the concrete gravity dam is more severe because the initial linear shear stiness of thenite elements containing the crack, which is used in the aK[K] term, is very high. Obviously,the damping force needs to be limited in some way in order for realistic results to be obtained.

Gravity dam undergoing cracking

This example is another demonstration of the problem with stiness-proportional dampingin a non-linear analysis when the initial linear [K] is used in the aK[K] term. Considerthe formation, opening and closing of cracks in a concrete gravity dam discretized with

nite elements. When a crack forms in an element, the stress n in the direction normal tothe crack is set to zero as long as the crack is open, this condition being enforced at theintegration points (smeared crack method). Frictional sliding criteria can also be imposed aftera crack forms. The strain rate n associated with opening and closing of the crack generatesan opposing damping stress d; n owing to stiness-proportional damping. The magnitude ofthis damping stress is approximately equal to

d; n = aKEcn (26)

where Ec is Youngs modulus for concrete, and where the coupling terms involving Poissonsratio have been neglected. Use of the initial linear stiness in the aK[K] term is assumed,i.e. the presence of the crack is not represented. In terms of the crack opening=closing ve-

locity voc, n equals voc=h, where h is the element dimension in the direction normal to thecrack. Based on previous studies [8], a possible value of voc is 50cm=s, which should not beconsidered an upper bound.

Using Equation (6b), Equation (26) is written as

d; n = 21

!

2

1 +R+ 2R

Ecten

1

hvocten (27)



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540 J. F. HALL

introducing the tensile strength of concrete ten. Inserting the expressions for ! and T1 fromthe previous example,

d; n = 1700

s=m 21 +R+ 2R

Ec

ten

H

hvocten (28)

Finally, substituting = 0:10, R= 4:5, voc=50 cm=s, and taking Ec=ten= 8000 and H=h=20,the damping stress d; n is found as 2:3ten.

This large value indicates that two signicant articial eects can occur regarding the extentof cracking. First, after the rst element along the edge of the dam cracks, the opening of thiscrack could be restrained enough to falsely impede the propagation of the crack further intothe interior of the dam mesh. Second, the tensile damping stresses carried across the crackas it opens could load the adjacent elements above and below the cracked element enough to

produce false cracks at these locations. An appropriate limit on the damping stress d; n wouldresolve these problems. Damping eects associated with sliding along a crack would have to

be dealt with simultaneously.

Penalty elements

Penalty elements are commonly used to impose constraints, such as those involving contactand sliding along interfaces. The simplest such application employs non-linear axial and shearsprings shown in Figures 10(a) and (b). The springs would be oriented normal and tangent,respectively, to the interface at the point of contact or potential contact. The stiness ka ofthe axial spring needs to be high to minimize penetration of the surfaces when in contact,similarly for stiness ks of the shear spring to minimize slip when the sliding condition isnot met. If the stiness-proportional damping term aK[K] is formed using such high stinessvalues, then large damping forces would be generated upon separation or sliding, which wouldgreatly restrain further movement between the surfaces.

The eect is similar to that demonstrated previously for base sliding and crack opening=closing of the concrete gravity dam example, only even more severe. If the h dimension of anite element of the dam mesh containing a smeared crack (element width normal to crack)were reduced to a small value, the element would act essentially as a penalty element. Asseen in Equations (25) and (28), the damping force or stress is inversely proportional to h.

Another application of penalty elements is using rotational springs to model plastic hingesin beam elements (Figure 10(c)). When the moment M carried by a spring is less than the

plastic moment capacity Mp of the beam, then the rotational stiness kr of the spring needsto be high to minimize its contribution to the exibility of the beam. If such a high stinessis included in the aK[K] term, a large damping moment will be generated when M reachesMp and the spring starts to develop a relative rotation, which will then act to greatly restrainfurther relative rotation.

The simplest solution to the stiness-proportional damping problem with penalty elements isnot to include any stiness contributions from these elements in the aK[K] term of the dampingmatrix. Since penalty elements are present to provide constraints and do not contribute to theelastic deformability of the structure, there is no physical reason why they should containdamping. On the other hand, if some damping is needed, say, to control spurious oscillations,then appropriate limits on the damping forces should be employed to avoid unrealisticallyhigh damping forces being generated.



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Figure 10. Force-displacement (or moment-rotation) relation of high-stiness penalty springs: Ra, a,ka= force, displacement, stiness of an axial spring; Rs, s, ks= frictional force, sliding displacement,stiness of a shear spring; = coecient of friction;M, , kr= plastic hinge moment, rotation, stinessof a rotational spring; and MP= plastic moment capacity: (a) axial spring on contact surface; (b) shear

spring on contact surface; and (c) rotational spring to model plastic hinge.

