RAM Concept Design Strips [TN]

Overview

This page is the landing page for user help for modeling design strips in RAM Concept. The page is divided into two main sections: Tech Notes and Frequently Asked Questions. The Tech Notes section includes links to web pages with in-depth discussion of other issues relating to design strips. The Frequently Asked Question section includes common questions relating to design strip modeling and properties.

It is recommended that users become familiar with the “Defining Design Strips” chapter in the RAM Concept Manual, which contains additional guidelines on design strips that are not included here.

Tech Notes

Generating Span Segments

Defining Manual Design Strip Boundaries

Cross Section Trimming including tips for Drop Caps and Drop Panels

Design Strips and Slab Openings

Frequently Asked Questions

When should design strips be modeled with column and ½-middle strips and when should they be modeled as full-width column strips?

RAM Concept uses a finite element analysis to determine design forces. These forces are determined by integrating (or averaging) the finite element forces across the width of the column or ½ middle strips. The wider the strip, the more the design force is averaged and the greater the difference to the peak design force in the strip.

Figure 1 shows a typical moment distribution. Note that the moments are highest near the column and decrease toward the middle of the span. Using the column/middle strip approach forces the program to integrate moments with similar magnitudes across the width of the strip. The design moment for the column strip will be closer to the peak moment than if a full-width design strip was used. It is important to remember that the use of column and middle strips only affects the integration of forces, but does not affect load distribution in the finite element analysis.

 Figure 1. Typical Moment Distribution Across Column and Middle Strips

In some cases, it may be desirable to use design strips modeled as full-width column strips instead of column and two half middle strips. If two columns are close together, for example, the moment distribution may not vary significantly over the width of the strip (see Figure 2). Averaging moments over the full tributary width may be justified.

 Figure 2. Typical Moment Distribution for Closely Spaced Columns

It is also acceptable to define full-width strips for the column strips and full middle strips manually in lieu of using column strips and ½ middle strips (see Figure 3). This allows for the middle strip to be designed as one strip instead of two separate strips. This approach may be the best option when the ½ middle strips are narrow. To model full-width middle strips, the span segments must be modeled independently using the span segment tool. RAM Concept will not generate the span segments for these strips automatically. Where the column support is large, you may want to may want to set the support width to the column dimension for the full-width column strip and 0 for the full-width middle strip. Assuming that the span segments are extended to the centerline of the support, this will force the program to locate the first cross-section at the support centerline as opposed to the centerline of the column (see Figure 3).

 Figure 3. Full-Width Column and Middle Strips

Full-width column strips are typically used for post-tensioned slabs for the following reasons:

• Capacity calculations of post-tensioned slab have generally assumed that the precompression in the slab is distributed over the full tribuary width.
• Tendons are often banded along column grid lines.
• The column/middle strip approach was developed for use for the approximate two-way slab method discussed in Chapter 13 of ACI 318-05. These do not apply to prestressed slabs per ACI 318-05 18.2.1.

Some design codes require column and middle strips for PT design and require some distribution of tendons in both the column and middle strips. Problems can occur in PT slabs with banded tendons if a column/middle strip layout is used and a tendon does not intersect the middle strip. See the following web page for more information:

Balance and Hyperstatic Loading

Are there any recommendations for modeling design strips for beams?

For design strips parallel to the beam, it is best to align the design strip with the axis of the beam. If the strip is skewed slightly, then the Orient Span Cross-Section tool should be used to keep the cross-sections normal to the beam.

Engineering judgment should be used to model the design strips perpendicular to the beam. Beams can be supported by columns or other beams. The end points of the span segment should be defined at the centerline of each support. When beams are supported as other beams, it will be necessary to uncheck the box for “Detect Supports and Edges Automatically” in the Strip Generation tab of the Design Strip properties dialog and manually enter support widths (See Box 1 in Figure 4). The Support Width defines where the first (and last) cross-section is located, i.e. the critical section for moment design. Typically, the critical section is taken at the face of the supporting column or beam, although some engineers take the critical section at the centerline of the support. When the critical section is taken at the face of the support, it is best to model the support width 2” larger than wider than the actual width to avoid any rounding or snapping errors. Note that when design strips extend past an intersecting beam, a portion of the cross-section may be trimmed by the inter cross-section slope limit. See the RAM Concept manual for more on this type of cross-section trimming.

Typically, design strips for beams have the Column Strip Width Calc set to “Code T-Beam” (See Box 2 in Figure 4) and the CS Design System set to “Beam” in the Strip Generation tab (see Figure 5). This ensures that the column strip includes the beam and its effective flange width and that the cross-sections are designed using the ACI provisions for beams as opposed to slabs. The Middle Strips tab will typically have the box for “Middle Strip uses Column Strip Properties” unchecked (see Box 4 in Figure 6). This allows for the beam and slab to be designed with different bar sizes. The MS Design System should be set to “Two-way slab” so that the code provisions for slab design are used for design (see Box 5 in Figure 6).

When modeling one-way slabs, it is convenient to use the “Design Column Strip for Column + Middle Strip Resultants (See Box 3 in Figure 4). When selected, forces are integrated separately over the column strip and each half middle strip, then added together, and used to design the column strip. This allows for the beam and its effective flange to be designed for the full design strip forces.

Figure 4. Typical Strip Generation Settings for Beam Design Strips

     Box 1. Option for manually defining support widths

     Box 2. Option for setting column strip width calc to "Code T-beam"

     Box 3. Option for designing column strip for column + middle strip resultants

Figure 5. Typical Column Strip Settings for Beam Design Strips

Figure 6. Typical Middle Strip Settings for Beam Design Strips

     Box 4. Option for choosing different properties for middle stips

     Box 5. Option for selecting design system for middle strips

Why does the Status Plan on the Design Status Layer show failures for ACI 318 18.3.3 (or other post-tension concrete code provision when the slab is a mild reinforced concrete slab and not a post-tensioned slab?

There is an option in the Default Span Properties dialog that controls whether the design strip is designed as a post-tensioned slab or a mild reinforced slab (see Figure 7 below). To change the setting for span segments that have been modeled previously, do the following:

1. Open the Latitude Design Spans Plan on the Design Strip layer.
2. Use the Selection Tool to select all span segments on the plan.
3. Right click in the plan window and choose "Selection Properties" from the menu to open the Default Span Properties dialog.
4. Click on the General tab.
5. Uncheck the box for "Consider as Post-Tensioned" (see Figure 7).
6. Repeat steps for the Longitude Design Spans Plan on the Design Strip layer.
7. Reanalyze model.

Figure 7. Consider as Post-Tensioned Option

See Also

Generating Span Segments

Structural Product TechNotes And FAQs