Florian Aalami* Soo Heon Lee**
* President & CEO ADAPT Corporation(http://www.adaptsoft.com)
** President BasisSoft, Inc (ADAPT software sales partner, http://www.basis.co.kr)
ADAPT Edge 2012-RC/PT 다층 구조물 해석 및 설계 프로그램
New Approach to Integrated Structural Analysis and Design for Multistory RC/Post-Tensioned Buildings
1. Summary
This article introduces ADAPT Corporation's latest software product for the integrated multistory analysis and design of concrete buildings, ADAPT Edge. Edge uniquely packages user-friendly modeling of multistory buildings, accurate gravity and lateral analysis, post-tensioning, and detailed slab and beam design, all in one software package.
The benefits of Edge over traditional concrete design software are explained.
2. Today's Design Challenges
Structural engineers today are expected to design complex structures with great analytical accuracy in very short timeframes.
To meet this challenge, and because steel structure can be generally represented as discrete linear elements, there has been a proliferation of software programs for the modeling, analysis and design of steel buildings. Some of these programs offer fully integrated approaches for steel projects that efficiently take the engineer from analysis to the creation
of detailed construction documents. For concrete structures, however, such integrated design and documentation workflows have been limited to software that can only model buildings with fairly regular geometries and employ simplified analysis approaches.
To take full advantage of concrete's inherent flexibility, developers, architects, and engineers would like to realize projects with longer spans and greater architectural expression.
To meet such non-traditional goals, drive down construction costs, and improve construction efficiency, the use of high- strength materials and innovative construction technology like post-tensioning has been widely adopted. Until recently, the integrated structural analysis software available in the market could not meet these new design challenges for concrete buildings, in particular those that included post-tensioning.
Engineers had no choice but to use specialized software for particular components of a concrete building that could model complex geometries and accurately design post-tensioning.
This lead to inefficient workflows requiring engineers to maintain multiple analysis models and potentially transfer data between software packages manually.
a) Slab model with post-tensioning
b) Slab model showing adaptive organic finite element mesh Figure 1 Two examples of slabs modeled in Edge with
irregular geometries
Figure 2 Organic 3D FEM mesh using quadrilateral shell elements for slab
3. ADAPT Edge Multistory 3D FEM Modeling
ADAPT Edge 2012 (Edge) is a new multistory 3D Finite Element Method (FEM) software developed by ADAPT Corporation specifically for the efficient modeling, analysis and design of concrete buildings. It is uniquely formulated to handle even the most complex geometries with ease and analytical precision. Structural elements can be conventionally reinforced or post-tensioned. Figure 1 shows two slabs modeled in Edge.
Edge can be used to model the true physical geometry of any single or multistory concrete structure. The program's automated meshing algorithm produces an adaptive, organic 3D FEM mesh using quadrilateral shell elements for slab and wall components and frame elements for beams. Figure 2 shows true 3D finite element analysis model created by all ADAPT FEM software, including Edge.
Whereas other FEM software flatten the analytical representation of models with components at different offsets, Edge uniquely maintains the analytical neutral axis of each component exactly where its physical model is created [Aalami,2000,2001]. Figure 3 provides examples of flattened analytic models and Figure 4 illustrates the analytic modeling approach taken by all ADAPT FEM software, including Edge.
The ability to maintain the differential offset of components is one of the main factors contributing to Edge's analysis accuracy even for the most complex structures. For example, where other software produce the same analysis result for an upturned beam as for a downturned beam, Edge produces distinctly different results, as one would expect based on first
principles.
The ability for Edge to properly analyze post-tensioning is also inherently tied to this underlying analysis capability.
Post-tensioning tendons are discretized as stick elements
Figure 5 ADAPT Edge model supported on foundation (i) Physical Model (ii) Flattened Analytic Model
a) Concrete section at column
(i) Physical Model
(ii) Flattened Analytic Model b) Step in slab
Figure 3 Examples showing how most structural analysis software 'flatten' vertical geometric offsets when creating
their analytic models
Figure 6 ADAPT Edge configuration screen Figure 4 ADAPT Software provides one of the most
accurate FEM analysis results for irregular concrete geometries by maintaining and accounting for the offset of
between each section of concrete and any embedded prestressing
where the calculated net prestress force after losses is applied to the concrete block it passes through at each shell/frame element intersection. Most other software that offer post-tensioning in their models simplify its effect by substituting the actual prestress force with an approximate uplift force [Aalami, 1990].
