Seismic Design Considerations

While Riyadh sits on the relatively stable Arabian Shield, seismic design for The Mukaab cannot be neglected. The Saudi building code requires seismic provisions for all structures, and a building of the Mukaab’s mass and public significance demands seismic engineering that exceeds minimum code requirements. The 1 million tonnes of structural steel in the mega-frame generate enormous inertial forces during seismic events, even at modest ground accelerations.

The Arabian Shield is a stable Precambrian craton that has not experienced significant seismic activity in recorded history. However, the East African Rift System lies to the west, and the Zagros seismic zone runs along the northeastern border of the Arabian Plate. Both sources are capable of generating earthquakes whose ground motions could reach Riyadh at attenuated but non-negligible levels.

For a structure of the Mukaab’s unprecedented mass and geometry, even minor seismic ground motion creates significant engineering demands. The cube form, with its uniform mass distribution across 400 meters of height, responds to seismic excitation differently than a tapered tower that sheds mass at upper levels. The mega-frame system must be designed to dissipate seismic energy through controlled deformation without compromising the structural integrity of the facade system, the internal spiral tower, or the holographic dome support structure.

The 1,200-pile foundation plays a critical role in seismic performance. Pile-soil interaction under dynamic loading differs significantly from static conditions, with soil liquefaction potential, pile group effects, and kinematic interaction creating additional demands on the foundation system.

Seismic Hazard Assessment for Riyadh

Saudi Arabia’s seismic hazard mapping divides the Kingdom into zones based on peak ground acceleration (PGA) with 475-year return periods. Riyadh falls within a low-to-moderate seismic zone with a design PGA of approximately 0.07g — significantly lower than seismically active regions like California (0.4-0.6g) or Japan (0.3-0.5g), but non-zero and relevant for a structure of the Mukaab’s mass and importance.

The Saudi Building Code (SBC) classifies structures by Seismic Importance Factor, with essential facilities (hospitals, emergency services) assigned higher factors than ordinary buildings. The Mukaab, as a structure accommodating up to 90 million annual visitors and containing residential, hospitality, and public assembly occupancies, likely qualifies for an Importance Factor of 1.25-1.5 — meaning the design seismic forces are 25-50 percent higher than those required for an ordinary commercial building.

The nearest significant seismic source zones include the Dead Sea Transform Fault system (approximately 1,200 km northwest of Riyadh), the Zagros Fold and Thrust Belt (approximately 900 km northeast), and the Red Sea spreading center (approximately 800 km west). While these distances provide substantial attenuation of ground motions, probabilistic seismic hazard analysis (PSHA) for a 2,475-year return period (the level typically used for buildings of the Mukaab’s importance) may yield design PGA values of 0.10-0.15g — still modest by global standards but meaningful for a structure containing 1 million tonnes of steel.

Performance-Based Seismic Design

For a structure of the Mukaab’s complexity and importance, the prescriptive force-based design approach of standard building codes is insufficient. Performance-based seismic design (PBSD) provides a more rigorous framework by defining specific performance objectives at multiple hazard levels:

Frequent Earthquakes (43-year return period, PGA ~0.02g): The building must remain fully operational with no structural or non-structural damage. All systems — elevators, climate control, fire safety — must continue functioning without interruption. The holographic dome projection systems and the spiral tower observation decks must maintain operational tolerances during and immediately after the event.

Design Earthquakes (475-year return period, PGA ~0.07g): The building experiences minor structural damage that does not compromise life safety or structural integrity. Repairable damage to secondary systems (partition walls, facade connections, piping) may occur but does not require building evacuation. The mega-frame remains elastic or experiences controlled yielding at designated locations without permanent deformation.

Maximum Considered Earthquakes (2,475-year return period, PGA ~0.12g): The building protects life safety through collapse prevention. Significant structural damage may occur but the building does not collapse or experience partial collapse. Controlled energy dissipation through designated yielding zones in the mega-frame prevents catastrophic failure. The foundation system remains stable without pile failure or excessive settlement.

Dynamic Response Analysis

The Mukaab’s seismic response characteristics differ fundamentally from those of conventional supertall towers. A tapered tower like the Burj Khalifa has its mass concentrated at lower levels, with decreasing floor plates reducing the seismic mass at upper stories. This mass distribution creates a relatively simple first-mode response dominated by cantilever bending.

The Mukaab’s cube geometry maintains its full mass at all levels — the 70th floor has the same floor area and therefore approximately the same seismic mass as the 10th floor. This uniform mass distribution creates a response spectrum that distributes seismic demands more evenly across the building’s height, rather than concentrating them at the base as in a conventional cantilever structure. Higher-mode responses — where different parts of the building move in opposite directions — become more significant in the cube form, requiring time-history analysis using multiple ground motion records rather than simplified modal response spectrum analysis.

The mega-frame’s lateral stiffness must be tuned to avoid resonance with the expected frequency content of ground motions from the relevant source zones. The Dead Sea Transform generates predominantly strike-slip earthquakes that tend to produce high-frequency ground motions. The Zagros subduction zone generates thrust earthquakes with potentially longer-period motions. The Mukaab’s fundamental period — estimated at 6-8 seconds based on its 400-meter height and mega-frame stiffness — places it well beyond the dominant periods of expected ground motions, providing natural period separation that reduces seismic demands.

Pile-Soil Interaction Under Dynamic Loading

The 1,200 foundation piles supporting the Mukaab interact with the surrounding soil in complex ways during seismic events. Under static conditions, the piles transfer the building’s gravity loads to competent rock or dense sand strata at depth. Under dynamic seismic loading, two additional phenomena emerge.

