SLENDER PRECAST WALL PANELS INTERACTED WITH STEEL PORTAL FRAMES UNDER EARTHQUAKE LOADS

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1 SLENDER PRECAST WALL PANELS INTERACTED WITH STEEL PORTAL FRAMES UNDER EARTHQUAKE LOADS Joo H. Cho 1 ABSTRACT: A case study was undertaken involving the previous Canterbury Earthquakes, by observing structural performance with failure modes for the slender precast wall panels involved in the low rise commercial warehouse and factory buildings. Numerical analysis was carried out on a fundamental dynamic theory, addressing the interaction response to the acceleration of the steel frames and its influence on the face load to the panels. Different failure patterns were investigated from the Canterbury Earthquake events by identifying what caused cracks, especially for the vertical crack occurring at mid-span. Relevant New Zealand standards were then reviewed to ascertain optimum design for the wall panels subjected to the earthquake load out of plane. A number of panels with different slenderness were utilised to look into the effect of slenderness and to propose design reinforcement. Finally, a design guideline with general recommendations was suggested for design practice. KEY WORDS: Slender precast wall panel, Interaction dynamic response, Canterbury Earthquakes, Standards 1 INTRODUCTION Slender precast concrete panels have been widely used for commercial warehouse and factory buildings in New Zealand. These wall panels, connected to steel frames, are normally used as boundary walls to protect from fire hazards [6,7,8]. The precast panels are simply constructed on site and are considered to be economical, as well as being efficient for fire proofing, compared to the steel framed walls which comprise girts, wall claddings and bracings. For the warehouse structures with wall panels, different systems are defined as to how the panels are connected to supporting structures and how the load path is determined. One particular system dealt in this research is a traditional portal frame & tilt panel structure commonly used in Christchurch and other regions of the South Island. For this system, a single panel is placed in between the steel frames and is restrained laterally at both side ends and at the base, through connections with weld plates and starter bars, respectively. In this respect, gravity loads are mostly supported by the portal frames and a small portion is taken by the panels. As per the lateral resistance, the panels tend to resist the in-plane earthquake load and the out-ofplane fire/wind loads, while the portal frames resist the in-plane earthquake load. For the wall panels loaded out of plane, designers are likely to focus on the vertical section of the wall, ensuring the fore load is resisted by the cantilevering action of the wall section thorough the starter bars coming from the foundation, whilst the horizontal sections are normally considered to be subjected to relatively low lateral loads from wind load and are routine to design, simply satisfying minimum code requirements for the shrinkage and temperature. The 2010/2011 Canterbury Earthquakes caused extensive damage to the tilt panels in the commercial buildings, with cracks occurring in various failure patterns, whilst the steel frames appeared relatively sound, except for some damage to the weld plate connections with wall panels. One particular pattern observed is a vertical crack occurring at mid span of the horizontal section, which is caused by a flexural mode against a face loading to the horizontal section. There was also an influence added from seismic movement of the steel frames which are connected to the roof frame in weak axis against earthquake excitation, so that the face loading to panel is likely to be increased. This is considered as the interaction response generated between two different accelerations of the panel and the steel frames, and is likely to increase the seismic load for the panels and thus dominate the design load out of plane [2,5]. In this respect, the horizontal span of the wall panel is deemed to be underestimated in design according to the current New Zealand standards, such as the wall design provisions in NZS 3101, which is mostly specified for the vertical cantilever walls to 1 Joo H. Cho, Harrison Grierson Consultants Ltd, Christchurch. j.cho@harrisongrierson.com

