Looking to contribute to this community? You must be signed in
Not a member? Register
Problems signing in? Check here
The American codes do not have explicit material factors. Instead, they use "strength reduction factors". These strength reduction factors account for unavoidable variations in material strength, design equations, fabrication and erection. For example, in the American steel code LRFD 2001, these factors are : 0.90 for limit states involving yielding 0.75 for limit states involving rupture 0.85 for limit states involving compression buckling For the American concrete code ACI 318-02, some of the values used are Tension-controlled sections - 0.9 Compression controlled sections, members with spiral reinforcement - 0.7 Shear and Torsion - 0.75 Bearing on concrete - 0.65 etc. These are requirements placed by the code. So, we do not have parameters for altering these.
STAAD performs concrete design for shear and torsion at locations defined by
(d + SFACE) from the start of the member
(d+EFACE) from the end of the member
respectively. The basis for this assumption can be found in Section 220.127.116.11 of ACI 318-99.
If these locations are beyond the mid-point of the member, that triggers the error message you encountered. In case you are not familiar with the parameters SFACE and EFACE, you will see in Chapter 3 of the Technical Reference Manual in Table 3.1 that these are values which the user may specify to convey to STAAD how far the face of the member is from the nodal point of the member. The default value for SFACE and EFACE is 0.0. "d" is the effective depth of the member.
So, this is what you can do. You can set the values for SFACE and EFACE to be negative quantities equal in magnitude to "d". That will result in (d+SFACE) and (d+EFACE) becoming zero, which means that the design will be performed at the nodal points of the member, thereby avoiding the situation of the design point being beyond the mid-point of the member.
So, in your input file, under the START CONCRETE DESIGN command, specify these parameters along the following lines:
START CONCRETE DESIGNCODE ACISFACE -d MEMB 110EFACE -d MEMB 110DESIGN BEAM 110END CONCRETE DESIGN
where "d" is the effective depth of the member.
The longitudinal reinforcement in the column must be extended into the footing so that the forces and moments at the base of the column can be properly transferred into the footing. However, since the construction sequence requires the footings to be constructed before the columns, reinforcement is placed in the footing and extends upwards. So when the column is constructed, it becomes part of the column bars. This reinforcement which comes up from the footing into the column is called the dowel reinforcement.
The definition of the word critical in the shear design output in not on the basis of which among the various load cases has a larger amount of shear force, but which one requires the largest amount of stirrup reinforcement.
To answer your question, in all likelihood, you will see this happen when both load cases require the same amount of stirrup steel.
Design is carried out for all the load cases. The steel area values for all the cases are then sorted in the ascending order from low to high. If more than one case ends up requiring that highest steel area value (same area required for multiple load cases), the first among those load cases is reported as critical.
Another possibility is that torsion in the load case reported as critical may be higher than the one which has the highest shear force. Stirrups are designed for shear and torsion, not just shear.
If you open the file in the STAAD editor (go to the Edit menu, and choose Edit Input Command File), and go to the end of the file, you will observe the following :
CLB 0.25 MEMB 1 TO 481DESIGN ELEMENT 1 TO 456 458 TO 481DESIGN COLUMN 457TRACK 2 MEMB 457END CONCRETE DESIGNFINISH
The TRACK command has to be specified before the DESIGN commands. In others words, the order of these commands must be the following :
CLB 0.25 MEMB 1 TO 481TRACK 2 MEMB 457DESIGN ELEMENT 1 TO 456 458 TO 481DESIGN COLUMN 457END CONCRETE DESIGNFINISH
If you make this change, you will get the interaction diagram.
STAAD performs concrete design for shear and torsion at locations defined by (d + SFACE) from the start of the member and (d+EFACE) from the end of the member respectively. In case you are not familiar with the parameters SFACE and EFACE, you will see in Chapter 3 of the STAAD.Pro Technical Reference Manual in Table 3.1 that these are values which the user may specify to convey to STAAD how far the face of the member is from the nodes of the member. The default value for SFACE and EFACE is 0.0. "d" is the effective depth of the member. The basis for this assumption can be found in Section 18.104.22.168 of ACI 318-95.
If you want the shear & torsion design to be performed using the member end forces (the nodal values) and not those at the location mentioned in the previous paragraph, you can set the values for SFACE and EFACE to be negative quantities equal in magnitude to "d". That will result in (d+SFACE) and (d+EFACE) becoming zero, which means that the design will be performed at the nodal points of the member.
