by Josh Taylor
Solutions Discussed: Structural analysis and design of concrete-framed buildings
Products Discussed: RAM Structural System
Sometimes a single setting within an engineering program can mean the difference between a well constructed building model that provides accurate analysis and design output, and one that is essentially meaningless. In this article we cover ten settings in RAM Structural System that can have a drastic impact on how your model behaves both analytically and during the design process. Many of the items listed are most relevant to concrete-framed buildings. If you are working on or reviewing a RAM Structural System model, consider double-checking the following settings before moving to the next step.
1) Replacing Gravity Columns/Walls with Springs in RAM Frame
This item applies to two-way floor slabs assigned as semi-rigid diaphragms in RAM Frame. Remember that the RAM Frame analysis considers only elements assigned as lateral members, as well as slabs assigned as semi-rigid diaphragms. So absent any other input, the slab will be unsupported at the gravity columns for the RAM Frame analysis, and thus may produce unexpectedly large slab displacements under gravity forces.
To support the slab against global vertical translation where gravity columns or walls occur, select the option "Include Gravity Columns as Springs" and/or "Include Gravity Walls as Springs". These settings are found in RAM Frame under Criteria -> Diaphragm -> Diaphragm Stiffness.
Floor slab properly supported by springs at gravity columns
Floor slab unsupported at gravity columns
2) Effective Length Factor for Columns Supporting Flat Slabs
RAM Concrete Analysis offers two options in determining the effective length factor for a concrete column: determination by use of the nomograph, in which the program performs the calculation automatically using the members that frame to a given joint, or by manual user assignment. The program does not consider the floor slab when doing the calculation according to the nomograph. Thus, for columns which support a flat slab floor with no beams, it is very important that the user assign an effective length factor directly to the columns.
3) Crack Factors for Two-Way Floor Slabs
The flexural crack factor for a two-way floor slab is defaulted to 1.0 at creation. It is important that the engineer set this to a realistic value. For example, ACI 318-08, Section 10.10.4.1 specifies that a value of 0.25 is to be assumed.
4) Gravity Forces on Lateral Members
Both RAM Concrete Beam and RAM Concrete Column allow you to choose the source of the gravity design forces on lateral members: RAM Frame or RAM Concrete Analysis. A detailed description of the differences between each analysis can be found in the RAM Concrete Analysis technical manual. In short, the best option to choose will depend on the type of structural system under consideration.
For example, RAM Concrete Analysis specializes in the pattern loading of floors. Since pattern loading is important for joist and beam-framed floors with one-way slabs, for this type of system gravity forces on lateral members are best suited to come from RAM Concrete. RAM Frame has the advantage of capturing systematic behavior (that can't always be captured by summing analyses conducted on discrete levels). For structures in which gravity forces produce large moments in columns, or induce significant lateral sway in the structure, gravity design forces for lateral members should come from RAM Frame.
5) Self-Weight of the Structure
This is a very basic one, but the impacts of not correctly accounting for the self-weight of the structure are so great it is worth mentioning again here. The option to consider the self-weight of the structure automatically is set in RAM Manager under Criteria -> Self-Weight. The self weight of beams, columns, walls, and slabs all have a separate self-weight toggle.
The surface load properties that are created in RAM Modeler should then be created with due consideration for the self-weight setting. If all self-weight is automatically considered, then dead loads need only account for the superimposed portion. Note also that surface loads have a separate specification for mass dead load. If SIDL is assumed to contribute to the mass of the structure for lateral analysis, make sure a non-zero value is entered here.
6) Snow Loads versus Roof Live Loads
Within the Criteria -> Member Loads menu in RAM Manager there is an option to consider either roof live loads or snow loads during the gravity analysis of the structure. Currently it is not possible to consider both types of loading simultaneously. Later this year (2010) this limitation will be removed.
7) Load Combinations - Response Spectrum Loads
Response spectrum load cases are not automatically added into load combinations in RAM Structural System. Load combinations considering RSA load cases need to be created manually in the Custom Combinations dialog.
8) Consideration of Sign for Response Spectrum Analysis
The RAM Frame General Criteria dialog contains a setting to consider/ignore the sign of modal response quantities when conducting a response spectrum analysis. Ignoring the sign will produce more conservative (possibly overly conservative) design forces.
9) Detailing Defaults for Lateral Beams
RAM Concrete Beam allows the user to specify a default beam reinforcing detail for gravity beams, gravity joists, and lateral beams separately. When designing an Intermediate or Special Moment Frame, leaving the detail for lateral beams to its default settings will often produce failures after the design is run. Normally one or more settings need to be more stringent than what is provided by default, such as the minimum number of continuous top bars, number of stirrup legs, or the stirrup type (hoop rather than closed).
10) Section Cuts in RAM Concrete Shear Wall
Section cuts for wall design can be auto-generated, in which case the program will place sections at locations within walls likely to be critical. Bear in mind though that the more sections you create, the longer the design will take to run and the larger the file will become. Being selective in where sections are placed in the model will improve program performance and also make the design output easier to process.