Examples of Phase Based Analysis for Construction
Phase based construction can include time-dependent variables such as creep, shrinkage, aging (change in elastic modulus with age), and tendon relaxation in addition to varying load patterns and support conditions. Further, phase based construction may be part of a sequence of nonlinear static or time-history analysis load cases. For linear load cases, the structural stiffness at a given construction stage may serve as the basis for analysis for materials such as concrete that have notable strength gain in the first seven days after being poured. An even more common aspect of phased construction entails the consideration of how structural components are assembled and how that impacts their design and coordination with other systems.
Base Plate Coordination
Reference the video from Unit 4 (Unit_4_Base Plate). This video highlights the coordination of a steel column, baseplate and anchor rods with a concrete foundation. Often times these connections are considered to be typical details and are shown in construction design drawings with a simple two-dimensional sketch and a note (see Figure 2). However, in this particular case, once the detail was modeled to a higher level of development it is easily seen that the anchor rod plates protrude out of the concrete, therefore this connection detail is not acceptable. Figure 3a demonstrates the two-dimensional base plate detail modeled, to better understand the connection the grade beam’s visibility is partially transparent.
In Figure 3b the anchor rod plates, highlighted in red, can be seen protruding out of the concrete. Although the connection may have been a sound solution numerically, geometrically a revision is required.
This coordination item needs to be discovered and revised before it is constructed in the field. Some of the solutions for this coordination issue will require changes to elements that impact the phasing and scheduling of the construction. For instance, the most obvious solution would be to widen the grade beam that the column is sitting on. In order to do that it must be known if the grade beam has been formed and poured in this area already in the field. Another solution could be to modify the configuration of the anchor rod plates. Lastly, the column size could be modified, reference Figure 3c. If analysis allows, switching from a W10x49 to a W10x45 gives a smaller flange width which creates more space for the anchor rods to be shifted in closer to the column. These items are all critical to consider when detailing connections. In summary, reviewing a connection detail at a higher level of development can unveil coordination items that may have been previously missed. If construction has begun on the project it is important to understand what phase it is in when producing a new solution.
Basement Wall Staged Construction
A classic example of staged construction can be seen in a typical building basement foundation wall. The basement wall is designed to resist the lateral forces from hydrostatic, soil and surcharge loads.
These lateral forces load the basement wall once the soil has been backfilled. It is the engineer’s responsibility to communicate in their design the assumptions that they made when calculating the forces and design components that resist the lateral loads on the system. Generally, for basement wall designs you will find that the assumptions that were made for design impact the construction sequences for the wall. Reference the video Unit_5_Phased Construction.mp4.
As shown in the video (4:19) the basement wall, designated as Pour 2, is supported at each end by a grade beam and a perimeter beam, pour 1 and pour 3 respectively. The basement wall itself can be supported to transfer lateral loads by detailing what is called a key way (2:08).
Keyways are commonly used in concrete construction. Keyways are typically created using a 2x4 placed in the formwork prior to the first pour. After the first pour has cured the 2x4 will be removed for the second pour to take place.
The concrete of the second pour would then be pocketed in the first pour. The reasoning for this is to increase the surfaces for friction between the pour elements to transfer forces from one element to the next.
Another assumption made in the design of the basement wall is the presence of a free-draining drainage system. A free draining system is created by placing a drainage pipe surrounded by gravel at the base of the wall. This system allows water to freely drain without building up pressure against the basement wall, which is known as hydrostatic pressure. If a free draining system is assumed in the analysis and design of the basement wall, than this information would need to be included in the LOD of the modeled design and noted in the construction documents. The free-draining system can be seen noted in CAD in Figure 4.
One of the last assumptions made during the design of the basement wall is that the top perimeter beam and slab (pour 3 in the video) are in place and have gained enough strength to brace the top of the basement wall prior to backfilling the wall. If the wall were to be backfilled prior to the placement and hardening of the perimeter concrete beam than the system would lose one of its supports and become unstable (10:45) resulting in the wall rotating and caving in. It would be noted on the drawings that the perimeter beam and slab system would need to be fully formed, poured, and cured to a specified concrete strength prior to backfilling the basement foundation wall.
The assumptions that the engineer made during the design and analysis of the basement wall need to be specified in the final design drawing of the wall system and noted in their calculations. These assumptions are considered to be dependent on the sequence of which the structural elements are installed, the type of detailing selected between the structural elements, and dependent on the time it takes for the concrete to cure to the proper strength.
This unit has reviewed two important topics for communicating design concepts to other teams on a construction project: level of development and phase based construction. 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.