Solutions Discussed: Analysis of concrete buildings

Products Discussed: RAM Structural System V8i, SELECTseries 6 (r14.06.01.00)

If you are using RAM Structural System to model a building that has a transfer slab, it is critical that the structure is modeled with due consideration for the analytical assumptions the software makes. This is necessary to ensure the vertical load path is complete, and all gravity loads are getting accounted for in the structure. Transfer slab in the context discussed here is a level at which a column or wall bears on a beam or bare slab, directly below which there is no column or wall. In this post, some of the more common transfer scenarios are discussed, how these situations should be handled, and any program limitations that come into play.

 

Case 1 - Flat slab structure with all beams, columns, and walls set as "Lateral" members.

This is the most straight forward case in RAM in terms of understanding the assumptions made by the software, the analytical model, and the resulting load path. In this approach, all elements of the structural model are built into the finite element analysis for all loading conditions. If there are any doubts as to whether other methods of modeling the building (the other cases discussed in this article) are accurately capturing the behavior of the system, this is the method to fall back onto. The possible disadvantage of this approach is that it does not capitalize on RAM's ability to separate the lateral-resisting framing from the gravity-only framing in the analysis. However, if this is not a concern, or if it is acceptable to account for this in other ways (such as through member fixities), this is a solid approach.

In this method all members and slabs are built into the finite element model and are considered effective for all load cases. Since this is a flat slab building, all decks are modeled as "two-way" rather than "one-way". Two-way decks are analyzed as shell elements in RAM Structural System, and so the gravity load applied to the floors is distributed to supporting members via shell elements. Columns and walls that stop on (are supported by) a two-way slab are meshed to the slab and the load is transferred via the finite element model connectivity. This is true of both gravity loads, as well as any overturning or vertical forces resulting from lateral loads.

Beams that lie within two-way slabs are meshed together with the shell elements composing the slab, with deformation compatibility between the beam and slab. As a result, if a column or wall bears on a beam rather than on bare slab, the load is not supported only by the beam, but rather shared between the beam and surrounding slab.

In RAM Frame, the choice between a Rigid diaphragm or Semi-rigid diaphragm impacts only the in-plane (membrane) stiffness of the diaphragm. In both cases, the flexural (plate) response of the diaphragm is considered finite and is a function of the slab concrete properties and flexural crack factor.

It would seem there is no need for Concrete Analysis with this approach since both gravity and lateral forces have been determined in RAM Frame for all members. There are a few considerations here however:

1. In order to design concrete beams and columns in RAM Concrete, Concrete Analysis must be run even if not using forces from this module. This is because certain design preparatory tasks are done during the analysis in Concrete Analysis.
2. Concrete Analysis performs the slenderness calculations and moment magnification prescribed in concrete design codes, whereas RAM Frame does not. RAM Frame considers only P-Delta (large delta) effects.
3. Concrete Analysis considers the patterning of live loads, whereas RAM Frame does not.

When designing beams and columns, users have a choice of using the gravity forces from RAM Frame or Concrete Analysis. This option is located under Criteria -> Beam (or Column) Design -> Design Checks/Forces. The wall design in RAM Concrete always uses the forces from RAM Frame, and there is no option to use Concrete Analysis forces.

 

Case 2 - Flat slab structure with both "Gravity" and "Lateral" beams, columns, and walls.

Now let's consider the same scenario described in case 1, except that rather than all members assigned as "Lateral", the model contains both "Gravity" and "Lateral" members.

Note: In RAM Frame, under Criteria -> Diaphragm -> Gravity Members Supporting Two-Way Decks, the setting Include Gravity Members should be used. This is important in ensuring that transfer loads are properly carried down the structure.

In this case, RAM Frame creates two different analysis models: one for which the gravity loads are analyzed (dead, live, roof, and snow), and one for which the lateral loads are analyzed (wind, seismic, and dynamic). The analysis conducted for the gravity loads is identical to the analysis conducted in case 1: all members are considered in the analysis. The analysis for the lateral loads excludes the "Gravity" members. The results for load combinations (whether in RAM Frame's load combination mode, or in the combined loads used in the design modules) are a combination of the results for each analysis, depending on whether the load case in the combination is gravity or lateral.