Edge building models can be supported on individual column and wall supports or mat foundation resting on soil springs, as shown in Figure 5. The foundation of a model in Edge can consist of any combination of structural mat slab, thickened areas, steps, and grade beams. Any combination of gravity and lateral loads can be applied to the model and analyzed.
4. Integrated Slab Design
The full slab and beam design capabilities of ADAPT Floor Pro (Floor Pro) are available in Edge models, offering the industry's first fully integrated slab design and multilevel analysis solution. With this combination, structural engineers can tap into ADAPT's 30 years of experience in detailed floor system design and leverage the capabilities of the industry's leading FEM software for advanced slab analysis and design, namely Floor Pro. Edge and Floor Pro can operate on the same building model and analysis solution in the same window, eliminating the need to maintain multiple models or manu
ally transfer data back and forth between systems. Figure 6 shows a platform of ADAPT Edge configuration screen.
Figure 9 A user can switch between building and single level mode with one click
Figure 10 Edge offers the unique ability to apply the internal reactions from Building loads to the re-analysis of
individual levels. In this illustration, the Gravity Loads are classified as generally applied and are always included in
the analysis Figure 7 ADAPT Edge model showing color contour of
calculated deflection values
Figure 8 Results of a Floor Pro advanced slab design in an ADAPT Edge model. The image shows the slab broken
down into design strips and associated design sections (one support line is highlighted). The total design moment
is shown along each support line
5. Substructuring: Building vs Single- Level Analysis
As design projects become more complex and detailed, it may be impractical and time-consuming to re-analyze or design the full building model for each design iteration.
Consequently, to work efficiently, Edge users are given the option of analyzing or designing their projects in either the entire building or single-level modes. The switch between modes is made with the click of an icon, as illustrated in Figure 9. Modeling, changes to an existing model, meshing, analysis and design operations are all valid in both states of
the program. With the introduction of substructuring, not all applied loads to a model may be relevant in every analysis configuration.
To address this situation, Edge distinguishes between generally applied load and building load cases. Loads classified as generally applied loads can represent any gravity or lateral load and are always applied to the model when analyzing in building or single-level modes. Building loads are only applied when analyzing the full structural model in building mode. These loads are intended to represent lateral loads that are typically applied to a structure in its entirety, such as wind and seismic.
As mentioned above, users can speed up their slab design iterations by modifying and re-analyzing a single level only.
In this case, what happens to the building loads that are only applied when analyzing the model in building mode? To address this condition, we have incorporated the automated
a) Advanced tendon modeling
b) Tendon property and profile
C) Force along tendon Figure 11 Images of a post-tensioning at single level of the building ability for Edge to substructure the analysis model and
solution. By doing so, building load cases can be available during the re-analysis of a single level as a set of applied loads (story force) at the slab level and internal reactions (see Figure 10). The internal reactions at the ends of upper and lower supports are in equilibrium with the applied story forces. This approach provides a reliable and accurate method for incorporating lateral load results obtained at the building level with the detailed gravity re-analysis at a single level.
6. Post-Tensioning in Multistory Structures
Post-tensioning has been successfully applied in building construction for over 60 years in the USA [Aalami, 2007].
Despite its widespread use in many global markets, its modeling, analysis and design are still regarded as somewhat mysterious and requiring special knowledge. Engineers have become accustomed to using sophisticated software tools to model and analyze, in detail the various structural systems
they are using in their projects. Through this process, they develop a better sense for the overall structural behavior of the system and can validate the structural performance of their proposed design configurations.
Before Edge, there were only two types of software available for the modeling and analysis of post-tensioned members: building software like ADAPT PT and Floor Pro that modeled one level of a building or high-end bridge design software that supported the modeling of prestressed cables. Single-level building software is the industry standard and most practical approach to design post-tensioned slabs or beams. It's drawback from the perspective of some engineers, however, is that it cannot give an engineer explicit feedback on the effects of post-tensioning on the overall building performance. On the other end of the spectrum, utilizing one of the high-end bridge programs is cost and time prohibitive.
