Cross Section Trimming [TN]


 Applies To 
 Product(s):RAM Concept 
 Version(s):Various
 Area: Analysis; Design

Overview

There are two types of cross section trimming in RAM Concept: Single Cross Section Trimming and Inter Cross Section Slope Limits. Single Cross Section Trimming considers one cross-section at a time and modifies the cross-section based on the user-specified trimming type. Inter Cross Section Slope Limits trims the top and/or bottom of cross-sections based on the adjacent cross-sections, their elevations, and the distance between the cross-sections.

It is important to understand “shear core” before using cross section trimming. RAM Concept defines the shear core as the parts of the trimmed cross section that include any vertical slices that extend from the top of the cross section to the bottom of the cross section. The one-way shear design calculations are based on the total shear force in the strip and the shear core. The nominal shear strength provided by the concrete (Vc) is calculated using only the area of the shear core; the rest of the cross-section is ignored for shear resistance.

Viewing the Latitude Cross Sections Perspective Plan and the Longitude Cross Sections Perspective Plan on the Design Strip Layer is a useful way of checking the validity of the trimmed cross section. Portions of the cross-section are displayed as follows:

  1. Areas highlighted in light blue represent the shear core
  2. Transparent areas represent sections included in the cross section but not in the shear core. These areas are included in the flexural designs, including Code Minimum design, but not in the one-way shear designs.
  3. Areas in black represent areas that are not part of a cross section and are ignored in both flexural and one-way shear designs.

Single Cross Section Trimming

There are 7 different options for single cross-section trimming: max rectangle, slab rectangle, beam rectangle, T or L, Inverted T or L, Max Shear, and None. The example below illustrates each of the different options. Note that the slab in the example contains both up-turned and down-turned beams and a slab step (see Figure 1).

Figure 1. Slab Section for Cross-Section Trimming Example

Max Rectangle

Max rectangle trims the top and bottom of the cross section and removes other pieces to produce a trimmed cross section with a uniform top and bottom elevation and a maximum area. The “rectangle” formed can be multiple separated rectangles with the same top and bottom elevations. See Figure 2.

Figure 2. Max Rectangle Trimming

Slab Rectangle

Slab rectangle trims the top and bottom to produce a trimmed cross section with a uniform top and bottom elevation and a maximum width. If multiple maximum-width rectangles are possible, the rectangle with the greatest area is used. See Figure 3.

Figure 3. Slab Rectangle Trimming

Beam Rectangle

Beam rectangle removes vertical slices of the cross-section so that the trimmed section is the maximum height rectangle possible. The “rectangle” formed can be multiple separated rectangles with the same top and bottom elevations. See Figure 4.

Figure 4. Beam Rectangle Trimming

T or L

T or L trims the top and bottom of the cross-section to produce a trimmed cross-section with a uniform top elevation and only two bottom elevations (flange bottom and web bottom). The trimmed section can include separated or joined rectangles. See Figure 5. Note that the webs of the down-turned beams are included in the shear core, and the upper slab is included in the flexural cross-section but is not included in the shear core.

Figure 5. T or L Trimming

Inverted T or L

Inverted T or L trims the top and bottom of the cross-section to produce a trimmed cross-section with a uniform bottom elevation and only two top elevations (flange top and web top). The trimmed section can include separated or joined rectangles. See Figure 6. Note that the webs of the down-turned beams are included in the shear core, and the bottom slab is included in the flexural cross-section but is not included in the shear core.

Figure 6. Inverted T or L Trimming

Max Shear Core

Max shear core trims the top and/or bottom of the cross-section to produce a trimmed cross-section with the maximum shear core area. See Figure 7. Note that the webs of the down-turned beams are included in the shear core, and both top and bottom slabs are included in the flexural cross-section but are not included in the shear core.

Figure 7. Max Shear Core Trimming

None

No single cross-section trimming is performed. In the example, the entire slab is included in the cross-section used for flexure design, but no portion of the slab is included in the shear core. See Figure 8.

Figure 8. None Cross Section Trimming

Inter Cross Section Slope Limit

Inter cross section trimming reduces design strip cross sections based on slope limits. This effectively trims the top and bottom elevations of adjacent cross section to limit the slopes between them. This is done because compression and tension forces cannot “flow” at sharp angles from one cross-section to the next. Inter cross section trimming will affect cross-sections in design strips that extend through intersecting beams, slab steps, and thickened slab areas like drop caps panels.

Refer to Figure 9. It would be unrealistic to use a design depth of t2 at cross-section A-A. If an inter cross trimming slope limit of 0.25 is defined, a portion of the thickened slab is trimmed using a 1:4 slope.

Figure 9. Inter Cross Section Slope Trimming

Engineering judgment should be used when deciding the slope limit to use in any model. A slope limit of 0.0 will not allow any change between adjacent cross sections’ top elevations and bottom elevations and appropriate for a drop cap whereas by code its projection below the slab cannot be used for strength calculations other than for punching shear. This effectively trims all the cross sections in a span segment strip to have the same top and bottom elevation. When a very large number, say 1000, is entered for the slope limit, the inter cross section slope limit is basically ignored. The RAM Concept Manual recommends a maximum slope limit of 0.25. Many engineers use a maximum slope limit of 0.5.

Notes on Drop Caps and Drop Panels

Drop panels extend a distance L/6 in plan from the centerline of a support and project below the slab at least ¼ of the slab thickness. The thickened portions can be used for flexural, one-way shear, and punching shear calculations.

Drop caps are thickened slab areas over columns that do not comply with the dimensional requirements of drop panels. Drop caps may be used for punching shear calculations, but should be ignored in flexural and one-way shear calculations.

RAM Concept does not directly distinguish between drop panels and drop caps. If a thickened slab is a drop cap, then the extra thickness should be trimmed from the cross sections. This can be done using an inter cross-section slope limit of 0 or by using the "Slab Rectangle" trimming for design strips. For single design sections, set the bottom ignore depth to the extra depth of the cap.

Note, when span segments are modeled from column to column, a portion of the thickened region will be trimmed based on the inter cross section slope limit as noted above. Setting the slope limit to 0 completely trims off the extra depth. A large slope limit allows for sudden increases in the thickness.

Design strips including drop panels should include a cross section at the face of the drop panel regardless of section spacing because design forces at this location could be critical. Additional cross-sections will be placed at this location by the program automatically.

Regardless of the design section properties, the extra thickness of drop caps and drop panels is considered in punching shear design.

Common Errors and Warnings

   

 

Figure 10. Out of Cross Section Errors

The errors in Figure 10 are associated with user reinforcement and post-tension tendons that intersect the cross section in an area that was trimmed by single cross-section trimming or the inter cross section slope limit. To fix the problem, first verify the cross section perspective plans and review the cross section trimming. It may be helpful to display reinforcement or tendons on this plan using the Visible Objects dialog to visualize the error. Reinforcement can be displayed from the Reinf. tab in the Visible Objects dialog; tendons can be displayed from the tendons tab in the Visible Objects dialog. After finding the problem reinforcement or tendons, revise the cross section trimming settings. In some cases, you may need to revise design strips in plan or move reinforcement and/or tendons.

No concrete remaining at one or more locations

If the trimming methods described above cause the entire cross section to be trimmed away so that there is nothing left for the program to design, the following error will occur:

This tends to happen when the strip crosses a step and the Slope limit prevents and concrete depth from being counted near the transition.

Simplifying the strip layout to avoid steps, or modifying the trimming settings is the solution to the problem.

More Information

Please see RAM Concept Design Strips for additional help links.