The following FAQ’s have been added to the Floor Pro page.
When the program calculates L/deflection ratios, what length is used?
The L/deflection ratios in Floor Pro are a function of the spans of each support line and their lengths. For example, if you run a support line from one slab edge to the opposite edge without any intermediate spans (i.e. “clicks”), the value for L/deflection ratio would be calculated based on the entire length of the support line.
When designing a PT slab, can the entire bay width be used in determining moment-capacity for a T-section at negative moment regions (i.e. drop panel at a column location)?
This response is specific for ACI-318. The question could be asked if there is any code provision that prohibits the used of full design section width in determining the design capacity? We are unaware of any such provision.
Since the precompression introduced into the system due to the PT is deposited over the entire tributary cross-section, most engineers will base the design capacity on the same section, which is typically defined as the most logical load path along support lines. ACI-318 clearly excludes provisions found in Chapter 13 (two-way conventially-reinforced concrete slabs) in the design of prestressed structures, unless clearly stated. Furthermore, Section 18.12.1 clearly states that factored moments and shears in prestressed slab systems “shall be determined” in accordance with the Equivalent Frame Method (EFM). Our interpretation would be that this means the full tributary.
The same provision excludes sections 188.8.131.52 and 184.108.40.206 which calls for the total negative moment to be distributed to column and middle strips. Some may apply column/middle strip design to PT structures or some similar line of thinking with respect to distribution of moment, etc. This does not apply to prestressed slabs and is explicitally excluded in ACI-318 Section 18.
Does disregarding column/middle-strip methodology as defined by ACI-318 when using a finite element analysis violate the code? Even when using FEM analysis, should I still proportion reinforcement based on code percentages?
ACI-318 specifically states that “column strip/middle strip” distribution does not apply to post-tensioned floor systems.
When using the program for conventionally reinforced two-way slabs, the program gives the user the option to distribute the reinforcement based on “column strip/middle strip.” The program DOES NOT apply the code-prescribed percentages to each strip, but geometrically sub-divides design strips based on the code provisions and designs each strip according to the nodal integration method.
The intent of the ACI recommendation for breakdown of reinforcement between column strip and middle strip has been for improved crack control in service condition. For the safety of the structure (Ultimate Limit State) the distribution is immaterial, as long as the tributary of each support line is provided with the associated rebar.
In summary, for better crack and deflection control, it is advisable to place most of the reinforcement of a design strip near the column when designing a conventionally reinforced concrete, regardless of the method of analysis. It is our opinion that this is the intent of ACI-318, although it is not explicitly stated in the code for designs based on FEM analysis. Since, using the reinforcement disposition, specified by a user, ADAPT-Floor Pro can calculate the cracked deflection of a conventionally reinforced two-way floor system, one can logically argue that the ACI’s column strip/middle strip disposition need not be adhered to, if cracked deflections meet the code’s deflection limit.
When calculating shear capacity of two-way prestressed slabs, does the program include the average precompression, fpc, as shown in ACI318-05/08 Equation 11-36?
Yes, however, the program conservatively assumes the minimum value of 125 psi regardless of the actual precompression in the slab. If a single tendon is included in a model, the program will apply Equation 11-36 for each column, regardless of its location with respect to the tendon location.
When a slab is overlaid on another slab region, does the program count the self-weight twice?
No, the program calculates the self-weight according to the true, three-dimensional representation of the model as input by the user.
How do I move a drop cap or drop panel such so that it is not centered on the column?
If you click on a drop and then select the handle (X in center) you can move it to any location away from center of column. This may require that you include some construction lines. The easiest approach is to highlight the component and then use the Copy/Move toolbar where you can enter the distance in X and Y directions.
If I modify the mesh after designing rebar do I have to run the analysis and rebar design again?
Anytime a change is made to the mesh, we recommend the analysis and design is rerun.