2 Longitudinal prevent out-of-plane buckling [14]. This does not seem to be sufficient to control the vertical cracks occurring in horizontal sections, unless vertical reinforcement is appropriately designed. In this research, the interaction responses between the panels and the steel frames are investigated by a dynamic theory, in order to determine optimum levels of earthquake face loadings to panels. Then, the horizontal spans are checked for reinforcement under the current standards, with panels of different thickness and slenderness. Finally, a design procedure is established to be suggested for the panel design, together with recommendations made for commercial warehouse and factory buildings. 2 STRUCTURAL SYSTEMS UNDER EARTHQUAKE LOADS 2.1 STRUCTURAL SYSTEM The structural system for the commercial warehouse buildings typically comprises steel portal frames, tilt-up panels and roof frames, as shown in Figure 1. The steel frames and wall panels are normally designed to resist in-plane loads in each direction, e.g. the transverse load to the steel frames and the longitudinal load to the wall panels. The steel frames typically made of hot rolled I-sections are the primary structure, supporting wall panels and roof frames by connections to portal legs and rafters, respectively, through weld plate connectors. Transverse Steel frame Roof Bracing Panel wall excluded from the column property in design for lateral capacity. As per the lateral resistance the longitudinal load is transferred from the roof frames to the panels in side walls through the roof cross bracing, whilst the transverse load is solely resisted by the steel portal frames. The wall panels are considered to act as cladding resisting face loads from the fire and wind load. Occasionally, the panel walls are designed to be used for the end walls with openings for door and windows, which are connected to the roof frames on top through the purlins spanning perpendicular to the walls. With a simple connection at base, the panels are deemed stable by ensuring the fixity on top and bottom. 2.2 EARTHQUAKE FACE LOADING TO WALL PANELS In earthquake events the wall panels sitting on the ground are likely to be subjected to the lateral load perpendicular to the wall plane, e.g. the out-ofplane face load, although the steel frames in orthogonal are intended to take all lateral loads in the transverse direction. However, the face loading to panels could be significant if the panels are large in size and thus create high acceleration from heavy self-weight. In this case, as observed in the Canterbury Earthquakes, the panel s responses to the earthquake face load present failure patterns a little differently compared to the wind and fire loads. This is due to the acceleration of the steel frames which is likely to influence on the panels performance. Unlike normal concrete structures for multi-storey buildings, which have a strong diaphragm from the concrete floor slab, the roof frames in these types of buildings consist of light weight purlins and cross bracing which are not strong enough to transfer the transverse lateral loads effectively to the steel frames in plane. This is likely to bring extra loads to the panels out of plane. In this respect, with no strong connection to roof frames the panels are deemed to be largely influenced by the steel frame s behaviour and subjects the panels to increased earthquake excitation out of plane. Figure 1: Typical structural system The precast tilt-up panels are also connected to the foundations, which are often simple pin connections made through the starter bars pulled out from the panels. The steel columns are sometimes concrete encased to withstand fire load when the connected panels are used as boundary walls. The concrete encasement is normally Crack Figure 2: Inertia loads from out-of-plane response Figure 2 illustrates earthquake loading to panels, combined with excitation of the steel frames. A

3 portion of inertia force is transferred from the steel frames to the panel out of plane, which is excited by the steel frame acceleration,, so that the wall acceleration is amplified by the interaction between the two systems [2]. This is the interaction response acceleration,, exciting the panel out of plane and thus the panel responds horizontally to the face force,, in which m is mass of panel. The magnitude of is the ratio of natural periods of steel frame to wall panel, which will be discussed further in next section. 3 FAILURE PATTERNS TO PANELS IN EARTHQUAKES 3.1 YIELD LINE PATTERNS The panels are supported on three sides through connections to portal knees at both ends and to the foundation at base and are structurally separated from the roof frame on top as free end. The yield line theory is introduced to investigate seismic performance of the panels out of plane as some patterns appear similar to those drawn from the theory [4]. Figure 3 illustrates a yield line pattern occurring to slabs supported on three sides with pin connections, presenting one vertical crack at mid span of the panel s horizontal section and two diagonal cracks at both corners. This is a pattern developed under ultimate deformation through uniformly distributed loads. one side when moving in the same direction with the steel movement. Figure 4 illustrates a yield line developing