Seismic performance analysis and BIM

Leo Salcé, Int Assoc AIA, LEED AP, and Joshua Gionfriddo, E.I.T.

The science and implementation of earthquake engineering is constantly evolving. As building fortification demands have increased due to recent seismic incidents in areas such as San Francisco, Mexico, and Chile, engineers are being warned to design more earthquakeresistant buildings using today’s performance-based design methods. Alternative technologies such as building information modeling (BIM), analysis methods, and software are providing engineers with the necessary tools to construct buildings that have a lower risk of earthquake-induced loss.

Seismic analysis methods Seismic analysis is the calculation of the response of a building structure to earthquakes. It is part of the process of structural design in regions where earthquakes are prevalent. Structural engineers can perform seismic analysis so that structures are built to resist seismic events, thereby protecting the structures and their occupants.

Fortifying buildings to resist earthquake forces has been regulated in the U.S. since the 1933 passage of California’s Field and Riley Acts. Early codes prescribed accounting for a percentage of a building’s mass as lateral shear forces acting on the base of the structure. In modern times, elastic design analysis, as described in the International Building Code (IBC), utilizes force levels obtained from the Design Response Spectrum (DRS). This design method has been used successfully for lower-risk buildings. A more exacting modern approach for high-risk construction is based on a static nonlinear, also known as dynamic analysis, of the building. This could include pushover analysis to provide a more accurate representation of the building’s likely performance during a seismic event.

BIM and seismic analysis In its current state, BIM software does not have the capability to account for so many different methods of fortification analysis without finessing a model through many pieces of software or custom API programming, which can be a cumbersome task. Design models used for construction documents must be passed from a BIM model to an analysis model. Even when this is done through a single company’s software suite, disconnects occur between BIM and analysis models while round tripping results. When attempting to use third-party analysis software, this disconnect can be even greater. Despite this, BIM models are still beneficial for building fortification in a number of ways:

Seismic retrofitting — BIM is especially useful for infrastructure when working with reality capture technology, whether via point cloud information (laser scanning capture) or photogrammetry (photo to 3D model). It is impressive to see how through the use of unmanned aerial vehicles (UAV) and an HD camera such as the GoPro we can render extremely accurate as-built 3D models of an existing building or structure. With the ability to create highly detailed 3D structural models of roads, bridges, tunnels, and any urban infrastructure element, seismic events and their impacts can be simulated.

BIM and code — BIM ensures that all seismic requirements are incorporated, eliminating the need for design modifications as the work in the field evolves. 3D views are especially critical when complying with seismic specifications, such as a regulation calling for a 2-inch space between pipes or ducts to prevent damage in the event of an earthquake. Basically, any hanger or brace can be modeled, penetrations and sleeves can be identified, and code validation can be established based on the BIM tool used. More tools are offering access to their APIs to further automate tasks or develop custom plugins that will streamline the code compliance checking.

Design coordination — A BIM approach can also improve systems design, supporting early discovery of potential problems in how these systems interact during the design-build stages. Visualization also often greatly speeds up the design coordination process.

Alternative techniques As of late, lateral systems such as Eccentric Braced Frames and Special Concentric Braced Frames are being replaced with higher performing systems, such as Buckling Resistant Braced Frames (BRBFs), which can deliver more predictable and resilient performance during seismic events. Universities such as Stanford now require designs that go beyond building codes and recommend structural designers use BRBFs for all capital improvement projects.

An alternative to building strong enough structures to resist all foreseeable earthquake forces is to attempt to reduce lateral forces imparted on the structure. Wave dampening techniques reduce a building’s sway to protect the structure during seismic events. These systems can include roller bearings or base isolators that impart friction and provide resistance to sway and change the frequency of the vibration.

More complex designs, such as Tuned Mass Dampers (TMD), consist of a large weight suspended high in the building to reduce the amplitude of the vibration waves in lateral loading conditions. These protect from both seismic and wind loadings and are found in some of the world’s tallest buildings. All of these modern structural systems require intense analysis methods to guarantee safety of built structures.

Software methodologies A number of software solutions in the market today allow engineers to leverage a 3D model for seismic analysis and simulation. Some applications allow analysis of the structure through a continuum and discrete stages of loading, including seismic events with some of the following methods:
• Applied Element Method tracks structural collapse behavior through the different stages of loading, including reinforcement yielding, crack initiation and propagation in tension-weak materials, etc. With this approach, engineers can analyze not only buildings but all structures, including bridges, stadiums, cranes, and pipelines. • Equivalent Lateral Force method is an alternative simplified approach for determining distribution of seismic-based shear force on the height of regular, multistory buildings.

Conclusion With new versions of software and better integration of analytical applications, BIM will eventually provide engineers with all the tools necessary to construct building-specific data that is needed to define what kind of damage can result from an earthquake. Today’s limitations in the way building seismic analysis is performed will continue to improve with the rapid evolution of current BIM software, and we can expect to see additional support for integration of advanced analytical applications such as RISA, RAM, and ETABS in the near future.

As the construction industry continues to adopt BIM, the cost-savings benefit is undeniable. However, for designers, namely structural designers, to fully release the benefits of BIM, collaboration between modeling and analysis software must be enhanced. Additionally, new sets of tools for the evaluation of complex buildings under both static and dynamic loading will need to be developed to protect from the next seismic event. The ability to analyze discrete members, entire buildings, and individual connections within a single analysis package will increase the productivity of engineers. Finally, as the industry progresses into the next decade, having the ability to perform all of this analysis from within a single BIM model will become the new norm and will enable true performance-based earthquake engineering.