Tsunami loading and structural guidelines

Tsunami loading has been the subject of numerical and physical investigation in recent years (e.g., Lloyd and Rossetto, 2010; Lukkunaprasit et al., 2008; Palermo and Nistor, 2008; Palermo et al., 2009; Yeh, 2007). The wealth of structural damage data collected from the 2011 Great East Japan tsunami are being used by the American Society of Civil Engineers (ASCE) 7 Tsunami Loads and Effects Subcommittee to generate new tsunami loading guidelines for structures, which would be implemented in the International Building Code (IBC) post-2020 (Chock, 2012). In the proposed ASCE 7 code, TVEB would be required to provide immediate occupancy for the maximum credible tsunami (1 in 2,500 y). Other buildings would be assigned a performance requirement based on their risk category and height (Chock, 2012).

A review of the Japanese guidelines (Okada et al., 2005) is also underway, with a proposed reduction in tsunami loading factors. For example, the horizontal tsunami force calculated in Cabi- net Office Government of Japan (2005) was derived using a co-efficient of 3.0 * water depth, based on earlier physical modelling. Analysis of structural damage compared to flow depth in T¯ohoku showed that this is very conservative, and that the co-efficient could be closer to 1.0 (Nishiyama et al., 2011). As a conservative procedure, in temporary guidelines, the coefficient was relaxed to 2.0 for buildings that are sheltered by other buildings, and 1.5 for buildings>500 m inland. Further relaxation of the Japanese guidelines is expected when accurate methodologies for estimating other tsunami loads have been established (Nishiyama et al., 2011). There is a movement in the Japanese coastal management community away from reliance on structural mitigation measures in favour of combined approach with non-structural measures for life safety (Section 3.2.1). Shibayama et al. (2013) proposed a three-tier classification of evacuation structures (Table 3.1).

Presently, two non-mandatory international structural guidelines for TVEB exist. Federal E- mergency Management Agency (FEMA) P646 presents the most complete set of guidance in terms of formulation of forces, and the companion report P646A (FEMA, 2009) presents guidance for community officials on building planning, design and construction capacity, maintenance, opera- tion, and funding. FEMA (2008) prescribes methods for calculating multiple tsunami loads (Table

52 3. Tsunami risk reduction and preparedness

3.2) but all loading calculations are based on those included in previous design codes that were developed for storm surge and riverine flooding (American Society of Civil Engineers, 2006a,b; City and County of Honolulu, 2000; FEMA, 2005; International Code Council, 2006). As the best available information, these were used in the development of FEMA (2008) but the guidelines recognise that tsunami flow characteristics include extreme amplitude fluctuations, flow velocity and mass of flow than riverine flow. Additionally, tsunami loads are likely to sustain high velocities and flow depth over longer timescales than riverine or storm surge flood waters, due to their long wavelength. Loading time histories are not considered in the guidelines, which base design guid- ance on maximum loads rather than the entire flow time history. Therefore, progressive weakening may be inadequately accounted for. Although potential combinations of loads are recommended in the guideline, as is the consideration of the impact of earthquake and subsequent tsunami load- ing in a local-source, these are not adequately addressed without reproduction of the full loading time-history accounting for multiple cycles of flow and loading (Park et al., 2012b).

The FEMA (2008) loading guidelines rely on derivation of maximum momentum flux (flow depth * velocity) at a site, which can be derived from numerical models. Numerical simulation requires topography data of sufficiently high vertical and horizontal resolution, and resolution of complex velocity structures onshore, which generally requires simulation to be conducted using a high-order non-linear model Boussinesq model (Section 4.1). With regards to vertical evacuation, safe elevation must be defined. Due to uncertainties in numerical modelling, a factor of safety (1.3 * maximum run-up at the site + 3 m splash-up) is used, but FEMA (2008) also states that this should never be taken as less than 80% of the values generated using the provided analytical approximations for maximum momentum flux and maximum flow velocity.