in the slabs with free ends in two sides, such as one at the steel and one at the roof, considering that one steel frame moves in the same direction with the panel out of plane, whilst the other frame moves in the opposite direction and reacts to the panel s inertia force. In this respect, a diagonal line develops in between the corners where the free ends are started and this appears to be different from the lines developing in Figure 3. It is therefore deemed that the panel s earthquake loads are different from those supported on a stable condition, involving complexity of the relationship with the seismic excitation of the steel frames. Figure 4: Yield line pattern supported on two sides 3.2 CRACK PATTERNS OBSERVED IN CANTERBURY EARTHQUAKES The Canterbury Earthquakes caused lots of damages to the commercial warehouse and factory buildings in this region, especially to the tilt-up panels which have cracked presenting different failure patterns, depending on their structural systems including connections to the walls and the foundations. Figure 3: Yield line pattern supported on three sides It is, however, noted that the yield line theory is based on a condition that the supporting structures are stable with no movement under loads. The drawn pattern is, however, likely to vary if the supporting structures, such as the steel frames, are in unstable condition with movement when subjected to earthquake excitation. The portal frames are connected to the roof frames through the purlins that have weak axis in the transverse direction, and are likely to move separately from one another by pushing and pulling the panel at both ends, in the same direction at one moment and in the opposite direction at another moment. In this moment, the panels are likely to have a free end in Figure 5: Vertical cracking to wall panel

4 One typical pattern is a vertical crack developing at mid span of the horizontal section of the panel, as shown in Figure 5, which is connected to the steel portal frames at both ends and to the foundation at the base of the wall. This reflects the boundary condition of the slab supported on three sides, as mentioned above in Figure 3. Two portal frames are likely to move together in the same direction but in the opposite direction to the panel s movement and tend to react against inertia force of the panel on a condition of pin connection. supporting sides. The negative stresses that resulted are much lower than the positive stresses. This represents that the weld plate connections are relatively weak to withstand stress transmitted. From the stress distribution, it is estimated that a vertical crack starts at the top centre and continue to develop down to the bottom, which has been observed in many cases from the Canterbury Earthquakes. Figure 6: Diagonal cracking to wall panel Trial investigation has been undertaken by the finite element analysis for the slabs with two different boundary conditions, as shown in Figures 7 and 8. Figure 7 shows a stress distribution for the wall supported on three sides, when subjected to uniformly distributed face loads. The boundaries were modelled for the connection with bolts and starter bars at the wall and the foundation, respectively. Figure 8 modelled the free end at two sides, reflecting the portal frame to move with the panel in the same direction. Figure 7: Stress distribution pined at three sides A large positive stress is developed at the top centre of the horizontal span which reduces towards the base, as shown in Figure 7, whilst maximum negative stresses are developed along the Figure 8: Stress distribution pined at two sides Figure 8 shows maximum stresses are developed towards along the two sides which reduces towards the free end corner where it is stress free, e.g. zero. The stress pattern is diagonally contoured, directing both corners where the free ends are started. From the stress distribution it is estimated that a diagonal yield line develops to connect the two corners, which is the pattern observed in the Earthquakes with the steel frames movement. It is therefore noted that the stress distribution resulted from the analysis is quite indicative to be able to estimate yield lines developing under earthquake loads and is useful to design the panels out of plane, especially for the connection details on a boundary condition. 4 INTERACTION RESPONSE BETWEEN WALL PANEL AND STREEL FRAMES Due to the multi-axial nature of the ground movement in earthquakes, the panel walls will be subjected to simultaneous vertical, horizontal, inplane and out-of-plane responses. In this case, the out-of-plane response of the wall is likely to be affected by an interaction with the in-plane response of the steel frame in orthogonal direction and so the wall response acceleration is amplified by the steel frame response acceleration.