So, in your input file, under the START CONCRETE DESIGN command, specify these parameters along the following lines :
The design of an element involves determination of the reinforcement for moments Mx and My at the centroid of the element. The reinforcement calculated to resist Mx is called longitudinal reinforcement, and is denoted in the output by the expression "LONG. REINF.".
The reinforcement calculated to resist My is called transverse reinforcement, and is denoted in the output by the expression "TRANS. REINF.".
The sign of Mx and My will determine which face of the element the steel has to be provided on. Every element has a "top" face, and a "bottom" face, as defined by the direction of the local Z axis of the elements. Mx will cause tension on one of those faces, and compression on the other. A similar effect will be caused by My. The output report of reinforcement provided on those faces contains the terms "TOP" for top face, and "BOTT" for the bottom face.
The procedure used by the program to arrive at these quantities is as follows :
For each element, the program first scans through all the active load cases, to find the following maxima :
Maximum positive MxMaximum negative MxMaximum positive MyMaximum negative My
The element is then designed for all those four quantities. If any of these moments happen to be zero, or if the reinforcement required to resist that moment is less than the capacity of the element with minimum reinforcement, only minimum reinforcement is provided. For the ACI code, the rules governing provision of reinforcement for shrinkage and temperature are used in calculating minimum reinforcement.
The rules applicable for design of a beam for flexure are used in calculating the steel areas. The width used in this calculation is a unit width of the element. For determination of the effective depth, the steel for longitudinal moment is assumed to be the outer layer, and the steel for transverse moment is the inner layer.
The output will consist of the steel area required for all of four maximas. As described earlier, they will be reported using the terms LONG, TRANSVERSE, TOP and BOTT.
When you ask for an element design or a slab design using the commands
DESIGN SLAB ..
STAAD designs the element for the moments MX and MY at the centroid of the element. By definition, MX and MY are termed as Moments per Unit width, since that is what they are. They have units of Force-length/length, as in 43.5 KN-mm/mm, or 43.5 KN-m/m. In other words, if you take a one metre width of the slab at the centroid of the element in question, the moment over that one metre width on that element is equal to 43.5 KN-m.
The design of that element hence has to be done on the basis of a unit width. Thus, in order to design an element for a 43.5 KN-m/m moment, one needs to use a one metre width of slab. The reinforcement required for that element is thus reported in terms of unit width of the element. The results are hence in the form Area of steel/unit-width of element, as in, "SQ.MM/MM".
The reinforcement report for many of those elements looks like the following:
Solution: In the above output, the word TOP and BOTTOM refer to the "local" top and bottom surfaces of the individual elements, and not in the global axis sense. The local top and bottom surfaces depend on the way an element is defined in its incidence statement. TOP is defined as the surface which coincides with the positive side of the local Z axis. BOTTOM is defined as the surface which coincides with the negative side of the local Z axis. Shown below are two examples in which the element incidence is numbered in two contrasting ways. In the first figure, the local Z axis of the element points in the vertically upward direction. Consequently, the local top and bottom surfaces have the same sense as the global top and bottom.
In the next figure, the local Z axis of the element points in the vertically downward direction. Consequently, the local top and bottom surfaces have the opposite sense as the global top and bottom.
You can verify the direction of the local axes of the elements in your model by doing the following. Click the right mouse button and select Labels. Under the Plate category, switch on Plate Orientation. The local axes will be displayed as shown in these figures above.
You can do the following to compute the capacity of the concrete section:
Model the strucuture.Specify the existing profile to the member propertiesSpecify all the required member specification and Support conditionSpecify the load on the strucutreSpecify the Concrete design parametersSpecify the parameter MinMain and Maxmain to the provided bar size Do the designCheck the results.Adjust the load and redo the design until the reinforcement matches with the provided steel.
The answer is unfortunately no. You can only specify if it is a Tied column or a Spirally Reinforced column.
You do not have to input any special request. As long as the section can be designed as a singly reinforced section (reinforcement in the tension zone only), STAAD will try to fit the bars in upto 2 layers. For each layer, the distance from the bottom of the section is reported. The number of bars required for each layer too is reported. It reports a failure only if more than 2 layers are required.
This means that though the program is able to come up with the value of area of steel required, it is unable to comeup with a bar arrangement which will satisfy the area requirement. Usually, this is because either because the MINMAIN and MAXMAIN limits might be too restrictive, or because the resulting bar spacing violates the minimum spacing requirements of the code.
Here is an explanation on the various design output items reported as part of the analysis output filecorresponding to the DESIGN ELEMENT command.
