This page outlines best modeling practices that are used in the RAM Concept PT Optimization Tutorial.The points below are important for producing successful optimizations in other models.
It is common to add tendons in ends spans that are approximately the same length or longer than interior spans. This is especially important in larger models, which have more interior spans. The Optimizer will determine the additional tendons only if the banded tendon is segmented at the end span. If a single banded tendon polyline is modeled, one tendon quantity would be optimized for the entire polyline, which would result in the same tendon quantity in each span after optimization.
An example is shown in the image below. Note that the banded tendon polyline is segmented at the first interior column on the left side, and each segment was manually designed with a different tendon quantity. The added tendon branch of 4 tendon strands is added automatically by the program to maintain consistency of tendon quantities at the intersection of the segments. When the model is optimized, the optimizer will design each tendon segment separately and may determine a different tendon quantity for each segment. The optimizer also automatically generates and calculates the tendon quantity in the added tendon branch.
Exterior spans of distributed tendons also often have added tendons. In the example model, an overlapping distributed tendon quadrilateral is modeled over the exterior spans to represent these added tendons. The optimization properties for these objects are defined with a minimum effective force of 0 kips/ft. When the minimum effective force or minimum number of strands is input as 0, the optimizer will consider a case with no added tendons as one of the possible solutions.
The minimum tendon quantity that is input in the Optimization tab should be based on the minimum ACI precompression limit (125 psi, see ACI 318-14 8.2.3). If a lower value is entered, the final optimized tendon quantity may be lower than the code limit.
The design strip associated with the banded tendon line is 15 feet wide. The minimum number of strands required to reach the 125 psi minimum compression threshold is:
Slab Thickness = 8”
Effective PT Force (one strand) = 26.8 k
Minimum Number of Strands = (125 psi)(8 in)(15*12 in)/26,800 lb = 6.7
As shown in the image below, 7 is entered for the Minimum Number of Strands for this banded tendon polyline in the example model. This ensures that the minimum precompression requirement is met. Note that the input values for number of strands do not need to be whole numbers and that we could have entered a value of 6.7 here.
4
The distributed tendon quadrilaterals are defined with a Minimum Effective Force of 12 kips/ft as shown in the image below. This quantity is calculated from:
(125 psi)(8 in)(12 in/1 ft)*(1 kips/1000 lb) = 12 kips/ft
Maximum tendon quantities should be based on practical maximum precompression limits. For example, many engineers use 300-400 psi as a practical limit for two-way slabs. Limiting the maximum tendon quantity in this way reduces the range of tendon quantities that are considered by the optimizer and improves performance.
At interior supports, tendons are almost always profiled at the highest possible elevation while respecting cover requirements for simplicity, repetitive use of standard details, and ease of construction. However, at cantilever spans, the profile at the support may NOT be placed at the highest possible elevation so that the span is not overbalanced.
In the tutorial model, all support polylines at interior supports (high profile points) are defined with the Optimize box unchecked. Not optimizing these polylines has the added benefit of reducing the number of optimizable properties and improving performance. Some of the support polylines at cantilever spans have been defined with the Optimize box checked with an appropriate range of maximum and minimum elevations. The image below shows one of these support polylines. The range of elevations for this polyline corresponds to mid-depth of slab (minimum) to 1” top cover (maximum). All span profile polylines (low profile points) are modeled with the Optimize box checked so that they can be adjusted by the Optimizer.
The Elevation Reference controls the reference point for maximum and minimum elevations defined for profile polylines. For example, if “Bottom Cover” is used for the Elevation Reference, then the Minimum and Maximum Elevation defined for the profile polyline are measured from the bottom of the slab. In general, it is best to reference low points from the bottom of the slab and high points from the top of the slab. This convention is consistent with the program default values and will avoid problems and minimize input effort if the slab thickness is changed.
It is common to model profile polylines continuous across multiple adjacent bays with approximately equal spans in order to reduce modeling effort, simplify the tendon layout, or reduce the number of optimized objects in the model. Caution should be exercised when doing this when running optimizations, however, as it can prevent the Optimizer from finding the optimal solution because only one elevation would be considered for the entire polyline.
In the tutorial model, two separate span polylines are modeled across the slab. The polyline on the left is associated with a 2-span condition, and the polyline on the right is associated with a 3-span condition. The Optimizer adjusts each polyline separately, and the best solution may have different low point elevations in each bay.
It is best to optimize span profile polylines with a wide range of possible elevations. In some cases, especially complex model, counterintuitive values may produce the best design. The wide range ensures that these unexpected solutions are considered by the optimizer.
In the example model, the minimum and maximum elevations for the span profile polylines are based on minimum cover requirements. See image below.
Also, note that an elevation increment of 0.25 is used in the example model, which is the common profile increment for slab in the United States. The elevation increment that is entered should be based on the common profile increment for a given locale. A larger increment can be used if less variation in elevations is desired. However, this may also prevent the program from finding the optimal design.
***Note that this modeling tip is not used in the tutorial model***
Generally, profile polylines are not required to be modeled at slab edges. When they are excluded, the program automatically places the tendon at the mid-depth of the element. However, if a model includes drop caps or beams along the slab edge, it is important to model a profile polyline so that the tendon is placed correctly.
For example, consider the addition of a 20” drop cap at the corner column in the lower, left corner of the example model as shown below. A profile polyline is needed at the slab edge so that the tendon is at mid-depth of the 8” slab. If this polyline was not modeled, then the generated tendon end would be dropped 10” from the top of the slab (i.e. mid-depth of drop cap). This would result in balance loading issues and likely result in design problems that could prevent the program from finding a valid design.