Ali Shan

I have a 3 story RCC Frame. During construction phase, it was revealed that columns of the frame were under-designed (provided reinforcement is 3% and required is 4.2%. Columns have been constructed upto first slab level and beams/slabs have not been laid yet. We are thinking to redesign the frame by releasing moments at ends of RCC beams (which have not been laid yet) so that moments in columns can be minimised and the previously provided 3% steel in columns may become sufficient. Is the approach of releasing beam end moments in RCC Frame (with monolithic beam to column joints) correct?? How we will detail the joint? What will be the degree of cracking at joints when beam ends will rotate?? Has anyone ever experienced such deign?

So do you mean that the increment factor 1.43 is also applicable for check floor deflections? Isn't it only for lateral wind drift? What could be the alternatives?

It is a common practice to apply stiffness modification factors on beams, columns and slabs as recommended by ACI-318 for strength design of members in ETABS. For checking deflection of floors (serviceability checks), we export slab to SAFE for cracked section analysis and checking corresponding deflections. I have two questions regarding this: 1. Some engineers multiply the stiffness modifiers by 1.43 (as per ACI-318) and check floor deflections within ETABS. But I have doubt that the multiplication factor 1.43 is only applicable for checking wind load drift checks. Am I right? Should we use SAFE for checking correct floor deflections as per cracked sections under serviceablity? 2. For the design of footing. We export base reactions from ETABS and import them into SAFE for footing modeling and design. Since in ETBABS model we apply stiffness modification factor for strength design conditions, hence the reactions imported from that model will not be applicable for deciding size of footing (again a serviceability requirement). So under what stiffness modifiers we should export the reactions from ETABS to SAFE?? I know the reactions obtained from ETABS using code recommended strength based modification factors will be fine for strength design of footing as well but what about footing serviceability checks????

With reference to this statement, I have a question regarding detailing of lift wells. If we design lift well existing in a building system for the gravity load only by ignoring its shear and flexural stiffness against lateral loading in the model and detail it with nominal reinforcement for gravity load only and putting all the lateral load on columns. Then how the wall will behave in case of seismic ground motions? Will it crack loosing the serviceability?

Alright. Have you ever experienced design of Strap footing (combined footing with beam) in CSI SAFE? I am confused about interpretation of slab moments.

@ANStructs So does it mean that we should prepare two models of building one with hinged base and other with fixed one? The hinged should be used for frame design and fixed for footing design?

I have a query regarding manual slab design, whether the co-efficient method of slab design given in ACI 318-63 considers role of supporting beam stiffness? I have gone through the code and could not find that the moment co-efficients are co-related with supporting beam stiffness. But whenever the beam stiffness will be varied, there would certainly be variation of moments in slab determined by aforesaid method in ACI 318-63. Exceptionally, I have seen somewhere on a webpage that for applicability of co-efficient method of slab design given in ACI 318-63, the slab has to be supported by walls or STIFF BEAMS? The question is again that HOW MUCH STIFF BEAMS?? Expert opinions shall be appriciated.

Ass per my comprehension, RCC slab supported by beams has moment of inertia and hence has out of plane bending stiffness. Therefore, in order to capture true picture as per site, the slab should be modeled as SHELL. Whats your judgement? Does membrane actually exist or is it a hypothetical invention?

@Simple Structures Thanks for nicely explaining the concept of base fixity and I have understood all that. You are right that even in case of hinged base, the column bars are required to go straight down to the footing for bar anchorage in compression. But I have seen in many books which present the attached reinforcement detailing (please see the attached image) of hinged column bases in which it seems that the column bars have been terminated at the face of footing and have not been developed into the footing. Your expert views on this are awaited please. Thanks

Very nicely explained. But I have never seen any practicing engineer in Pakistan who provides detailing of column bases as HINGED CONNECTION in case of RCC buildings. Most of them simply design the footing for AXIAL LOAD ONLY and let the column bars go straight into the footing pad. Could you please reveal that myth?

Hi everyone. I have seen many structural engineers using Pinned column bases for RCC buildings in ETABS and many others are using FIXED ones (soil being of compressible nature in both cases). I want experts to comment on the right approach and also please explain what are the detailing requirements for both cases as I have seen that people using both approaches use same detailing pattern in which column bars go straightly down to the footing.

Please clarify my confusion. While modeling a frame structure with shell in ETABS, we perform centre to centre modeling, means all the members are connected together at their geometric centres and this is a common practice in market of structural engineering. In the same way, beams and shells are connected together. My question is that, during analysis, while calculating moments etc, does software considers composite behaviour of beam with shell like a T-beam by taking into account the effective flange width OR it analyses it as rectangular beam with no composite action? If it accounts for the composite action, then what type of forces (compressive / tensile) will be generated in that effective part of slab? I think there should be no force in the slab under composite action because the slab is assigned at the geometric centre of the beam. An elaborated answer is expected.

Please clarify the following confusions one by one:- 1. If we run P-delta analysis in ETABS, then should we ignore stiffness property modifiers for beams and columns? I have heard that if we perform P-delta analysis and apply stiffness modifiers at the same time then the moment magnification process is doubled...? 2. ETABS considers selenderness of a column by applying moment magnification factors. If we run P-delta analysis also, does it mean that the selenderness of column is being over-estimated? I mean once the moments are magnified in P-delta analysis process and again through moment magnification process? Please help me understand the software myth and clarify above confusions.

How to check Punching Shear in a flat slab in SAP2000? Is there some direct command as in SAFE?

I have modeled a simple 6m long concrete circular column of 760mm diameter. I have applied a joint live load at the top of the column. Selecting ACI-318 as design code with frame type as sway special, I get the flexural and shear reinforcement. However, when I change the design code to AASHTO Concrete 2007 in the design preferences with zone 3 in overwrites, I get nearly same flexural reinforcement as by ACI but I observe a huge difference in shear reinforcement. It is to be noted that I have used same combination while designing by both codes. Please check the file attached and explain. G.SDB