5 Interaction amplication factor (D) 4.1 DYNAMIC RESPONSE RATIOS In order to estimate the influence of dynamic loading from the steel frame, a fundamental dynamics theory of Clough and Penzien (1993) is introduced for a SDOF (Single Degree of Freedom) system [1]. Considering an undamped system subjected to a harmonically varying load of sine-wave form having amplitude and circular frequency, the equation of motion is (1) in which and are mass and stiffness of the system, and and are its acceleration and displacement. The general solution of Eq. (1) is obtained as (2) in which P/k is static displacement, β is frequency ratio between two systems and ω is the natural freevibration frequency. The influence of dynamic loading is measured as the ratio of the dynamic displacement response to the static displacement response, i.e. (3) The response ratio of an undamped system in atrest condition is thus the steady-state harmonic response. Since the transient response damps out quickly, it is usually of little interest [1]. Introducing the steady-state harmonic response for a system with viscous damping, the equation is β β β ω (6) in which is damping ratio. This is expressed in a simple form ω (7) in which a dynamic response amplitude is β ω β β (8) The dynamic amplification factor is the ratio of dynamic response amplitude to static displacement; thus β β (9) The resultant response is amplified with frequency ratio β and damping ratio in which the frequency ratio is practically used for the natural period ratio of two systems. This amplification factor is now introduced to the interaction response between two structures, such as the panel wall and the steel frame, and this is called the interaction amplification factor, D. (4) Figure 9 illustrates the initial response to be increased by the response of dynamic loading. Two responses are combined with different natural period of vibrations in which the frequency ratio is defined as (5) % damping 5% damping 10% damping 20% damping 30% damping R(t) Stead state response Total Response Period ratio ( Ts/Tw ) Figure 10: Interaction response between panel and steel frame Transient state response Figure 9: Initial response amplified by dynamic loading 4.2 DYNAMIC AMPLICATION FACTOR Considering a system with viscous damping, the equation of motion has the total response resulted with two terms such as the transient response and Figure 10 shows the interaction amplification factors varying with natural period ratios, Ts/Tw, in which Ts is natural period of steel frame and Tw is natural period of wall panel. In terms of the out-ofplane acceleration of the wall, the wall s natural period is normally longer than the steel frame s period responding to in-plane acceleration. The interaction responses increase as the period ratios increase, up to infinity at Ts/Tw of 1, the resonant response. Nevertheless, this is not practical to be used for this particular type of system with the panel connected to the steel frames. It is also noted

6 Max. bending moment (knm) that the amplification greatly relies on the damping ratios, especially for the high period ratios. From the figure it is noted that the interaction responses are low in the low period ratios, which can be the case that a flexible, or slender, panel is connected with a relatively stiff steel frames, towards the lateral loads in the same direction to the panel out of plane. On the other hand, one same panel would be more influenced by a flexible steel frame with the higher period ratio than with a stiff frame. Due to the complexity of seismic behaviour between the two different systems, it is difficult to obtain an exact amplification factor for practical use, particularly for the high period ratios ranging between 0.8 and 0.9, or higher. Unless a rigorous analysis was conducted, it is therefore recommended by Paulay and Priestley (1992) to use factor of 1.5 to 2.5 for a simple procedure [2]. Similarly, AS/NZS1170 sets criteria for the part structures influenced by acceleration of main structure in which maximum spectral factor of 2.0 is specified for the short periods, such as 0.75 seconds or less [11], which can be applied to a system with relatively stiff walls, however the longer period of structures are still largely influenced by the interaction response with the high period ratio, as shown in the figure. 