RISAFloor implements AISC's vibration calculations and automatically calculates floor framing accelerations and frequencies as a standard part of every design. This method is based on walking resonance and is described in detail in the AISC publication, Design Guide 11: Floor Vibrations Due to Human Activity. For floor with steel joist framing, RISAFloor follows SJI Technical Digest 5: Vibration of Steel Joist - Concrete Slab Floors guidelines on calculating floor accelerations and frequencies.
For additional advice on this topic, please see the RISA Tips & Tricks webpage at risa.com/post/support. Type in Search keywords: Vibrations.
The Vibration Results Spreadsheet displays the results for each beam that has been checked. The pull down list at the top of the spreadsheet allows you to toggle between floors.
The Vibration code and the damping ratio are set on the Codes tab of the Model Settings. If you do not want any vibration checks to be performed you may specify “none” as the vibration code.
The Beam Label column displays the beam label.
Two Moments of Inertia are reported (Ij and Ig). Ij is the moment of the inertia for the beam being considered. Ig is the moment of inertia of the supporting beam if there is one. If there are two supporting beams, Ig will be based on the more flexible of the two supporting beams per section 4.3 of the Design Guide.
Note
The midspan deflections (Deltaj and Deltag) are calculated from the supported vibration loads. Where Deltaj is the deflection for the beam being considered and Deltag is the deflection of the most flexible supporting girder if there is one.
The natural frequencies of the beam and girder (fj and fg) are based on their midspan deflections. They are used to calculate the panel frequency (fn). Refer to the AISC Design Guide for more information.
The Equivalent Panel Zone Weight (W) is calculated from the effective beam panel weights (Wj and Wg). The load used to calculate W, Wj and Wg is a combination of member self weight, deck self weight and the VL load category.
Note
Vibration calculations are performed for steel joist framing floor per SJI Technical Digest 5: Vibration of Steel Joist - Concrete Slab Floors.
Note
The program will automatically include the self weight of all members and deck when calculating the vibration loads. However, the portion of the Super Imposed Dead and Live Loads that should be included in the vibration loads must be specified by the user. This is accomplished by assigning the load to the "VL" Load Case. Therefore, the total vibration load will be the summation of the self weight loads and the VL loads.
The total vibration load is converted into an equivalent uniformly distributed load over the length of the member. The total load is assumed to be distributed equally over the length of the member as a uniform load as describe in section 3.3 of the AISC Design Guide.
The loads are then used to calculate the beam deflections (Delta j and Delta g). The only exception to this is when a girder supports a single point load at the mid-span of the member. When this occurs, the program uses a deflection equal to 4/pi times the regular equivalent load deflection as described in section 3.3 of the AISC Design Guide.
Note
AISC Design Guide #11 and SJI Technical Digest 5 only addresses vibrations on floors with composite deck or concrete slab. Therefore the program only provides vibration check when the Deck Type is selected as Composite Deck. Under Composite Deck type, one can select a specific composite deck system or concrete deck from the Database / Manufacturer drop-down list.
The floor stiffness requirements recommended for panels that have a natural frequency greater than 9 or 10 Hz are not checked within RISAFloor.
The excitation force Po that is used to calculate the walking excitation is always assumed to be 65 lbs. Per Table 4.1 of the design guide, this is an appropriate value for offices, residences, churches, and shopping malls. However, it may not be appropriate for footbridges.
Equation 3.5 of the design guide includes a deflection contribution for columns that applies for tall buildings. This is not currently accounted for by the program.
Section 3.4 of the design guide is not taken into account for continuous beams.
AISC Design Guide #11 does not have specific guidelines for cantilevered beams and outriggers, so currently there is no special consideration for these members by the program.
The program does not limit the Bg (effective width) calculation of equation 4.3b to 2/3 of the floor length.