CAPPED VISCOUS DAMPING

As demonstrated in the previous examples, under certain conditions problems can occur withboth the mass and stiness-proportional parts of Rayleigh damping. In this section, a modieddamping formulation is examined in which the mass-proportional contribution is eliminated

and the stiness-proportional contribution is bounded. This is an attempt to overcome theproblems that have been discussed.

Consider as an example the simple model used earlier for the 10-storey building shown inFigure 3. A capped viscous damper is placed in each storey along side the shear spring there.The amplitude of the shear force in the damper in storey j is computed as

Rd; j(t) =min(aKkj| xrel; j(t) xrel; j1(t)|; 2Rsy; j) (29)



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542 J. F. HALL

2 4 6 8 10-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Time (sec)

Normalizedfo

rce

Figure 11. Time histories of the shear-spring force in the rst storey (solid) and the damp-ing shear force in the rst storey (dashed) for the full-scale Olive View ground motion(non-linear response). The forces are normalized by the yield strength of the rst storey.

These results are for capped viscous damping.

where aK= 2=!, is the damping parameter, != 23

(2=T1), Rsy; j is the yield strength ofthe shear spring in storey j, and xrel; o(t) is taken as zero. The sign of Rd; j(t) is chosen sothat this force opposes the shearing velocity of the storey. For a low level of excitation,the building will respond as if it had linear stiness-proportional damping, i.e. [C] = aK[K]with [K] computed using the initial stinesses of the storey shear springs. At higher levelsof excitation, the storey damping forces will be capped by the 2Rsy; j term, so the damping

becomes non-linear. The 2 multiplier comes from the fact that, with aK computed as 2=!,the peak damping shear force in a storey equals 2 times the peak shear-spring force in that

storey when the building undergoes harmonic linear response at the frequency !. So, with= 0:05 as taken previously, the damping force in each storey will be capped at 10% of theshear-spring yield strength of that storey. This seems like a reasonable limit to place on thedamping forces.

The previous 10-storey building example is now repeated with all mass, stiness andstrength parameters as before, but with capped viscous damping at = 0:05. Figure 11 corres-

ponds to Figure 6 and shows the time history of the capped viscous damping force in the rststorey along with the time history of the shear-spring force there. The capping of the dampingforce at 0.10 of the shear-spring yield strength is evident. This reduction in the damping forceleads to a higher rst storey displacement, which is reected by an increase in the post-yieldforce in the shear spring (compare to the spring force in Figure 6). The displacement timehistories of all ten oor levels of the building are shown in Figure 12 for two cases: linear

viscous damping (the previous case) on the top and capped viscous damping on the bottom.The plotted displacements are relative to the ground. Large increases occur in the shearingdisplacements of the rst three stories of the building when capped viscous damping is used.For the rst, second and third stories, peak shearing displacements are 12 :2; 8:2 and 6.0cmfor the linear damping case and 16:9; 12:6 and 9.5 cm for the capped viscous damping case.These signicant increases demonstrate that the unrealistically high linear viscous dampingforces generated by the Rayleigh damping matrix produce very unconservative results.



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2 4 6 8 10-50.0

-25.0

0.00

25.0

50.0

Floordisplacemen

t(cm)

Linear viscous damping

2 4 6 8 10-50.0

-25.0

0.00

25.0

50.0

Time (sec)

Floordisplacement

(cm)

Capped viscous damping

Figure 12. Time histories of displacements of the oors of the 10-storey building for the full-scaleOlive View ground motion (non-linear response). The top plot is for linear viscous damping, and

the bottom plot is for capped viscous damping. The ten curves in each plot correspond to the tenoor levels of the building and are motions relative to the ground.

The concept of capped viscous damping can be generalized to more complex systems.In solid mechanics where stress and strain are used, the mass-proportional term aMui inEquation (7) would be dropped, and bounds would be placed on the stiness-proportionaldamping stresses given by the aKKijklkl term in Equation (8). For a situation where plasticyielding is the dominant non-linear mechanism, appropriate caps could be based on the yieldstrengths of the material scaled down by the 2 factor. With other types of non-linearity, suchas interfaces where opening, closing and sliding occur, the choice of appropriate caps for thedamping stresses is less obvious. For example, the sliding strength of the interface depends on

the instantaneous value of the normal stress, which could vary within a wide range, includingbeing zero when separation occurs. Such issues, as well as other damping formulations thatovercome the problems demonstrated in the previous sections, are the subject of continuinginvestigation.

