Advances in Steel Structures - part 58 ppt

Shear analysis for asphalt concrete deck pavement of steel bridges Xudong Zha 1 and Qiuming Xiao 2 1Department of Highway and Bridge Engineering, Changsha Communications University, Chan

550 X.-S Li and Y Goto

Figure 6 shows that the ductility factor/1 p of the pier increases with the increase of yield force ratio Qy/Py for most values of Kbl/Kp, while the ductility factor/1 b of the isolator decreases A different tendency, however, is observed for Model-2 with Kb~/Kp=0.7 as shown in Fig.6(c) In this case, the maximum value of /.t p is obtained for the yield force ratio of Qy/Py=0.2 that is the smallest one This phenomenon is identical with that observed in Fig.5(c) and is also identical with those obtained for concrete piers by Kawashima & Shoji (1998) It can be concluded that an isolator with a higer stiffness and a smaller yield force may result in much damage of the pier when subjected to wave Type 2- Ill

Fig.7: Relations between Energy Ratio Ep/Ef and Period Ratio Tp/Tf

Effect of Elongation of Natural Period

The main difference between isolated bridges and conventional bridges is characterized by a significant increase in the natural period of isolated bridges due to the introduction of the isolator that may also effectively absorb energy As a result, the inertia force induced by earthquake is considerably reduced in isolated bridges In order to find an appropriate period range for the isolated bridge, an energy ratio Ep/Ef is used to evaluate the damage of the pier, where Ep = 4Ppdup and Ef = ~Pfduf are the energy absorbed in the pier with the isolator and that without the isolator, respectively And, a period ratio Tp/Tf is used as the abscissa, where Tp = 2nx/m/K e and Tf 2nx/m/K p are the fundamental periods of the isolated bridge and the conventional one, respectively K e is the equivalent stiffness of the isolated bridge and defined as Ke=KBKp/(KB+Kp), where KB is the equivalent stiffness

of the isolator as shown in Fig.2 Figure.7 shows the relations between the energy ratio Ep/Ef and the period ratio Tp/Tf for Model-1 and Model-2 It can been seen from Fig.7 (a) that a general tendency is that the energy ratio Ep/Ef decreases as the period ratio Tp/Tf increases, except for a few points The three points over 0.4 are corresponding to the cases: (Qy/Py=0.3, Kbl/Kp=0.8, 0.9) and (Qy/Py=0.2, Kbl/Kp=0.7) for Model-2 subjected to wave Type 2-Ill If both Qy/Py and Kbl/Kp are limited to the

Seismic Analysis of Isolated Steel Highway Bridge 551 range from 0.3 to 0.7 that are favorable in the practical design, the maximum value of Ep/Ef will be less than 0.3 as shown in Fig.7 (b) This result implies that the range from 0.3 to 0.7 is appropriate for both Qy/Vy and Kbl/Kp of the isolator The corresponding period ratio Tp/Tf is between 2.1 and 3.7 Furthermore, by noting that Ep/Ef approaches zero when Tp/Tf > 3.0, the range from 2.1 to 3.0 of the period ratio Tp/Tf may be more appropriate This is based on the fact that too much elongation of the natural period may lead to an undesirable large displacement of the deck

Application of 'Property of Displacement Conversation'

As previously described, the interaction between the pier and the isolator results in the computational difficulty due to the nonlinearity that occurs in both the pier and the isolator To avoid the difficulty, some simple methods have been proposed for predicting the dynamic response of the pier, such as the method based on the 'Energy Conservation Principle' For the conventional bridge pier that has a shorter fundamental period, the method has been verified applicable and widely used in practical design For the isolated bridge that has a longer fundamental period, however, it overestimates or underestimates the response of the pier Here, based on the 'Displacement Conservation Principle' that states 'the maximum elasto-plastic deformation of a system with a long fundamental period is approximately equal to the maximum elastic deformation of the same system', the maximum responses of steel piers are predicted by elastic dynamic analysis In this method, the maximum elastic response displacements of both the pier and the isolator are first dynamically calculated for the isolated bridge with the elastic stiffness Kp of the pier and the equivalent stiffness KB of the isolator Then, the maximum lateral force of the pier can be obtained by equating the elastic response displacement to the static elasto-plastic response displacement

