Document Information

Document Type: FAQ  

Product(s):  STAAD.Pro

Version(s):  All

Original Author: Bentley Technical Support Group

If using an American code for code check, is there any parameter to define the material factor or is it already included?

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.

I am performing concrete design for a beam per the ACI code and I encounter an error message : "LOCATION FOR DESIGN FOR SHEAR AT START OF MEMBER 2 IS BEYOND THE MIDPOINT OF MEMBER. DESIGN FOR SHEAR AND TORSION NOT PERFORMED." How can I get around this situation?

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. The basis for this assumption can be found in Section 11.1.3.1 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 DESIGN
CODE ACI
SFACE -d MEMB 110
EFACE -d MEMB 110
DESIGN BEAM 110
END CONCRETE DESIGN

where "d" is the effective depth of the member.

I am doing a footing design in STAAD.Pro 2002. I am unfamiliar with the term "dowel reinforcement". I am guessing that this is a term used by American engineers. Could you explain what that is?

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. 

 My input file contains 2 load cases - case 1 and 2. For member 43, case 2 produces a larger value of shear force along local Y axis than case 1. However, the concrete design report indicates case 1 as being critical for shear design, and not case 2. How do you explain this?

 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.

Why is it that the concrete column interaction diagram is not plotted in the output although track 2 was specified?

 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 481
DESIGN ELEMENT 1 TO 456 458 TO 481
DESIGN COLUMN 457
TRACK 2 MEMB 457
END CONCRETE DESIGN
FINISH

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 481
TRACK 2 MEMB 457
DESIGN ELEMENT 1 TO 456 458 TO 481
DESIGN COLUMN 457
END CONCRETE DESIGN
FINISH

If you make this change, you will get the interaction diagram.

 I am performing concrete design for a beam per the ACI code. At the start as well as the end nodes of the member, the value "Vu" which is reported in the shear design output does not match the shear force Fy from the member end force output. Why is that?

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 11.1.3.1 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 :

START CONCRETE DESIGN
CODE ACI
SFACE -d MEMB 110
EFACE -d MEMB 110
DESIGN BEAM 110
END CONCRETE DESIGN

where "d" is the effective depth of the member.

When I perform concrete design on an element, the output contains expressions such as "LONG. REINF.", "TRANS. REINF.", "TOP", "BOTT.", etc. Can you explain what these terms mean?

 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 Mx
Maximum negative Mx
Maximum positive My
Maximum 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 I perform concrete design on an element, the output reports reinforcement in terms of "SQ.MM/MM". Can you please explain why?

When you ask for an element design or a slab design using the commands

DESIGN ELEMENT ..

or

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".

A floor slab has been modeled using 4-noded plate elements. The elements are subjected to pressure loading in the vertically downward direction. A concrete design has been performed on the elements. (See below for the reinforcement report for many of those elements.)
Why is it that the moments as well as reinforcement are appearing on the top and not on the bottom of the plates?

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.

For an existing concrete member, I need to compute the capacity of the section. How do I do this?  

You can do the following to compute the capacity of the concrete section:

Model the strucuture.
Specify the existing profile to the member properties
Specify all the required member specification and Support condition
Specify the load on the strucutre
Specify the Concrete design parameters
Specify the parameter MinMain and Maxmain to the provided bar size
Do the design
Check the results.
Adjust the load and redo the design until the reinforcement matches with the provided steel.

Can I change the strength reduction factors in the program? For example: For a tied concrete column, I assume that the current value is 0.70. Can it be changed to 0.65?

 The answer is unfortunately no. You can only specify if it is a Tied column or a Spirally Reinforced column. 

 In concrete design per the ACI code, if the size of the concrete beam
member which I am designing is limited and I need to have 2 rows of reinforcement in the top or the bottom of the beam, how do I input this request? Or Does Staad automatically output the data with the second row? have been trying to find this in the Manuals. I have seen LEVELS BUT IT DOES NOT SAY WHAT I NEED.

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.

In concrete design per the ACI code, what does the following expression in the STAAD output file mean: BAR SIZE CAN NOT BE MATCHED TO MEET ALL REQUIREMENTS

 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.

See Also

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