RCDC-Beam

• 1. Can RCDC design Curved beams? Will it convert the curved beam composed of many parts to a single physical beam?
  • Yes. If curved beam is modelled in sufficient small parts then RCDC identifies it as single beam as per the geometry and support conditions. The beam is designed for Bending, shear and torsion as per regular beam.

• 2. Can RCDC provide bottom bar without curtailment in single span?
  • RCDC has the option of detailing bottom reinforcement as Best fit and Max dia. user can use option of Max diameter to maintain the same number of bars at bottom most layer. For Best fit it curtails the bars at support if it is allowed as per detailing requirements.

• 3.Can number of bars for the given width of beam be edited?
  • Yes, User can set the number of bars as per width of section in "zone bar setting" option. It also checks the minimum and maximum spacing criteria at the same time. For more information refer Topic “Preferred Bar Spacing” for beam from help Content.

• 4.Can RCDC provide the detailing of beam with lowest diameter throughout the length and balance area of steel with higher diameter in other layer?
  • Yes. It can handle this only for top reinforcement. To maintain the lowest bar at top user can select “min dia.” option in general setting. Lowest Bar diameter and numbers will be provided across the length of the beam and remainder will be provided at next layer of reinforcement. This option will result in detailing like 2-T12 at first layer and 2-T25 at second layer. For more information refer Topic “General and Reinforcement Settings” for beam from help Content.

• 5.Does RCDC allow to provide only 2 bars for top reinforcement?
  • The Number of Bars suggested by RCDC are as per beam width and maximum spacing criteria given in codes. User can change the numbers of bars prior to design subjected to spacing criteria given in code. For zero bending moment zone RCDC can provide minimum two bars with minimum bar diameter. For more information refer Topic “Preferred Bar Spacing” for beam from help Content.

• 6. What is the purpose of mirror & straight option in group/ungroup tab?
  • Mirror grouping option can be used if beams are mirror in arrangement with same geometrical properties. So beam on one side can be mirror in detailing of the beam on other side. Similarly if beams are identical in plan i.e. beam geometry is similar at a given level and repetitive then these beams can be grouped. For more information refer Topic “In Plan Grouping” for beam from help Content.

• 7. Why specifically only 13 stations (for beams) can be imported from E-tabs to RCDC?
  • 13 stations divides the element in to twelve parts. As per standard practice of all codes, curtailments of reinforcement lies at L/4, L/6 and L/3 of span where L is the span of the element. Dividing the element in to 13 station satisfies all the requirements of curtailments. Curtailment of reinforcement helps in beam detailing thus it is mandatory to provide 13 stations to all beams in the analysis. Staad automatically provides the results at 13 stations. E-tabs can provide results at any stations as per user requirements Thus in E-tabs user has to assign output stations as 13 before exporting the results. For more information refer topic “Technical Discussion” of beam and Column from help Content.

• 8. Side face reinforcement (SFR) is designed for beams even though the depth is less than 750mm, whether there are any options to enable/ disable side face reinforcement?
  • In RCDC, SFR is calculated as per design requirements including torsion. User can provide the SFR even it is not required in design. In many cases for beams with depth less than 750 mm, there may be torsion, which would result in SFR. User may check the detailed calculation report for clarification.

• 9. Please give clarification of failure type for beam element. Failure type – Shear –Tc max
  • Tc max failure means Tv exceeds the maximum permissible shear stress.

• 10. Even after opting best fit for top reinforcement, why does RCDC provides higher reinforcement (as per minimum Pt) at top at mid-span?
  • This would be typically doubly reinforced section which causes more reinforcement at top even when moment at that location is nominal. In the calculation report it can be clearly checked where Asc-required at mid-span is captured. This is the top reinforcement required at that section.

• 11. Why default 0.2% is provided for nominal steel in RCDC? Does it have any reference?
  • RCDC provides the nominal steel where bending moment is ZERO. Default value provided in RCDC for nominal steel is 0.2 %. This is an approximate Value and we have kept the value close to minimum steel in beam. The above value is editable and can be put as per user requirements. For zone with no Bending Moment, RCDC uses nominal steel and for detailing, it would adopt 2 bars with minimum diameter possible.

