staad concrete design

14. 05. 2013. STAAD.Pro Concrete Design [FAQ] - Structural Analysis and Design Wiki - Structural Analysis and Design - Be Communities by Bentley

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STAAD.Pro Concrete Design [FAQ]

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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 anyparameter to define the material factor or is it alreadyincluded?

The American codes do not have explicit material factors. Instead, they use "strength reduction factors". Thesestrength reduction factors account for unavoidable variations in material strength, design equations, fabricationand erection. For example, in the American steel code LRFD 2001, these factors are : 0.90 for limit statesinvolving yielding 0.75 for limit states involving rupture 0.85 for limit states involving compression buckling For theAmerican concrete code ACI 318-02, some of the values used are Tension-controlled sections - 0.9Compression controlled sections, members with spiral reinforcement - 0.7 Shear and Torsion - 0.75 Bearing onconcrete - 0.65 etc. These are requirements placed by the code. So, we do not have parameters for alteringthese.

I am performing concrete design for a beam per the ACIcode and I encounter an error message : "LOCATION FORDESIGN FOR SHEAR AT START OF MEMBER 2 ISBEYOND THE MIDPOINT OF MEMBER. DESIGN FORSHEAR AND TORSION NOT PERFORMED." How can I getaround this situation?

STAAD performs concrete design for shear and torsion at locations defined by

(d + SFACE) from the start of the member

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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. Incase you are not familiar with the parameters SFACE and EFACE, you will see in Chapter 3 of the TechnicalReference Manual in Table 3.1 that these are values which the user may specify to convey to STAAD how far theface of the member is from the nodal point of the member. The default value for SFACE and EFACE is 0.0. "d" isthe 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 inmagnitude to "d". That will result in (d+SFACE) and (d+EFACE) becoming zero, which means that the designwill be performed at the nodal points of the member, thereby avoiding the situation of the design point beingbeyond the mid-point of the member.

So, in your input file, under the START CONCRETE DESIGN command, specify these parameters along thefollowing lines :

START CONCRETE DESIGNCODE ACISFACE -d MEMB 110EFACE -d MEMB 110DESIGN BEAM 110END CONCRETE DESIGN

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

I am doing a footing design in STAAD.Pro 2002. I amunfamiliar with the term "dowel reinforcement". I amguessing 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 momentsat the base of the column can be properly transferred into the footing. However, since the construction sequencerequires the footings to be constructed before the columns, reinforcement is placed in the footing and extendsupwards. So when the column is constructed, it becomes part of the column bars. This reinforcement whichcomes up from the footing into the column is called the dowel reinforcement.

My input file contains 2 load cases - case 1 and 2. Formember 43, case 2 produces a larger value of shear forcealong local Y axis than case 1. However, the concretedesign report indicates case 1 as being critical for sheardesign, 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 loadcases 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 sameamount of stirrup steel.

Design is carried out for all the load cases. The steel area values for all the cases are then sorted in theascending 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 thehighest shear force. Stirrups are designed for shear and torsion, not just shear.

Why is it that the concrete column interaction diagram is notplotted 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 tothe 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 thesecommands 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.

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

STAAD performs concrete design for shear and torsion at locations defined by (d + SFACE) from the start of themember and (d+EFACE) from the end of the member respectively. In case you are not familiar with theparameters SFACE and EFACE, you will see in Chapter 3 of the STAAD.Pro Technical Reference Manual inTable 3.1 that these are values which the user may specify to convey to STAAD how far the face of the memberis from the nodes of the member. The default value for SFACE and EFACE is 0.0. "d" is the effective depth of themember. 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 notthose at the location mentioned in the previous paragraph, you can set the values for SFACE and EFACE to benegative 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 thefollowing lines :

START CONCRETE DESIGNCODE ACISFACE -d MEMB 110



EFACE -d MEMB 110DESIGN BEAM 110

END CONCRETE DESIGN

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

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

The design of an element involves determination of the reinforcement for moments Mx and My at the centroid ofthe element. The reinforcement calculated to resist Mx is called longitudinal reinforcement, and is denoted in theoutput by the expression "LONG. REINF.".

The reinforcement calculated to resist My is called transverse reinforcement, and is denoted in the output by theexpression "TRANS. REINF.".

The sign of Mx and My will determine which face of the element the steel has to be provided on. Every elementhas a "top" face, and a "bottom" face, as defined by the direction of the local Z axis of the elements. Mx willcause tension on one of those faces, and compression on the other. A similar effect will be caused by My. Theoutput report of reinforcement provided on those faces contains the terms "TOP" for top face, and "BOTT" for thebottom 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 thereinforcement required to resist that moment is less than the capacity of the element with minimumreinforcement, only minimum reinforcement is provided. For the ACI code, the rules governing provision ofreinforcement 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 inthis calculation is a unit width of the element. For determination of the effective depth, the steel for longitudinalmoment 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 bereported using the terms LONG, TRANSVERSE, TOP and BOTT.

When I perform concrete design on an element, the outputreports reinforcement in terms of "SQ.MM/MM". Can youplease 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 andMY 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 centroidof 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 anelement for a 43.5 KN-m/m moment, one needs to use a one metre width of slab. The reinforcement required forthat element is thus reported in terms of unit width of the element. The results are hence in the form Area ofsteel/unit-width of element, as in, "SQ.MM/MM".

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

The reinforcement report for many of those elements looks like the following:

ELEMENTLONG. REINF

(SQ.IN/FT)MOM-X /LOAD

(K-FT/FT)TRANS. REINF

(SQ.IN/FT)MOM-Y /LOAD

(K-FT/FT)

134 TOP : 5.944 1474.13 / 12 6.914 1679.58 / 12

BOTT: 1.296 0.00 / 0 1.296 0.00 / 0

Solution: In the above output, the word TOP and BOTTOM refer to the "local" top and bottom surfaces of theindividual elements, and not in the global axis sense. The local top and bottom surfaces depend on the way anelement 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 asthe 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 localtop 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, thelocal 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 rightmouse button and select Labels. Under the Plate category, switch on Plate Orientation. The local axes will bedisplayed as shown in these figures above.

For an existing concrete member, I need to compute thecapacity 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 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.

Can I change the strength reduction factors in the program?For example: For a tied concrete column, I assume that thecurrent 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 theconcrete beam member which I am designing is limited and I need to have2 rows of reinforcement in the top or the bottom of thebeam, how do I input this request? Or Does Staadautomatically output the data with the second row? havebeen trying to find this in the Manuals. I have seen LEVELSBUT 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 reinforcedsection (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 isreported. It reports a failure only if more than 2 layers are required.

In concrete design per the ACI code, what does thefollowing expression in the STAAD output file mean: BARSIZE CAN NOT BE MATCHED TO MEET ALLREQUIREMENTS

This means that though the program is able to come up with the value of area of steel required, it is unable tocomeup with a bar arrangement which will satisfy the area requirement. Usually, this is because either becausethe MINMAIN and MAXMAIN limits might be too restrictive, or because the resulting bar spacing violates theminimum spacing requirements of the code.

See Also

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