Fig.8: Comparison of Maximum Responses of Piers Obtained

by Simple Method and Analytical Method

The comparison of the maximum responses of the piers obtained by the simple method and by the elasto-plastic dynamic method are shown in Fig.8 (a), where RF and ~ are the maximum lateral

552 X.-S Li and Y Goto

force and displacement of the pier, respectively The subscripts e and p denote the elastic dynamic response and the elasto-plastic dynamic response It is observed that the maximum lateral force ratio RFJRFp is closer to 1.0 when Tp/Tf is smaller than 3.0, whilst the maximum lateral displacement ratio

d d d p is in the range from 0.77 to 1.86 Similarly, if both Qy/Py and Kbl/K p are limited to the range from 0.3 to 0.7, RFe/RF p and ~ o/~ p will take the values between 1.0 to 1.81 The approximate results obtained by the 'Displacement Conservation Principle ' may be considered to be reasonably safe and accurate for practical design of the isolated steel piers

SUMMARY AND CONCLUDING REMARKS

From the above analysis, the following conclusions are obtained

The response of an isolated bridge is greatly influenced by the initial stiffness and yield force of the isolator The both ductility factors for the pier and the isolator increase with the increase of the initial stiffness ratio Kb~/I< v of the isolator The maximum response displacement of the pier increases with the increase of yield force ratio Qy/Py for most values of Kb~/l ~, while the ductility factor r b of the isolator decreases An isolator with a higher stiffness and a smaller yield force may result in much damage of the pier when subjected to wave Type 2-m'

As a general tendency, the energy ratio Ep/Ef decreases as the period ratio Tp/Tf increases Within the range from 0.3 to 0.7 for both Qy/Py and Kb~/Kp of the isolator, the energy ratio Ep/Ef will be less than 0.3 Furthermore, by noting that Ep/Ef approaches zero when Tp/Tf > 3.0, the range from 2.1 to 3.0 of the period ratio Tp/Tf may be more appropriate

The applicability of the 'Displacement Conservation Principle' for predicting the maximum responses

of steel piers of the isolated bridges is numerically confirmed The numerical results show that both RF~RFp and d d ~ p will take the values between 1.0 and 1.81 when Qy/Py and Kbl/Kp are limited to the range from 0.3 to 0.7 The approximate results may be considered to be reasonably safe and accurate for practical design of the isolated steel piers

REFERENCE

Kawashima, K and Shoji, G (1998) Interaction of Hysteretic Behavior between Isolator/Damper and Pier in an Isolated Bridge, J Struct Engrg JSCE Vol.44A, pp.733-741

Li, X.S and Goto, Y (1998) A three-dimensional nonlinear seismic analysis of frames considering panel zone deformation, Struct Eng.~Earthquake Eng., JSCE, Vol.15, No.2, pp.201-213, 1998, Oct Ministry of Construction (1992) Manual of Menshin Design of Highway Bridges, Civil Engineering Research Center, Tokyo, Japan (In Japanese)

Shear analysis for asphalt concrete deck pavement of steel bridges

Xudong Zha 1 and Qiuming Xiao 2 1Department of Highway and Bridge Engineering, Changsha Communications University,

Changsha, 410076, PRC 2Department of Highway and Bridge Engineering, Changsha Communications University,

Changsha, 410076, PRC

ABSTRACT

This paper is based on the elastic layered theory of rigid support The horizontal shear stresses between the bridge deck pavement of asphalt concrete and steel box girders, the maximum principal stresses and the maximum shear stresses at the bottom of the deck pavement layer has been analyzed under the normal motion and the emergency break of motor vehicles Based on this result, the shear resistance index of binder course has been put forward so as to supply the binder course design of steel bridge deck pavement with a scientific basis

KEYWORDS

steel bridge, bridge deck pavement, asphaltconcrete, horizontal shoving, binder course, elastic layer, principal stress, shear stress, shear strength, shear resistance index

