Keywords: vertical ground motion, near-field, far-field, axial force, 3-D steel building, building code.

A vertical ground motion component is one of the characteristics of near-field earthquakes. By increasing the distance from the source of the earthquake, the vertical acceleration decreases more severely than the horizontal acceleration [1,2], so in the near-field, this component acceleration is greater than in the far-field. In the past, usually the vertical component acceleration of strong earthquakes were not considered in seismic design, but the Northridge (1994) and Kobe (1995) earthquakes brought the attention to researchers to perform extensive studies to learn the reasons for structural damage especially those near the seismic source and to add new instructions to building codes.

This paper aims to study the effects of vertical acceleration on structural response due to near-field excitations. To gain this objective, a 3-D steel building of five and thirteen stories which were designed according to the Iran Building Code [3] were selected. The sample buildings were subjected to eight near-field and three far-field ground motions recorded at Iran, Taiwan and the US. All the earthquakes were categorized into two groups, that is, horizontal component only and horizontal and vertical component simultaneously.

Based on nonlinear analysis, the vertical acceleration and velocity of the storey, and the base axial force have larger values in the near-field than the far-field. Studying the characteristics of near-field vertical ground motions, such as the peak ground velocity / peak ground acceleration ratio (PGV/PGA) and the peak ground displacement / peak ground velocity ratio (PGD/PGV), spectral velocity, duration of strong motion, ground input energy, duration and amplitude of pulses, show that if PGV/PGA and PGD/PGV ratios of vertical component have lower values, and the spectral velocity (S_V), input energy (E_i), and timeshift (T_S) in vertical direction have higher values, the response of the structures will increase. Thus, with regards to the characteristics of near-field earthquakes, it can be concluded that the response of structures in near-field regions is extremely greater than in far-field. Also, near-field earthquakes with pulses of greater amplitude (greater PGV) or greater duration (less T/T_P) results to increase in structural response. Although the PGV/PGA ratio for the horizontal component is greater in the near than in the far-field; this ratio is visa versa for the vertical component [4].

To compute the duration of strong ground motions, the Page et al. and Bolt approach was used [5]. The simplest way to define the duration of the pulses is to determine the pulse duration using the zero crossings of the pulse wave form. An alternative approach is the utilization of the peak value of the velocity response spectrum to indirectly define the pulse period [6]. The peak ground velocity is considered as the amplitude of the pulse.

The axial force in central and lateral columns in two exterior and middle frames indicate that axial force in columns closer to central part of buildings is consequent only of vertical acceleration component and the average ratio of axial force due to vertical ground motions to designed force for building's central column is about 1.65 for near-field and 1.08 for far-field excitations. Due to this increase, vertical acceleration must be considered in structural design in near-field and both response spectra for vertical and horizontal acceleration should be included in building codes.

1

Bozorgnia, Y., Niazi, M., Campbell K.W., "Characteristics of Free-Field Vertical Ground Motion during the Northridge Earthquake", Earthquake Spectra, Vol. 11, No.4, 515-525, 1995. doi:10.1193/1.1585825

2

Bozorgnia, Y., Mahin, S.A., Brady G., "Vertical Response of Twelve Structures Recorded During the Northridge Earthquake", Earthquake Spectra, Vol. 14, No. 3, 411-432, 1998. doi:10.1193/1.1586008

3

"Iranian Code of Practice for Seismic Resistance Design of Buildings, Standard No.2800", 2nd Edition, Building and Housing Research Centre, 1999. (In Persian)

4

Rahim Labafzadeh M.S., "Nonlinear Dynamic Behaviour of Structures Due to Near-Field Ground Motions", Master's thesis, AmirKabir University of Technology, Tehran, Iran, 2004. (In Persian)

5

Naeim F., "The Seismic Design Handbook", 2nd edition, Kluwer Academic Publishers, 2001.

6

Mavroeidis G.P., Papageorgiou A.S., "Near-Source Strong Ground Motion: Characteristics and Design Issues", 7th US NCEE, Boston, No. 0240, 2002.

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