4.3 SLENDER PANELS WITH INTERACTION RESPONSE If a panel is designed with insufficient horizontal reinforcement and it is connected to steel frames at both ends, it is hard to prevent the vertical crack to occur at mid span of the horizontal section. Provided that the panel is predominantly supported by the steel frames, the horizontal slenderness becomes one major factor to determine the earthquake load, which is defined as the slenderness ratio of the panel length to its thickness, L/t Interaction response included Interaction response excluded Horizontal slenderness ratio (L/t), 120mm Figure 11: Influence of interaction response on slender panels Figure 11 plots the maximum bending moments required for the panels subjected to the face loads with and without the interaction response from the steel frames, in which 120mm thick walls are typically used for square panels with different lengths. When the interaction response is ignored in the assessment, the moment demands smoothly increase as the slenderness ratios increase, whilst the demands that the interaction response is included are much higher for the low slender ratios less than 25. This is because the less slender, or thicker, panels have relatively short natural periods of vibration, which gives rise to the high period ratios resulting in and thus the high interaction amplification, as shown in Figure 10. On the other hand, the more slender, or thinner, panels have the lower amplification factors due to relatively short natural periods. As a result, it is noted that the interaction response to the steel frames acceleration demands higher bending moments and flexural reinforcements in the panel design, than in the conventional design where the response is ignored. 5 STRENGTH DESIGN FOR SLENDER WALL PANELS Current NZ standards, which are relevant to the concrete walls, are reviewed to ensure the steel frames dynamic loads are adequately considered in determining the wall panel s earthquake loads out of plane. Series of the slender panels with different thickness were used to design of the loads including the interaction response and then the design horizontal reinforcements were plotted in the charts with the slenderness ratios. Finally, a design procedure is suggested for the slender panel connected to the steel frame, together with recommendations for the design of the commercial warehouse and factory buildings. 5.1 REVIEW OF RELEVANT NEW ZEALAND STANDARDS AS/NZS :2004, Standard Design Actions Part 5, Earthquake Actions Sections 8 provides earthquake actions for the parts and components in which the panel walls are considered as the claddings attached to the main structure, such as the steel frames subjected to inplane loads. The standard also states that the part supported directly on the ground floor shall be designed as a separate structure [11], which means its design actions are derived as main structures. This implies that the wall panels are treated as normal structures in design for the face loads because most wall panels are directly supported on the ground. It is also noted that, according to the standards, the part structures are likely to have the design actions higher than normal structures, due to the high value of coefficients required, such as floor height, spectral shape, etc. This is likely to lead the panel walls, which are designed as normal structure, to

7 Horizontal reinforcement ratio, P Horizontal reinforcement ratio, P have the design actions underestimated than the part structures NZS 3101:2006, Concrete Structure Standard As a minimum requirement for concrete structures, the shrinkage and temperature reinforcement is specified in Section 8 for the ratio of reinforcement area to be at least 0.7/fy, but equal to or greater than , in which fy is yield strength of reinforcement. This requirement is normally used for the secondary section spanning perpendicular to the primary section towards design actions and so it is a prerequisite to identify the primary section first, in order to use this requirement. Section 9 provides for one-way slabs, limiting minimum longitudinal reinforcements to be As = ( f c / 4fy ) bw d but equal to or greater than 1.4 bw d / fy, in which f c is compressive strength of concrete, bw is width of web and d is distance from extreme compression fibre to centroid of tension reinforcement. These criteria may be major requirements for the wall panels which are considered as one-way slab spanning between steel frames, when subjected to earthquake face load. In the mean time, Section 11 provides for structural walls and their design requirements, especially for the stability at the ultimate limit state by limiting eccentric loads. However, these provisions mostly account for the vertical stress occurring in the walls, based on the assumption that the walls are load bearing walls supporting gravity load, as well as lateral loads. As a result, for the wall panel in the storage building the vertical sections are designed primarily for the longitudinal reinforcement, whist the horizontal sections are treated as secondary span with the transverse reinforcement designed for minimum shrinkage and temperature reinforcement only. 5.2 HORIZONTAL REINFORCEMENT FOR WALLS Design horizontal reinforcement is plotted in Figures 12 and 13 for yield strengths of 300MPa and 500MPa, respectively, when the panels are subjected to earthquake face load with the interaction response to the steel frame s in-plane acceleration. The