Document Type: FAQ
Original Author: Bentley Technical Support Group
Document Type: FAQ
Original Author: Bentley Technical Support Group
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 18.104.22.168 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 22.214.171.124 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.
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 ELEMENT ..
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:
134 TOP :
1474.13 / 12
1679.58 / 12
0.00 / 0
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.
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The procedure advised to compute the capacity of an existing concrete member is quite lengthy and boring. Please advise some quicker method.