Kinematic interaction occurs when the soil surrounding the piles deforms due to seismic wave propagation. The stiff piles resist this soil deformation, generating bending moments and shear forces in the piles that add to the inertial forces transmitted from the superstructure. For large-diameter piles in layered soil profiles — where soft layers overlying stiff layers create differential soil displacement — kinematic interaction can govern pile design at depths where inertial demands are small.

Inertial interaction occurs when the superstructure’s mass generates inertial forces during seismic shaking that must be transmitted through the piles to the soil. The Mukaab’s unprecedented mass — approximately 1 million tonnes of steel plus concrete floors, facades, and contents — generates base shear forces that the 1,200-pile group must transfer to the ground without pile failure, excessive pile-head displacement, or foundation rotation that would tilt the building beyond serviceability limits.

The pile group effect complicates both kinematic and inertial response. The 1,200 piles are not independent — they interact through the soil that surrounds them, creating a coupled system whose dynamic response depends on pile spacing, soil properties, loading frequency, and pile group geometry. Three-dimensional finite element modeling of the complete pile group under seismic loading, validated by centrifuge testing of scale models, provides the analytical framework for pile design.

Seismic Isolation and Energy Dissipation Options

For structures of extreme importance and sensitivity, seismic isolation systems — flexible bearings placed between the foundation and superstructure that decouple the building from ground motion — offer significant performance benefits. Base isolation has been successfully applied to buildings of 20-30 stories, though its application to mega-scale structures remains an active research topic.

The Mukaab’s scale makes conventional base isolation challenging. The 1 million tonnes of superstructure mass would require bearing systems of extraordinary capacity — hundreds of isolation bearings, each supporting thousands of tonnes, with displacement capacities of 300-500mm. The cost and spatial requirements of such a system may prove prohibitive, but the life-safety and business-continuity benefits of isolation — maintaining immediate occupancy performance even during maximum considered earthquakes — could justify the investment for a building of the Mukaab’s economic significance.

Alternative energy dissipation strategies include tuned mass dampers (TMDs), viscous dampers installed in the mega-frame, and buckling-restrained braces (BRBs) that provide stable energy dissipation through controlled steel yielding. The Mukaab’s large enclosed volumes — particularly the holographic dome space and the atrium surrounding the spiral tower — provide potential locations for massive TMDs that use the building’s own suspended structures as tuned mass elements.

Instrumentation and Monitoring

The Mukaab’s seismic design will be validated through a comprehensive structural health monitoring system that records the building’s response to actual ground motions. Accelerometers installed at the foundation, mid-height, and roof levels will capture the building’s dynamic amplification characteristics. Displacement sensors at expansion joints, pile-head gages, and tiltmeters at the building’s corners will track foundation response.

This monitoring data serves dual purposes. In the short term, it validates the design assumptions and analytical models used during design, providing feedback that benefits future mega-structure engineering. In the long term, it provides early warning of any structural deterioration that might compromise seismic performance — corroded connections, settled foundations, or damaged dampers — enabling preventive maintenance before seismic events test the structure’s capacity.

The monitoring network for the Mukaab will be one of the densest seismic instrumentation arrays ever installed in a single building. With sensors distributed across the building’s full 400-meter height at multiple positions on each instrumented floor, the system captures the three-dimensional seismic response of the cube geometry — including the torsional modes that arise when seismic waves approach the building at oblique angles relative to its faces. The data processing infrastructure must handle simultaneous recordings from thousands of channels at sampling rates of 200-1000 Hz, with automated event detection algorithms that distinguish seismic signals from the ambient vibrations caused by wind, traffic, and the building’s own vertical transportation systems operating within the 1 million tonnes of structural steel mega-frame.

Non-Structural Component Protection

In a building valued at $50 billion, damage to non-structural components during seismic events can create losses measured in billions of dollars even when the primary structure remains intact. The Mukaab’s non-structural inventory — facade cladding spanning 640,000 square meters, the holographic dome projection systems, thousands of kilometers of piping and ductwork for the climate control system, elevator guide rails for the vertical transportation network, and the interior finishes across 2 million square meters of floor area — represents an investment exceeding the structural steel cost.

Acceleration-sensitive components (mechanical equipment, suspended ceilings, light fixtures, computing infrastructure) require seismic bracing and flexible mounting systems designed for the floor accelerations predicted by the dynamic response analysis. For the Mukaab’s cube geometry, floor accelerations at upper levels during a design earthquake may reach 0.15-0.25g — modest by California standards but sufficient to topple unbraced equipment, rupture rigid pipe connections, and shatter brittle partition systems.

Drift-sensitive components (facade panels, partition walls, piping that crosses story boundaries) must accommodate the inter-story drift ratios predicted during maximum considered earthquakes. The mega-frame’s stiffness limits drift ratios to approximately 1/400-1/500 under design earthquake conditions, but the 240-millimeter thermal expansion movements superimposed on seismic drift create combined displacement demands that facade connection details must accommodate without failure. The triangular cladding panels of the Mukaab’s exterior surface require connection hardware that permits both the slow thermal cycling of daily temperature changes and the rapid oscillatory movements of seismic response — a dual-performance requirement that challenges conventional facade engineering practice.

The AtkinsRealis and Bechtel seismic design program for the Mukaab integrates structural and non-structural protection into a comprehensive performance framework. Every component in the building — from the primary mega-frame columns carrying thousands of tonnes to the suspended ceiling tiles in a hotel corridor — must be designed, detailed, and installed to achieve specified seismic performance objectives. The result is a seismic engineering program whose scope and complexity reflect the Mukaab’s status as the most ambitious building ever attempted in a region where seismic hazard, though moderate, cannot be ignored for a structure containing 1 million tonnes of steel and serving populations measured in hundreds of thousands.

Seismic Design Considerations