LONG REINF – Reinforcement required along the longitudinal direction ( along local X axis of the plates ). This reinforcement is reported in terms of area required per unit width of slab
TRANS REINF - Reinforcement required along the transverse direction ( along local Y axis of the plates ). This reinforcement is reported in terms of area required per unit width of slab
MOM-X – Longitudinal moment, corresponding to which LONG. REINF is calculated. This is reported per unitwidth of slab.
MOM -Y - Transverse moment, corresponding to which TRANS. REINF is calculated. This is reported per unitwidth of slab.
LOAD – Critical load case for each moment
FY – Yield stress of reinforcing steel
FC – Compressive strength of concrete
Cover ( TOP) – Top cover for reinforcement. The surface in the direction of the positive local Z axis of theplate is considered as top.
Cover ( BOTOM ) - Bottom cover for reinforcement. The surface in the direction of the negative local Zaxis of the plate is considered as bottom.
TH – Thickness of the slab
TOP : Longitudinal direction – Only minimum steel required - means that only the minimum amount of reinforcementas prescribed by the code is good enough for the top surface along the longitudinal direction. All such faces/directions for which minimum steel can be provided,is listed one after another. In the above example, minimum reinforcement is ok for top face in the longitudinal direction and both top and bottom faces in the transverse direction. Only the bottom face in the longitudinal direction needs more than the minimum steel.
The required reinforcements are reported next.
The first 0.54 mm2/mm corresponds to Longitudinal steel at top surface ( comes from minimum reinforcementcriteria )
The 0.00/ 0 data corresponds to ( MOM-X/LOAD ) and indicates that there is zero moment in the longitudinal ( X direction ) at the top face and so no load case is listed as critical.
Next 0.54 mm2/mm corresponds to Transverse steel at top surface ( again comes from minimum reinforcementcriteria )
0.48 /1 indicates that Moment Y ( transverse moment ) for the top surface is 0.48 KN-MM/MM and thecorresponding critical load is load case 1.
Similarly one can interpret the reinforcements for the bottom surface.
These are all required reinforcement areas and based on these one should decide on a suitable bar arrangement ( #size bars @ xx spacing ). The software does not suggest bar arrangements.
Yes it does convert the nominal strengths to design strengths and checks the factored column loads/moments against those values. It designs against combined axial load and biaxial bending. The Phi factor for column design is based on compression controlled section and on the type of transverse reinforcement being used.
As far as X and Y direction are concerned, these are displayed on the slab itself when you go to the Concrete slab > Summary page within RC Designer. You can always change the direction of X and Y, if you want to, by going to the menu option Slab > Slab Axes. The resulting dialog box would provide option to define the X direction either by selecting two existing slab nodes or by selecting a component plate, in which case the X is aligned along the local X of the selected plate. Once you make the change, the graphics may not refresh. To refresh it, right click on the graphics window, choose Diagrams > General > uncheck the box for Design Axis> click Apply which should remove the axis plot. Now recheck the Design axis check box and click Apply. You will now see the modified orientation of X and Y. Outer bar simply denotes the bars which are closest to the top and bottom faces of the slab. Once the X and Y are established, you can use the right hand rule to find positive Z. The face in the direction of the positive Z would be the top face of the slab just like in STAAD.Pro. The opposite face would be considered the bottom face.
So if you have Outer bar direction set to “X” under Top Bar Criteria, it means that for the top face of the slab, the bars which are placed nearest to the face, are oriented along the X direction. The bars in Y would be inside the X direction bars.
STAAD.pro designs the columns for all load cases. The reporting of critical case is not based on the highest member forces but it rather based on the highest reinforcement requirement. The load case that requires the Maximum Reinforcement is reported as the critical case.
Now , if it so happens that the reinforcement requirement is minimum for all the load cases, then only the first load case gets reported as the worst case. The reason being, no other load case that follows the first load case, requires higher reinforcement than what is required by load case 1. That is what is happening in your case.
To verify this, use separate load list & column design commands for a few load cases which you feel may be critical & check the reinforcement. You will find that the reinforcement requirements for all of these subsequent load cases are either the same as that required for load case 1 or are lesser than that required for load case 1.
Product TechNotes and FAQs
Structural Product TechNotes And FAQs
Bentley Technical Support KnowledgeBase
Bentley LEARN Server
Bentley's Technical Support Group requests that you please confine any comments you have on this Wiki entry to this "Comments or Corrections?" section. THANK YOU!
The procedure advised to compute the capacity of an existing concrete member is quite lengthy and boring. Please advise some quicker method.