design bending moments were resulted from that horizontal action of the panels with different thicknesses in which the earthquake load was determined based on Christchurch region for Z = 0.3. The panels were sized as square with different lengths, and the design compressive strength of concrete chosen was 30MPa typically. As single panels are connected with steel portal frames at both ends, the face load induced bending moments act horizontally and the flexural modes are sustained by the horizontal reinforcement. This is because the vertical cracks at mid span cannot be controlled by the vertical reinforcements. The panels adopted in this research are fixed at base to the extent that the fire induced face load is resisted as cantilever, including the in-plane earthquake shear load mm thk 180mm thk 150mm thk 120mm thk 100mm thk NZS3101, fy=300mpa Horizontal slenderness ratio (L/t) Figure 12: Horizontal reinforcement for fy = 300 MPa mm thk 180mm thk 150mm thk 120mm thk 100mm thk NZS3101, fy=500mpa Horizontal slenderness ratio (L/t) Figure 13: Horizontal reinforcement for fy = 500 MPa As seen in the figures, large horizontal reinforcement is required for the high L/t ratios, such as slender panels, in which L is panel length and t is panel thickness. In the same slenderness ratio, the thick panels demand more bending moments than the thin panels, especially for the panels with large slenderness ratios. This is because the higher inertial force is generated from the panel self weight. For the less L/t ratios than 25, where trends appear differently, greater reinforcement is required for the lower L/t ratios. This is because a high interaction response acts in the panels, as shown in Figure 10, in which the thin panels, such as 100mm thick, on the same slenderness ratio are

8 more excited with the interaction response due to the short period and thus the larger period ratio. From the high slenderness ratio zones, however, it is noted that a high horizontal response is mainly caused by the out-of-plane acceleration of panel itself rather than the interaction response. From the figures, it is noted that minimum horizontal requirement of NZS 3101, such as As = f c/(4fy) bwd but equal to or greater than 1.4 bwd/fy, is much lower than the reinforcement designed for the higher L/t ratios such as 45 for 200mm and 65 for 100mm. For L/t ratio of 70, greater horizontal reinforcement is required for 200mm, such as 0.01 and 0.06 for concrete strengths of 300MPa and 500MPa, respectively. However, the large slenderness ratios are not practical on site, while the minimum code requirements are deemed to be reasonable, limiting the panel lengths to be 9m and 6.5m for 200mm and 100mm, respectively. Therefore, it is noted that the one way slab requirements in NZS 3101 is deemed to be applicable for the wall panels loaded out of plane. 5.3 DESIGN PROCEDURE FOR HORIZONTAL REINFORCEMENT Although an inelastic behaviour of panels is likely to occur out of plane by the yielding of the vertical reinforcement at base, the inelastic design is not practical due to the instability issue of slender panels. Therefore, the design focuses on an elastically responding panel out of plane: (a) Calculate the natural period of panel out-ofplane: Seismic mass of panel, Mp Stiffness of panel, kp = 3EI / H 3 (E: modulus of elasticity of concrete, I: moment of inertia, H: panel height) Natural period of panel, Tw = 2π (Mp / kp) (b) Calculate the natural period of steel frame inplane: Seismic mass of steel, Ms Force in serviceability, Fs = Cd(S) W (Cd(S): seismic coefficient in serviceability state, W: seismic weight) Deflection limit, = H / 400 Stiffness of steel, ks = Fs / Natural period of panel, Ts = 2π (Ms / ks) (c) Calculate the interaction amplification factor: Period ratio, B = Ts / Tw Set the damping, e.g. 5% Amplification factor, D (Figure 10) (d) Calculate the bending moment for horizontal section: M* = 1/8 Cd(U) W L 2 D (Cd(U): seismic coefficient in ultimate state) (e) Design horizontal reinforcements: Design the flexural strength for one-way slabs. Meanwhile, according to the minimum requirements for the one way slabs specified in NZS 3101, maximum panel lengths are set in Table 1 for the slenderness ratios at which the reinforcement demands meet the minimum code requirements, such as the minimum design reinforcement. For the purpose of analysis, the steel frames are set to span 30m and spaced each other to match the length of the panels. The panel lengths in Table 1 vary with a range between 6.5m and 9m according to their thickness, in which the design reinforcement also satisfy code requirements, especially for the space which should be more than three times the wall thickness or 450mm on centres, whichever is the least. For the panels with thickness of 150mm and 120mm, which are widely used for the commercial warehouse buildings, maximum lengths of 7.5m and 7m are