PREFACE

In the late 70s, China started building steel bridges with orthotropic plate decks, and the relative decking technology research has been done for 20 years From Mafang Bridge to recently completed Xiling Bridge, Humen Bridge and Qingma Bridge, the deck pavement technology has been continuously improved and developed However, up to now, bridge deck pavement engineering of steel bridges in China has not achieved a complete success

553

554 X Zha and Q Xiao

There are three main reasons for the distresses of bridge deck pavement of Xiling Yangzi Bridge, Chongqing Highway Scientific Research Institute (1998) The first is that the deck pavement system was not perfect, the second the qualification of construction teams could not meet the requirement for the works, and the third the influence of heavy truck vehicles used for the Three Gorges was not fully estimated The local temperature conditions was not completely occupied during the designing of Humen Bridge, and the worse thing was that the deck pavement thickness and structure were changed

at the time of construction, thus reducing the material quality, and the thermal stability distress appeared due to the poor construction control Although Qingma Bridge built in accordance with the English bridge deck pavement improved the softening point of hard asphalt to 70~ the deck failed

to pass the test of high temperature in Hong Kong In the meantime, the use of Eliminator waterproof adhesive agent left the hidden trouble of air bubbles

Now, China is building a lot of steel bridges with orthotropic plate decks The bridges under construction are Jiangyin Yangzi Bridge, Xiamen Haicang Bridge, Nangjing Yangzi Bridge II, Chongqing Egongyan Yangzi Bridge, and etc The bridge deck pavement of asphalt concrete was employed in most of bridge construction Therefore, the bridge deck pavement technology should be urgently solved for steel box girders The paper has put forward the shear resistance index for asphalt concrete bridge deck pavement through the theoretical analysis as far as the horizontal shoving problem between asphalt concrete of the bridge deck pavement and the steel sheets are concerned This has provided the design of binder course for bridge deck pavement and program alternatives with a scientific basis

BASIC THEORY

Due to the strength and rigidity of asphalt concrete is far from those of the steel box girders, the geometric sizes of the wheel load is much less than the length and width of the steel box girders In the meantime, under the wheel load, the reducing speed of the horizontal distribution of the asphalt concrete stress is rapid Therefore, the asphalt concrete deck pavement of steel bridges can be regarded

as a elastic layer, while the steel box girders can be simplified as rigid support Thus, the theory of elastic layers on the rigid support can be used for the stress analysis of deck pavement, while the motor vehicle load simplified as the uniformly distributed loads of double vertical and horizontal circles,

Asphalt concrete deck | pavment E, h, ~t 1 r / / / Binder course / / , < \ V / , , K \ \ ~z Steel girder

Figure 1: Mechanical model of bridge deck pavement analysis

Shear Analysis for Asphalt Concrete Deck Pavement of Steel Bridges 555 Highway Planning and Design Institute of Ministry of Communications (1997) The mechanical model

is seen in Figure 1, x showing the longitudinal direction and y the horizontal one

SHEAR STRESS ANALYSIS

Nowadays there exits a trend to apply SMA material for bridge deck pavement of asphalt concrete in China The pavement will be built in two layers The size of the aggregate for the upper layer is 2-5mm larger than that for the lower layer, the general thickness of the pavement 6cm and the modulus value of SMA measured in laboratory 1400 -1600MPa Therefore, in Figure 1, when the analysis is carried out, the layer thickness h = 6cm, the modulus E = 1500MPa, the Poisson's ratio ~t = 0.25 The load of motor vehicles will be the standard axle load of 100kN, and the radius 5 = 10.65cm, the vertical load p = 0.7MPa The horizontal load will be determined in accordance with the friction coefficient f between wheel and deck pavement, i.e.:

Where: q is the horizontal load (MPa); p the vertical load (MPa); f the friction coefficient, 0.2 represents normal motion of motor vehicles, and 0.5 accidental emergency break, Fang Fusen (1993)

According to the elastic-layered theory and the above data chosen, this paper has analyzed the maximum principal stress cr 1 at the bottom of the pavement (z = 6cm), the maximum shear stress Xmax and the horizontal shear stress Xxz The different stress distributions will be computed under normal motion of motor vehicles and emergency conditions, as shown in Figure 2 Figure 7 The maximum and minimum value of various kinds of stress will be seen in Table 1 From Figure 2-Figure 7, the maximum and minimum value of various stress at the bottom of the deck pavement layer will always appear on x-direction in the center of the singular wheel The relevant profile distributions in x- direction will be seen in Figure 8-Figure 10

