Unit 5: Phase Based Structural Analysis

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An introduction to Phase Based Construction and Level of Development

One of the key elements to a successful project is clear communication for all design and construction teams involved. The LOD Specification is a means to provide a common language for communicating design. Phase based construction requires critical thinking by the designer to fully understand and communicate to the construction team the assumptions made during design for the sequence of construction

 

 

 

Learning Objectives

After completing this unit students will have an introductory grasp of:

  • The Level of Development Specification
  • Phase Based Construction
  • Coordinating a base plate design with other systems
  • Work through the construction sequence for a basement foundation wall

Introduction

In this unit we will review key features and points to consider when coordinating phased based construction and designing for sequential loading of structures. Phased construction, or staged construction, is a static modeling, analysis, and design approach that looks at building a structure in sequential steps as it is actually assembled in the field. When staged construction is analyzed in structural application, the definition of the sequence of construction stages are established. These include the structural system itself and load patterns which are added, removed or combined.  Referencing back to Unit 1 and Unit 2’s example of man building shelter, it is understood that there will be a sequence in which the shelter is assembled and that Man cannot build the roof of his shelter until the walls are properly erected and braced. 

Level of Development

AGC BIM Forum Level of Development Specification

Used with permission; copyright of BIMForum https://bimforum.org/lod/

Recently, the BIM Forum released a specification that contains vocabulary for communication between all teams involved on a construction project, including structural engineers.  The Level of Development (LOD) Specification  is a reference that has changed the industry by giving designers the tools required for communicating and clarifying their design.  This tool assists in telling the story of the structural design and can assist in communicating the elements that impact the phases of construction.  The LOD is a collection of graphics and model examples, paired with definitions that describe the level of information being communicated.  This level of information can range from conceptual, preliminary, designed and coordinated or as installed in the field.  The LOD Specification applies to an individual system and not the entire model.   These graphics and definitions give a common vocabulary for use between all teams involved.

[1] BIMForum. (2013). Level of Development Specification for Building Information Models. The graphics reproduced in this text are copyrighted information. They were reproduced with permission from the BIMForum.

The following table defines LOD in terms of Modeled Elements and illustrates the LOD definitions with a graphic of a structural steel column as it progresses from an LOD 100 through an LOD 500.  The text in quotations comes directly from the Level of Development Specification.


Images used with permission; copyright of BIMForum https://bimforum.org/lod/

It is imperative that a designer communicate at what level their design model is developed to.  This allows other teams who are coordinating their designs with the structural design understand what information is known and provided, and what information is possibly shown as a place holder but may not be finalized.  To take the previously used shelter example: The roof of the shelter may be sketched out generically to represent that the shelter will have a roof.  The sketch has a low level of development and therefore communicates that more information needs to be attained before coordinating the shelter or building it.

In summary, the LOD specification is an asset to communicating design intent to other designers, builders, and owners.  This tool provides a means to specify the level of development of the modeled element being communicated.  This provides the required information for understanding how to stage the design and construction of a system and how to coordinate the design with other systems.

 

Designing for Staged Construction



Figure 1

Understanding the stages a structure is built and erected is a key element in understanding exactly which forces will be acting on the structural members.  As discussed in the previous units, the pattern, timing and magnitude of these forces, known as loads, must be analyzed and combined, utilizing different equations outlined by the governing design code.  Each of the different forces acting on a member must be determined for the different stages of construction and occupancy before member design can begin. One example of staged construction is a multi-story hotel (see Figure 1).  The hotel is made up of an 8’-0” wide corridor with two 24’-0” deep hotel rooms on each side of the corridor and a wide open first floor lobby with no columns except on the exterior wall. In order to achieve the open lobby, a structural element known as a transfer girder must be designed.  A transfer girder is a horizontal member that supports columns from above, and transfers those loads into the foundation.  It is important to note that the amount of the load on the girder, and consequently its deflection, will increase as each story of the hotel is constructed.  Additionally, the types of load that the transfer girder handles can vary over time.  For instance, the live load from a construction crew is different than the live load of a hotel room and corridor.  The stability and safety of all structural elements must be understood and designed for each phase of construction.

To further explore the idea of staged construction, we will take a closer look at the design approach for the concrete transfer girder.  In this particular example the transfer girder is post-tensioned.  It will have multiple tendons that will be stressed to balance the forces loading the girder.  However, the tendons cannot all be stressed at once.  The engineer will have to analyze the girder at each construction stage to design the amount of tendons to be stressed at each phase. Robot Structural Analysis is a finite element analysis program that is capable of providing staged analysis through their Phase function found under the Geometry tab. This is a built-in advantage that Robot provides, allowing the engineer to design the transfer girder for its final loading condition while also providing analysis at each phase.  Robot is set up for the engineer to input different load phases for the element.  Also, in some cases a support condition may not be present until the final stages of constructions. The engineer can model the final phase of the building but analyze the elements based off of which support conditions are present at each stage.

In summary, the stability and safety of the structural elements must be understood during each phase of construction.  The post-tensioned concrete transfer girder requires special attention to the different construction phases for its design.  It is the engineer’s responsibility to communicate, through the construction documents, at which stage the different tendons are to be stressed.  Structural Analysis programs, such as Robot, have the built-in capability to design an element for multiple-phased loading and support conditions.  Attached to this unit is a Revit Structure file that has a sampling of the hotel example mentioned above.  We encourage the user to export this file into Robot and explore setting up phases for analysis and design.

 

Examples of Phase Based Analysis for Construction

 See examples of phase based construction and its structural design considerations. Examples include a base plate and a basement wall.