Horizontal and Vertical Ground Motion Studies

Legeron and Sheikh (2009) presented a theoretical approach to calculate support reactions of bridges under vertical earthquake ground motion. Based on this theoretical approach, an efficient method has been developed that enables the calculation of the support reactions with less computational effort. The method can be easily adopted in the seismic design of

49

highway bridges. They said that, generally, three methods are used in practice for the calculation of support reactions:

1. Rayleigh method: recommended by the American and Canadian bridge design codes for irregular ordinary multi-span bridges and regular essential or emergency- route bridges. The Rayleigh method is generally not recommended for vertical ground motion since the choice of deflection shape for a complex system is not straightforward and does not provide results close to the exact calculation in most cases.

2. Modal analysis: most suitable for structures with irregular geometry, mass and stiffness. This method does not take into account non-linear effects. However, non- linear effect is not considered significant for vertical earthquake ground motion, as mentioned earlier. Moreover, the ductility of bridge piers under vertical earthquake ground motion is not well known and is usually considered to be low.

3. Time history analysis: very complex and time consuming, and usually carried out only for critical structures in high seismic zones. The choice of representative ground motion may add complexity in applying this method. Hence, elastic modal analysis has been considered sufficient for the calculation of the support reactions under the vertical earthquake ground motion in this study.

The need for definable levels of reliability in structures increases with time, as society becomes increasingly dependent upon infrastructure. This is exacerbated in safety-critical structures such as nuclear installations, particularly those constructed in seismic regions of the world. For this reason there is international interest and participation in the production and dissemination of research regarding earthquake-resistant structures. Consideration of vertical earthquake motion has often been neglected in the past, although nowadays it has begun to take priority amongst the engineering community.

Measurements of ground motions during past earthquakes indicate that the vertical acceleration can reach, or even exceed, values comparable to horizontal accelerations. Furthermore, measurements of structural response show the possibility of significant amplification in the response of bridges in the vertical direction that can be attributed to the vertical component of ground motion (Saadeghvariri & Foutch, 1991). Analyses of actual bridges indicate that, in general, the vertical motion will increase the level of response and the amount of damage sustained by a highway bridge. Vertical motion

50

generates fluctuating axial forces in the columns, which cause instability of the hysteresis loops and increase the ductility demand.

Kim et al. (2008) presented a paper concerning analytical assessment of the effect of vertical earthquake ground motion on reinforced concrete (RC) bridge piers. A bridge structure damaged during the Northridge earthquake and a Federal Highway concept bridge design were examined. The effects of a suite of earthquake ground motion records with different vertical-to-horizontal peak acceleration ratios on the two bridges were presented and the results compared with the case of horizontal-only excitation. The effects of the arrival-time interval between horizontal and vertical acceleration peaks were also reported and compared to the case of coincident motion. It was observed that the inclusion of the vertical component of ground motion had an important effect on the response at all levels and on all components. It was therefore concluded that vertical motion should be included in analysis for assessment and design, especially when there are no particular challenges impeding its inclusion.

Many researchers in the field of earthquake engineering are now recommending further study regarding vertical earthquake ground motion. Saadeghvariri and Foutch (1990) described how vertical motion can produce high magnitude forces in abutments and foundations that are not accounted for by the current seismic design guidelines. Therefore, it is important to consider this ground motion component in the design of highway bridges, especially for those located in regions near seismic faults where the effect is greatest. Collier and Elnashai (2001) stated that the vertical component of earthquake ground motion has generally been neglected in the earthquake-resistant design of structures. Elnashai and Papazoglou (1996) described some field evidence and results from dynamic analysis on possible structural effects of strong vertical ground motion. These collectively confirm that structural failure may ensue due to direct tension or compression as well as due to the effect of vertical motion on shear and flexural response.

In many earthquakes, the vertical component of the ground motion was found to exceed the horizontal component, which directly contradicts the current general code provision that assumes the value of the vertical ground motion to be between one- and two-thirds of the horizontal component. After almost every destructive earthquake some engineers postulate that structural damage was due to strong vertical ground motion. Therefore,

51

seismic design of the structure without the consideration of the vertical ground motion component may result in unquantifiable risk of collapse, especially those in constructed in close proximity to the fault. However, there seems to be little consensus as to the methods of quantifying the contribution of vertical motions in overall damage caused. Moreover, little has been learned from the recent earthquakes in Loma-Prieta, Northridge or Kobe, which indicate conclusively that damage to structures was predominantly caused by vertical motions (Shrestha, 2009).