How to Accurately Model and Analyze PT Transfer Beams with Stub Columns in ETABS to Control Deflctn 1

[Prk_B]

I have a situation where my RCC structure is not exhibiting the desired behavior in ETABS due to the PT transfer beam at the first-floor roof level. Below are the details of the structure:

Number of Floors - G+4
Ground Floor Height - 3.2m (Parking)
First Floor Height - 6.2m (Mezzanine Floor at 3.2m) (Banquet Hall)
Second Floor Height - 3.6m (Accommodation)
Third Floor Height - 3.6m (Accommodation)
Fourth Floor Height - 3.6m (Accommodation)
Code - Indian
Framing Type - Ordinary
Type of Analysis - Equivalent Static Analysis
Importance Factor - 1.5

Since there is a banquet hall on the first floor, I cannot provide columns in the span of the building, which is 16.5m center to center. For the accommodation from the second floor level upwards, I raised stub columns from the second floor level from PT transfer beams (beams spanning 16.5m). I defined PT beams as normal RCC beams for the analysis purpose in ETABS; later on, PT beams will be designed separately. While modeling stub columns, I have released M22, M33, T, V2 & V3 at the base of stub columns.

The problem is, as I defined PT beams as normal RCC beams, the deflection I am getting is more because of that the entire structure above those PT beams is not behaving as it is supposed to. I aim to restrict the long-term deflection of the PT beam to 17-18mm and short-term deflection to 7-8mm, but here in my model, I am getting around 60-75mm deflection for the gravity load combination.

I tried to control the deflection of PT transfer beams by assigning hinge and roller support to the base of the stub column, but doing that reduced my column reinforcement considerably (from around 3.5% to 0.8%). I even tried assigning a point spring (restricting to 5mm deflection) to the base of the stub, but the result was the same. I want to know the right practice to model and analyze a structure that has PT transfer beams with stub columns on top and how to control the deflection of PT beams without affecting the column reinforcement.

I have attached a few screenshots and the ETABS model for reference. I am looking for a solution to this problem. Your comments are valuable and greatly appreciated. Thank you
Link

 

[EZBuilding]

I tried to control the deflection of PT transfer beams by assigning hinge and roller support to the base of the stub column, but doing that reduced my column reinforcement considerably (from around 3.5% to 0.8%). I even tried assigning a point spring (restricting to 5mm deflection) to the base of the stub, but the result was the same. I want to know the right practice to model and analyze a structure that has PT transfer beams with stub columns on top and how to control the deflection of PT beams without affecting the column reinforcement.

With the fixed based columns - these columns are participating as a vierendeel truss in transferring the loads to the supporting columns.

Unless you are utilizing ETABs construction sequencing analysis, the ETABs analysis assumed that the building is all built and then applies all of the loading in it's analysis. This is not an accurate representation of the behaviors as the construction sequencing will impose demands on the transfer beams without the benefit of the vierendeel truss behavior.

When I design transfer beams - I prefer to ensure that the transfer beam is stiff and strong enough to support the loads without the benefit of that behavior. Accordingly, I would tend to create a save as file of my ETABs model and delete all elements below my transfer level. I would then use a pinned support at the transfer columns to define the transfer loads, and I would design the transfer beam for those loads.

 

[Prk_B]

Thank you for the reply.

As you suggested, I've made the PT transfer beam stiffer by changing the moment of inertia about 2 & 3 to 30 in the property modifier section. I arrived at the value of 30 by restricting the maximum deflection of the PT transfer beam to 18mm. Is this the right practice to ensure that the transfer beam is stiff, or is there another method? I'm restricting the long-term deflection of the PT transfer beam to 16-17mm and the short-term deflection to 7-8mm. If this is the correct approach, should I limit the deflection considering short-term deflection or long-term deflection?

After changing the moment of inertia, the structure above the transfer beam is not exhibiting Vierendeel truss behavior, but the area of the column supporting the transfer beam has been reduced to 1.07% from 3.7%. Also, I have kept the column size up to the transfer beam as 600x900mm and after that reduced it to 375x900mm, and the moment Mu3 in the bottom of the column above the transfer beam (375x900) is almost the same as the Mu3 at the top of the column below the transfer beam (600x900). Due to this, I'm getting almost 1.5 to 1.75 times the reinforcement in the upper column base compared to the lower column. Is this how the column will behave, or is there any mistake in my approach? By any chance, am I underestimating the column reinforcement?

I've released V2, V3, T, M22 & M33 at the base of the stub column at transfer beam level. Is it okay to release these at the start?

I've attached few screenshots for the reference. Thank you