• 12. Is there any option to disregard the contribution of concrete in resisting shear for beam when IS: 13920 is used?
  • No. Beam shear design is as per IS 456. Shear induced due to Sway action is calculated as per IS 13920. For Sway shear also the part of shear is arrived from the dead and live load. Thus RCDC does not allow to ignore the concrete for shear design.

• 13. When IS: 13920 is selected, does RCDC consider required rebar or provided rebar to calculate capacity (plastic) shear force (or moment capacity) for beam?
  • Yes. RCDC designs the beam shear as per Clause 6.3.3 of IS 13920 and consider the reinforcement provided to calculate the moment capacity. For more information refer Topic “Calculation of Ductile Shear” for beam from help Content.

• 14. Does RCDC calculates moment capacity for sway shear calculations as per rectangular section or flanged section for beam?
  • For moment capacity of section RCDC always considered as a Rectangular section as Flange section is not applicable at support. Also to get the flange action, flange has to at compression side which is not the case at support. For more information refer Topic “Flanged Beam” for beam from help Content.

• 15. If Beam is designed for axial plus biaxial forces, ideally if axial force in beam is less than the permissible value given in IS 13920 code, the member should be designed as beam only both for strength and crack width and check as per columns should not be applicable.
  • Whenever design for ‘Biaxial Bending’ is selected, RCDC would treat the design of beam like a column. There is possibility where axial forces would be negligible but lateral moment and shear would be high. In this case this member should be designed as a column only. Axial forces will not govern whether to details the member like beam or column.IS 13920 suggest to design/ details member like column as per clause 7 if axial stress exceeds 0.08ck. here the section will be detail as a column. shear (ductile) links should be calculated as per column.if axial stress less than 0.08 fck the section would be details as beam. All the links calculations would be as per beam. Further depending on Pu-threshold value set by user (the value is considered as compressive), RCDC will design the section with Pu, Mu-major and Mu-minor. If Pu for a given combination is less than Pu-threshold, it simply assumed.Pu = 0. This is only for ‘design combinations’ or limit state of collapse.For crack-width, there is no input for threshold value (currently there is no provision for it) of axial force. The crack-width is checked for service combinations. In this case, the values as per combinations are used for crack-width check.

• 16. We accept that concrete capacity shall be ignored as per clause 6.3.4 of IS 13920-2016 but this can be logically ok if only Ductile shear governs the design.In cases where DL+LL shear governs the design (i.e. no plastic hinge evidently formed) it will be very conservative to ignore shear capacity of concrete in beam.
  • The requirement of ignoring concrete capacity is due to unpredicted forces during earthquake. The earthquake forces are instantaneous, the concrete will crack, and plastic hinge will be formed. Even after the earthquake loads, the beams will be loaded with the Dead and live loads. The cracking of concrete, the shear capacity of the bean will be depending on the shear reinforcements only. Thus, ignoring concrete capacity of the beam for shear check is applicable to all load combinations. So even the critical combination is DL+LL, the concrete capacity should be ignored

• 17. In Beam design output of stirrups, what is 2L-T8 (H)?

  • 2L-T8 (H) is the horizontal link (stirrups) provided in the beam in case if Beam is designed for Axial+ Biaxial forces.

    Based on the axial stress (0.08 fck) type of beam detailing is performed in RCDC. If section selected for Axial + Biaxial design in RCDC then section will be designed as a column. If the section requires stirrups for horizontal shear in case of beam detailing, RCDC provides horizontal links. In case of section is detail as a column, RCDC provides the horizontal link to tie all the rebars along all faces of the section.

• 18. If the Beams exists at support level of the structure, can it be design in RCDC?
  • Yes. if the Beams are present at support level, RCDC can read and design these beams. After reading the analysis file for beams, RCDC shows all the levels available in analysis file. User can select the support level for beam design. Column above the support level are considered as support for beam and accordingly beam drawings are generated.