designed, respectively. The designed lengths appear to be a little bigger than practical use for the single panels spanning between steel frames, whilst the reinforcement is designed to be spaced longer than the practical design. It is therefore noted that the one way slab requirements of NZS 3101 are applicable to design for the horizontal one-way panel walls subjected to the earthquake face load. Table 1: Panel design with minimum reinforcement to NZS 3101 (Steel frames typically spanning 30m) Panel thk. (mm) fy (MPa) Max. panel length (m) Minimum horizontal reinforcement D12@200 or D16@ HD12@350 or HD16@ D12@250 or D16@ HD12@400 or HD16@ D12@300 or D16@ HD12@400 or HD16@ D10@250 or D12@ HD10@300 or HD12@ D10@ HD10@300 6 DISCUSSION AND RECOMMENDASTIONS From the observations in the Canterbury Earthquakes, it is worthwhile to note that the commercial warehouse buildings suffered damage mainly to the panel walls, whilst the steel frames were generally sound with no significant structural damage. This necessitates the structural system of those buildings to be further enhanced to ascertain the integrity against earthquake loads. The panel walls need to be secured through adequate connections with steel frames to ensure no excessive stress is transmitted to the panels. It is recommended that more consideration should be

9 taken into the designing of commercial warehouse structures as stated below: The panel s earthquake face loads should be determined for the part and component structures, taking the part spectral shape coefficients into account, as specified in AS/NZS The horizontal reinforcement for the panels should be designed according to NZS 3101 for the one way slabs. Panels should be rigidly connected to supporting structures, such as steel frames and foundation, in which sufficient weld connectors and starter bars are used for the connection at both the portal knees and the base. This will help the face load stress to be distributed to the supporting structures in-plane. Portal frames should be stiff enough for the period ratio and interaction response to be reduced and thus less face loads are applied to panels. The side wall panels should be strongly connected to the roof frame to ensure fixity to top. This would help the panel s face load to be partially supported by the steel frames in orthogonal direction. Provide roof cross bracing all along the side walls at one side, which will tie the portal frames together in the transverse direction, making the roof frame stiffer and thus preventing any localised load from being applied to the panels. 7 CONCLUSIONS Out-of-plane earthquake loads could cause damage to slender panels that are connected with steel frames for warehouse type buildings. Dynamic response of the panel is amplified by interaction response to steel frame acceleration, depending on the ratio of natural periods of steel frame to panel. The horizontal slenderness is a major factor influencing the dynamic response of the panels restrained by the portal frames. The horizontal reinforcement should be designed as one way slabs as specified in New Zealand standards. REFERENCES [1] Clough, R. W., and Penzien, I.: Dynamics of Structures, McGraw-Hill, 2 nd ed., [2] Paulay, T., and Priestley, M.: Seismic Design of Reinforced Concrete and Masonry Buildings, John Wiley & Sons, [3] Park, R., and Paulay, T.: Reinforced Concrete Structures, John Wiley & Sons, [4] Johansen, K. W.: Yield line theory, Cement and Concrete Association, London, [5] Joo H. Cho: Out-of-Plane Earthquake Load on Slender Precast Concrete Wall Panels connected to Steel Frames, ASEA-SEC-1, Perth, November 28 December 2, [6] Beattie, G. J.: Design Guide Slender Precast Concrete Panels with Low Axial Load, Building Research Association of New Zealand, [7] Lim, L.: Stability of Precast Concrete Tilt Panels in Fire, Fire Engineering Research Report 00/8, University of Canterbury, [8] Poole, R. A.: Review of Design and Construction of Slender Precast Concrete Walls, Department of Building and Housing, New Zealand, [9] Davidson, B. J.: Determination of Seismic Design Forces for Slender Precast Slab Structures, Paper number 14, 2004 NZSEE Conference, New Zealand, [10] Guidance on Detailed Engineering Evaluation of Earthquake Affected Non-Residential Buildings, Part 3, Technical Guidance: Engineering Advisory Group, April, 2013 [11] Standards New Zealand. 2004: Structural Design Actions Part 5: Earthquake actions New Zealand NZS :2004. SNZ, Wellington, New Zealand. [12] Standards New Zealand. 2004: Structural Design Actions Part 0: General Principles AS/NZS :2004. SNZ, Wellington, New Zealand. [13] Standards New Zealand. 2004: Structural Design Actions Part 1: Permanent, Imposed and Other Actions AS/NZS :2004. SNZ, Wellington, New Zealand. [14] Standards New Zealand. 2006: Concrete Structures Standard Part 1 The Design of Concrete Structures NZS 3101:Part 1: 2006, SNZ, Wellington, New Zealand.