Edge now offers engineers an easy, integrated solution for the design of post-tensioned buildings where the effects of post-tensioning on the overall building structure are implicitly
Figure 12 Edge model with post-tensioning at all levels of the building
Figure 13 Edge model responding to different wind loads
analyzed and accounted for. Edge reports the prestress force in the concrete members as well as the secondary, hyperstatic reactions. Edge has seen particular applications in the design of transfer slabs and beams.
7. Effects of Differential Shortening
Another factor of great concern to structural engineers when designing multilevel structures is the effect of support shortening. In addition to calculating the distance formwork should be super-elevated to account for long-term shortening, engineers want to determine the effect of differential support shortening on horizontal elements attached to the supports.
Since the building analysis results in Edge implicitly account for the instantaneous component of support shortening, slabs and beams designed using the building solution automatically include these effects. Bending and shear reinforcement calculated by the program will provide adequate capacity to resist these forces. If a user does not want to include these forces in their design, they can re-analyze individual floors in single-level mode.
8. Wind Analysis
Edge includes a user-friendly wind load wizard that gives users the choice of auto generating applied wind loads based
Figure 14 Vibration analysis result for the first three modes on code formulas or entering a user-defined wind pressure
profile. In both cases, the program lets the user select the primary angle of attack for the wind, calculates the exposure width of the structure at each level, and applies the calculated loads to the structure. Loads are applied as line loads to the exterior edges of the slab in the user-specified primary windward direction, optional 90-degrees angle, and corresponding leeward sides. An optional incidental torsional moment can also be specified and automatically applied by the program at each level. All values in the wind load wizard can be easily overwritten by the user. The wizard can also be used to easily update previously defined wind load cases.
9. Seismic Analysis
For projects in seismic zones, Edge offers a practical seismic load wizard. Similar to the wind load wizard, this utility gives the user a choice of using code specified formulas to calculate base shear or enter user-defined values.
User-defined seismic loads can be entered as individual story force or a base shear. If a base shear is entered, Edge automatically calculates the mass of the structure and distributes the base shear according to a user entered distribution coefficient, and relative weight of each level, or ACI 318 formula. The story force from seismic effects is applied as a uniformly distributed load on the slab. Due to the FEM formulation of slabs as shell elements, the inherent eccentricity of each level with respect to the stiffness of its supports is implicitly accounted for in the analysis. If required, the user can specify a % for the calculation and addition of an additional torsional force due to any incidental eccentricity of the structure.
10. Integrated Gravity and Lateral Design of Floor Systems
The greatest workflow improvement Edge offers is the explicit combination of gravity and lateral design in one integrated model. Prior to Edge, engineers were forced to model and analyze concrete buildings in one software to design the lateral load resisting system and the slabs in a separate software like Floor Pro. To qualify our statement, we are referring to concrete projects of a more complex nature
and with post-tensioning that could not be adequately analyzed and designed using traditional concrete building design software available in the market. The integrated design workflow Edge supports extends beyond slabs and also includes the foundation systems of concrete buildings.
11. Vibration Analysis
An inherent capability of ADAPT's 3D FEM analysis is modal vibration analysis. This is a useful tool in the assessment and evaluation of the vibration performance for sensitive structures. With Edge, modal vibration analysis can be carried out on the entire building, or any user-selected subset.
12. BIM Workflow
The adoption of intelligent, interoperable modeling approaches as part of a Building Information Modeling (BIM) strategy can save consulting companies time, improve coordination quality and reduce errors. To this end, ADAPT
Figure 15 BIM based integrated design process
maintains a good link with Autodesk Revit Structure. The integration link allows users to easily transfer smart model information from a Revit concrete building model to Edge.
After completing the design in ADAPT, slab rebar and basic tendon geometry information can be transferred back to the original Revit model.
References
1. ADAPT Corporation, software documentation and tutorial videos, www.adaptsoft.com.
2. Aalami, B. O. (2007) Critical Milestones in Development of Post-Tensioned Buildings, ACI, Concrete International, pp 52-52.
3. Aalami, B. O. (2001) Software for the Design of Concrete Buildings, American Concrete Institute, Concrete International Journal, pp. 28-35.
4. Aalami, B. O. (2000) Structural Modeling of Post- Tensioned Members. American Society of Civil Engineers, Structural Journal, pp. 157-162.
5. Aalami, B. O. (1990) Load Balancing A Comprehensive Solution to Post-Tensioning, ACI Structural Journal, V.
87, No. 6, pp. 662-670.
[담당：윤장근 편집위원]