TABLE 1

MAXIMUM AND MINIMUM VALUE OF VARIOUS KINDS OF STRESS (MPa)

Items

Maximum value

Minimum value

f= 0.2

Gl "~m~x "l~xz

f=0.5

From the above figures and tables, the maximum principal stress is mostly under compressive condition The maximum tension stress under both conditions will often be less than 0.1MPa, while the cleavage strength of SMA between 1.2-1.6MPa This shows that the fatigue cracking will not generally appear in the paving layer The maximum value of the maximum shear stress will be less than 0.40MPa Since SMA is of excellent stability at high temperature The test shows that the shear strength is between 1.0-1.4MPa Therefore, the fatigue shear distress will not normally occur inside

556 X Zha and Q Xiao

Figure 2: Distribution of principal stress Cl (f = 0.2)

Figure 3: Distribution of maximum shear stress Xm~x (f = 0.2)

Figure 4: Distribution of horizontal shear stress Xx~ (f= 0.2)

Shear Analysis for Asphalt Concrete Deck Pavement of Steel Bridges 557

Figure 5: Distribution of principal stress c~ (f= 0.5)

Figure 6: Distribution of maximum shear stress Xm~x (f = 0.5)

Figure 7: Distribution of horizontal shear stress x= (f= 0.5)

558 X Zha and Q Xiao

Figure 8: Distribution of principal stress t~ l at center of singular wheel in x-direction

Figure 10: Distribution of horizontal shear stress Xxz at center of singular wheel in x-direction

Shear Analys& for Asphalt Concrete Deck Pavement of Steel Bridges

the paving layer

559

As to the horizontal shear stress between the paving layer and steel sheets, the maximum absolute value will appear at the place of 9.585cm (0.95) from the center of singular wheel along the driving direction The relevant value of the normal motion and emergency break of motor vehicles will separately be -0.189MPa and -0.283MPa Under normal driving condition, the area of reverse direction for the horizontal shear stress will happen at the bottom of the front edge of singular wheel (Figure 10) The maximum value is 0.068MPa Through analysis, the larger shear stress will produce between the deck pavement layer at 0.96 from the center of singular wheel and the steel sheet under wheel loads, and this will result in horizontal shoving Therefore, the design of the binder course should meet the requirement of the shear action Because the deck pavement layer will bear the repeated load of motor vehicles, with the consideration of overloads and heavy loads, the shear resistance strength of the binder course should be of enough safety The shear resistance index is as follows, Fang Fusen (1993):

1;

K,

In which 1;a is the computed horizontal shear stress value (MPa) ; 1;R is the allowable horizontal shear stress (MPa); 1; is the shear resistance strength (MPa); I~ the structural coefficient of the shear resistance strength, this is related to the acting times of axle loads

K~ should be determined in line with the traffic conditions, the application conditions and the importance of the steel bridge through the fatigue shear tests I~ will be 2-3 at the time of normal driving, while I~ will be 1.5-2 when in emergency From it, the shear resistance strength index of the binder course will be 0.4-0.6MPa

CONCLUSION

This paper analyzes the maximum principal stresses, the maximum shear stresses and the horizontal shear stresses in the deck pavement layer of asphalt concrete for steel bridges under the load of motor vehicles The analyzing result shows that the fatigue tension cracks and fatigue shear distress within the paving layer will not generally happen However, the greater horizontal shear stresses will take place between the deck pavement and the steel box girders This is the main cause for the horizontal shoving of the deck pavement of steel bridges The maximum absolute value will appear at 0.98 from the center of singular wheel Therefore, in order to prevent larger horizontal shoving of the deck pavement, the shear resistance in the binder course should be controlled In consideration of the material, the thickness of the deck pavement of asphalt concrete for steel bridges and the traffic and application conditions in China, it is suggested that the shear strength of the binder course between the deck pavement and steel sheets at high temperature season should not be less than 0.4-0.6MPa