• 19. Three beams are failed due to shear since no links were designed and also, I am not able to add in any by manual means.

  • If the beam shown failure due to stress exceeds the maximum permissible stress for Shear and Torsion, then it’s a section failure. User can increase the size or Material grade to pass the beam in design. As it is section failure, there is no option available to change the reinforcement diameter to pass the design.

• 20. While designing beams of a group a floor together for a multi-story building, the software error. There is no problem if I design for each floor level separately.
  • There are some missing beams marked in snap below. Please note that the levels can be grouped only if the beam arrangement is similar at all floors.As RCDC is design and detailing software, it generates the Beam elevation. If the Beam are not same at all grouped levels, then it would not be possible to perform the beam design and generate the design calculation reports including drawings.

• 21. It is noticed that when ductile design is done (even only near supports), that there is a big jump in shears in the “Mid” span.  Let me know if this is unexpected

               Reply:

              For Non ductile and Ductile beam, Shear force at end is depend on the following settings,

              .

As per above settings, for ductile beam the end shear would be considered as per 2xDepth of beam. For Non ductile it would be L/3. It is purely depend on the depth of the beam and length of the Beam. If the Beam L/3 is more than the 2xD then it is possible that  at mid zones shear would be less as compared to ductile beam.

RCDC provides following two options for Sway shear check in Beam design.

For Ductile Shear at Support      : Sway shear would be calculated only at ends.

For At All Station                           : Sway shear would be calculated at all 13 stations for which beam is designed. Maximum of Shear due to sway at mid zones would be considered as critical in design.

It is not clearly mentioned in the IS code up to which location the sway shear is to be considered. The Option is added as per our discussion with the IIT professors. Professors is in opinion that, Earthquake loads are impact loads and shear due to Lateral loads might extend up to the mid span of beam. To avoid this kind on un-foreseen condition he has suggested us to add the option of checking sway shear at all stations.  Thus, these two options are available in RCDC. But It would be user’s choice

22.How to calculate reinforcement of beam at face of column in RCDC?

In STAAD generally we design the beam as centre to centre of column and from output we interpolate the beam reinforcement at the face of the column. How we can do the same in the RCDC. Because in boiler supporting structure where column size is 900x900 mm difference in beam reinforcement at centre of column & at face of the column is around 1000 mm².  Like in attached STAAD file for beam member no. 105, reinf. At centre of column is 3297 mm². Whereas by interpolation at face of the column its around 2722 mm².

Reply :

RCDC design the Beam based on geometry and forces available from the analysis file. It doesn’t modify the forces. In you case, the beams are modelled up to centre of the column and beam need to design at face of the column.

We would suggest following methods to add the beam at face of the column, so that beam will get design at moment available at face of the column,

User can assign Member offset command available in STAAD. RCDC will read the beam length and force available in STAAD as per the offset beam length.

User can define the rigid member within the column extent. The property of this member can such that the stiffness is more than the beam stiffness. i.e. YD = 3m and ZD = 0.45 m

The density of this member can be assigned as Zero. RCDC auto ignores the member within the column and with zero density.

The beam can be designed as per moment available at face of the column. Refer below snap

Below is the snap from the RCDC,

• 23.The Building is designed including EQ loads and STAAD RCDC is giving more reinforcement at bottom near supports as curtailed bars. Why?
  • RCDC design the beams as per section forces available from analysis. it designs the beams at 13 stations for given load combinations. when lateral loads are applied, due to sway effect end moment exists at end of the beam. So generally for +ve Eq and -ve Eq loads, the heavy moment exists at top or bottom of the end of the beam. we would request you to check the detailed design calculation report to get more information about the design of the beam. You will get the information about the for which moment end reinforcement is calculated.

• 24. I showed best fit beam reinforcement drawings to client, but he categorically said that I have never seen this type of detailing and it’s not practically possible. Why RCDC gives this type of detailing? 
  • Best fit detailing option is used by most of the structural consultant in Metro cities. it is generally used for the commercial, office and shopping malls where beam spans are larger. Best fit option can be used to save the reinforcement for long span beams. there are other options available in RCDC. i.e. Maximum Dia & Min. Dia which gives flexibility to users to detail the beams as per design and detailing requirements. Beams having shorter spans, maximum or minimum diameter option can be used to avoid lapping.