The Japanese government guidelines for designation of building for vertical evacuation are that they meet adequate construction standards and minimum heights (Cabinet Office Government of Japan, 2005). The proposedStructural Design Method of Buildings for Tsunami Resistance(Oka- da et al., 2005) requires buildings to be designed for seismic resistance according to the standard building code. Subsequently, tsunami loads are estimated and pressure-exposed surfaces and struc- tural frame are designed accordingly for hydrostatic, hydrodynamic and buoyancy forces based on maximum flow depth at the site (Okada et al., 2005). Overturning and sliding failure are also cited in the guidelines as a required analysis. The seismic safety requirement of TVEB in Japan was achieved by designating buildings constructed to post-1981 seismic standards (Textbox 3.2).

Based on tsunami load analyses, design guidelines and damage observations, several design concepts have been proposed for tsunami-resistant buildings. The principles defined by FEMA (2008) are: strong systems with reserve capacity to resist extreme forces; open systems that allow water to pass through with minimal resistance; ductile systems that resist extreme forces without failure; and redundant systems that can experience partial failure without progressive collapse. These include Reinforced Concrete (RC)/steel moment frame and RC shear wall systems. Particu- lar concepts outlined in FEMA (2008) are: the use of round columns to reduce hydrodynamic force

3.2. Structural mitigation 53

Tab. 3.1:Proposed classification of evacuation areas (Shibayama et al., 2013). Category Description

A

Hills (higher terrain) that are adjacent to the coast but continue to increase in elevation for a long distance. Not be isolated low hills, include those that form part of larger geographical features and have a large hinterland region.

B

Robust buildings that have≥7 storeys, or small hills that are more than 20 m in height. Such buildings would generally ensure the safety of anybody taking shelter in them and could be considered ‘critical lifeline’ structures. This cat- egory would have the inherent risk of being isolated during the worst tsunami, but would likely be safe for most events. All new Evacuation Buildings should be at least Category B.

C

Robust buildings that are>4 storeys high. This category, however, would have the risk of being overtopped during the worst tsunami events, as described earli- er. The use of such a category is not recommended, but in areas where Category B or A do not exist they could be used while better evacuation points are not available. No new Evacuation Buildings should be built in this category.

compared to square columns; fixing of columns to every storey; orienting shear walls parallel to flow to minimise hydrostatic loads; designing floor systems for upward forces (uplift, buoyancy), as well as downward forces (gravity loads); piles that are designed to withstand scour of foundation around the pile cap; and use of breakaway non-structural walls, designed to fail under loading, thus reducing hydrostatic loading on the building.

Textbox 3.2 (Key criteria for TVEB in Japan) The key criteria from the 2005 guideline for official designation of buildings as tsunami evacuation buildings (Cabinet Office Government of Japan, 2005):

• Are of a minimum height according to estimated maximum inundation depth

<1 m depth = 2-storeys or higher required

2 m depth = 3-storeys or higher required 3 m depth = 4-storeys or higher required

• Are RC or steel reinforced concrete composite Steel Reinforced Concrete (SRC) con-

struction

54 3. Tsunami risk reduction and preparedness

Tab. 3.2:Description of tsunami loads considered by FEMA (2008).

Load Description

Hydrostatic

Acts on a wall when the water surface is different levels on either side. This force will likely be reduced when the ground floor contains door and window openings that reduce unequal water level inside and outside the building.

Buoyant forces The vertical (upward) equivalent of hydrostatic forces, acting on a water tight structure.

Hydrodynamic (drag) forces

Lateral force acting on structural components and the whole struc- ture, comprising friction forces and pressure forces from the mass of water flowing through and around the building.

Impulsive forces

Caused by the leading edge of the water surge (i.e., before drag forces begin to act). Calculated by FEMA (2008) as a factor of the drag forces.

Debris impact forces

Vital in the assessment of critical infrastructure, and have been shown to be important in influencing the level of damage sustained by any structure in previous events. FEMA (2008) presents several for- mulations, indicating the current levels of uncertainty around debris forces.

Debris damming forces

Caused by the accumulation of waterborne debris, which can enhance the hydrodynamic force by effectively widening the wall surface.

Uplift forces

Vertical upward force acting on a floor level which is below the water level but where the building exterior displaces water above that floor level. It is a combinations of vertical hydrodynamic forces and some buoyant forces.

Additional gravity loads

Apply when tsunami water is retained in a building during and after drawdown, and is dependant on inundation exceeding the elevation of each floor.

Combination of loads

Several combinations of loads provided for the whole structure and individual components. Multiple combinations are provided as not all loads will act at the same time on the same part of the building.