One of these potential damping formulations that is sometimes employed is to use thecurrent tangent stiness matrix in the stiness-proportional part of the Rayleigh dampingmatrix. This will reduce the damping forces when the tangent stiness reduces during yielding,



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544 J. F. HALL

sliding, opening, etc., which would seem to alleviate some of the problems discussed in thispaper that occur when stiness-proportional damping uses the initial linear stiness matrix.However, use of the tangent stiness is an ad hoc approach since there is no physical basis tosuch a damping mechanism. Furthermore, the damping forces can change rapidly in time and

even be discontinuous when there is a sudden change in tangent stiness. For the exampleof the cracked gravity dam discussed earlier, during crack closing the damping stress d; nwould jump from zero to a possibly large value, dependent on the closing velocity, as soonas contact is made. Such behaviour can cause convergence diculties. Use of the tangentstiness matrix in the damping formulation is not recommended.

CONCLUSIONS

The use of Rayleigh damping in nite element analyses of structures under dynamic loadscan result in damping forces that, under certain conditions, are unrealistically large and, thus,unconservative. Assessment of these damping forces is aided by a physical interpretation ofthe mass and stiness-proportional parts of the damping matrix.

The mass-proportional part of Rayleigh damping corresponds physically to linear viscousdampers that connect degrees of freedom of the structure to external supports. Althoughsuch a mechanism cannot possibly exist, mass-proportional damping is commonly includedin dynamic analyses because of the increased control it allows over modal damping ratios.However, problems can occur. Formulation of an earthquake analysis in terms of total motioninvolves a component of rigid body motion that generates extraneous damping forces if mass-

proportional damping is present. An alternate formulation using motion relative to the groundsolves this problem. Another example occurs in analysis of a base-isolated structure if theRayleigh damping matrix is constructed using properties of the relatively sti superstructure

alone. The damping ratio imparted to the overall structure consisting of the superstructure andthe exible isolators can be very high owing to the mass-proportional damping term. A thirdtype of problem arises if a portion of a structure breaks loose and thereby develops a highvelocity; in which case, large mass-proportional damping forces can also develop. An exampleof this is a structure sliding on its base such as a concrete gravity dam. Quantication of theundesirable eects in all of these cases is detailed in the paper.

The stiness-proportional part of Rayleigh damping corresponds physically to linear viscousdampers that interconnect the degrees of freedom of a structure. In a non-linear analysis wherethe non-linearity is of the softening type, limits on the restoring forces are imposed by variousmechanisms such as yielding, cracking, sliding and buckling. If the initial linear stiness matrixis employed to construct the stiness-proportional damping term, then the damping forces ina softening element can reach unrealistically high values compared to the elements restoring

forces as the velocity gradient across the softening element increases. The greater the initialstiness of the softening element and the higher the velocity gradient, the greater is the eect.As quantied in the paper, this eect can be very signicant.

One suggested remedy to the problems with Rayleigh damping is to eliminate the mass-proportional damping contribution and bound the stiness-proportional damping contribution.This keeps the damping forces within reasonable limits set by the analyst. In some situations,such as non-linear contact, appropriate cap values may not be obvious. Formulation of damping



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strategies that ensure reasonable damping forces for a wide variety of non-linear structuralbehaviour is an area that needs further research.

REFERENCES1. Bathe K-J. Finite Element Procedures. Prentice-Hall: Englewood Clis, NJ, 1996.2. Chopra AK. Dynamics of StructuresTheory and Applications to Earthquake Engineering (2nd edn).

Prentice-Hall: Englewood Clis, NJ, 2001.3. Weaver W, Johnston PR. Structural Dynamics by Finite Elements. Prentice-Hall: Englewood Clis, NJ, 1987.4. Fung YC. A First Course in Continuum Mechanics. Prentice-Hall: Englewood Clis, NJ, 1969.5. Hall JF, Ryan KL. Isolated buildings and the 1997 UBC near-source factors. Earthquake Spectra 2000; 16:

393411.6. Fenves G, Chopra AK. Simplied analysis for earthquake resistant design of concrete gravity dams. Report

UCB=EERC-85=10, Earthquake Engineering Research Center, University of California, Berkeley, June 1986.7. Chavez JW, Fenves GL. Earthquake analysis and response of concrete gravity dams including base sliding. Report

UCB=EERC-93/07, Earthquake Engineering Research Center, University of California, Berkeley, December 1993.8. El-Aidi B, Hall JF. Non-linear earthquake response of concrete gravity dams part 2: behavior. Earthquake

Engineering and Structural Dynamics 1989; 18:853865.


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