• 25. STAAD adv. Concrete designer RCDC provides top bars with lapping in output beam reinf. near supports but I want through bars at top without lapping and with curtailed bars near supports. How to do it?
  • Please select Min OR Max diameter detailing option in design settings in RCDC. refer below snap,
    
    

26.Can you provide clarification regarding the curtailment of 100% top reinforcement at a certain location from column as per IS code?

Reply:

RCDC design the reinforcement as per the forces available at all sections in the beam. It takes 13 stations (i.e. 12 zones) for design of beams. Based on the forces available in each zone, reinforcement is calculated. Lapping locations are available as per user inputs. Default location for lapping of top reinforcement is L/4. As RCDC design the beam for 12 zones, the options of L/3,L/4 and L/6 are available for lapping at top and bottom reinforcement.  User can change the lapping location as per design requirements.

Lapping of top reinforcement is available to optimized the reinforcement. Curtailment location is available in SP 34. Refer below snap,. As far are rebar are lapped with the development length, lapping is permitted for top reinforcement.

RCDC provided various option for top and bottom reinforcement detailing. This option is available in General and reinforcement settings,

If user doesn’t want to provide lapping for top reinforcement, we would suggest you to use Max Dia and Min Dia option of detailing.

27.In BBS for ductile beams, there should be no lap splice within the joint and within 2D from the face of the column. (ACI 318M – 14 cl. 18.6.3.3)

(Currently, lap splices are still drawn within the restricted locations.)

Reply:

RCDC design the reinforcement as per the forces available at all sections in the beam. It takes 13 stations (i.e. 12 zones) for design of beams. Based on the forces available in each zone, reinforcement is calculated. Lapping locations are available as per user inputs. Default location for lapping of top reinforcement is L/4. As RCDC design the beam for 12 zones, the options of L/3,L/4 and L/6 are available for lapping at top and bottom reinforcement

At top location, the lapping is provided at curtailment locations. i.e. L/3,L/4 or L/6 distance from column face. Here the distance 2xD is not checked for curtailment. If we start providing curtailments for beams as per the beam depth, it would be complicated. The curtailment location will vary as per the beam depth. The location of curtailments might fall between two stations. Checking reinforcement curtailments at any location as per 2D would not be simple and might not give the desire result. Interpolation of forces is not possible for in between values. Even if we consider on higher side, it would not be consistent to all beams and curtailments will not match the distance 2D for all beams. Also, it is very difficult to handle Ast calculations and shear check location for individual beams.

For Bottom location, RCDC provides the lapping at face of the beam. The higher diameter rebars extends to lower diameter to make sure that the it satisfies the bending and shear check criteria. Also, the confining of the reinforcement exists up to twice the distance from the column face.

If we provide lapping away from the twice the beam depth, it might reach in the middle zone of the beam which is also needs to avoid as it is governed by permanent loads. In this case we might need to handle ductile and Non-ductile beams separately. For ductile beam lapping will place away from the 2D and for Non-ductile it is possible to provide within 2D.

For such cases, we would recommend user to use detailing options available in RCDC.

For Top rebars use max dia. Option: here the same diameter rebar is continued at outer most layer to avoid lapping.

For Bottom rebars use max dia. Option and try to provide same rebar numbers and diameter to adjacent beams to avoid lapping.

28. Does RCDC perform the Serviceability check for allowable deflections from ACI 318M-14 Table 24.2.2 by using the effective moment of inertia from ACI 318M-14 cl. 24.2.3.5 ?

Reply:

Presently deflection values are not read by RCDC and deflection check is not performed. There would be simple check for deflection as per the deflection values available from analysis for given load cases. This would-be short-term deflection check.

Other is long term deflection check which is depend on the creep and shrinkage of concrete.

 We will try and take this as an enhancement in RCD for future release.