WR1

Sorry guys, I have been very busy lately and also the current situation leads to this quietness.

Yes, 1st mode is torsion and it is good that translation and torsional modes are well separated (1.23s and 0.48s). However, I can see the sum of mass participation is still not 90%. Bring it up by including more modes, and then check results. There is no problem having 1st mode as torsion as long as you design for it. However, the building framing will be working very hard and is not very efficient against lateral loads but no is stopping you to not have 1st torsional mode. Hi Asif, I am very much interested to see if there is any reference for keeping Rz below 5% and 10% for 2nd mode?

I'd do it separately due to following reasons: 1. You will get weird dynamic and mode shapes due to a very rigid concrete structure and very flexible steel structure above. So difficult to interpret time periods etc by most users (will tend to believe software's values are correct when they will not be). 2. It is usually not very straightforward to combine R factor for two systems with variable stiffness in a single model. I'd analyze the steel structure and apply it's worst loads on concrete structure (taking caution about horizontal loads and directions) on top of concrete columns as point loads (and include these point loads in seismic weight for concrete structure).

1. Pattern loading (continuous beam assumed as simply supported) is used for live load only. 2. It is necessary in a real building to account for variability in loading conditions. 3. If you are trying to do just a simple beam, make pattern live load = 0

ETABS does this in a way by a pattern live load factor. Have go through its manual on pattern live load factor.

ETABS does give correct values of reo (it does take into account the lower/upper limits of reo for a particular code). Totally agree and recommend first 2 points. However, I do not think, ETABS in 2019 has bugs for such simple things. Yes, it does have few bugs like any other package but for sure not related to these basic things. Please check what code says when you provide As = 1.33 x Asreq ??? And why ETABS giving less moment and reinforcement than what you getting manually is totally a different question and depends if you are modelling it right or if you are manually calculating it right.

Also, take care that the torsional shear reinforcement = area of only outermost 1 closed stirrup. In other words, make sure the outermost continuous big stirrup (have 4 legs all around) is sufficient to carry all torsional shear. You can't distribute in among internal stirrups. However, you can provide multiple stirrups (verticals legs only effective) for flexural shear. Also, if one stirrup (flexural or torsion) is not enough, provide 2 bundled stirrups side by side along beam length or reduce spacing.

Maximum reinforcement in beams (flexure only members) are controlled by maximum net tensile strain. The above-mentioned limit corresponds to 8% max reo (4% on each side), similar to column. Beams usually have reo less than 2%. If you are trying to put more reo in a beam using ETABS, change that section property from beam to column. I always love the idea of putting beams as columns in ETABS. A frame section in ETABS defined as a 'column' will be designed for flexure in both direction + axial loads, a true interaction diagram. If defined as beam, ETABS wil ignore any axial load (if present) in beam design.

I can discern this is due to the moment as your base is fixed and probably the governing combination include lateral loads. Few checks that you need to perform: 1. Are columns sway special in ETABS? If yes then why? 2. Can you tie these columns with tie beams at PARKING or BASE level to reduce critical length of column? 3. If PARKING to BASE height is 1m then from scale of the attached image, PARKING to 1ST FLOOR is just 2.5m. Is it right? Base on the grid spacing of 2.5m, I do not see any problem having 300x300 columns with 2 to 3% reo giving 1800 to 2800 square mm. Or you are not concerned about sizing and just curious why the increase in reo? If so, check the bending moment on governing load combination and you will get a fair idea what's happening here.

Depending on the plan, you can try strap beams, connecting beams etc.

moment

I think I misunderstood your question initially. I thought of it as a simple cantilever beam. However, from the trailing discussion I can discern the said beam was part of a building or frame. And yes, that's where the torsion stiffness play a role.

I would not do that for a cantilever structure. Even if the post installed epoxy bars are okay strength wise, they would be carrying all the loads alone and deflect too much. Imagine the full balcony resting from these steel bars at one end?? The section at junction of old to new concrete is essentially fully cracked.

Not Fixed. Apply pin. (Depends on your connection details though). Apply for all supports not just first and last.

I would be designing them as corner columns with load (or moment) = sum of all column loads and geometric properties of 1 composite section (it also depend of the distances between the columns) so just rationalize it with any appropriate assumption.

This is it. So in short, if Mcr has not reached, strength based design equation does not apply.

Never came across such an issue. Share the results may be?

Yes, put the dead and live loads manually to the columns to keep things simple and if resulting column reo is not that much.

True, and that's where my question kicks in. How did you determine these columns are not slender? By the way I am talking about effective lengths and cantilever column effective length = 2 x L

What do you mean when you say columns are not slender. You mean the section capacity = member capacity for given lengths?

Depending on what you are trying to achieve with tie beams? Reducing bearing pressure? Or reducing forces in pile cap? Modelling it as line element or shell?

I am currently using steel designers handbook 8th edition based on AS4100.

Please elaborate. No one is going to download and check your model unless you post some snap shots and details of your problem.

This topic has been discussed to death. Please search past posts on the same topic.

This topic has been discussed to death. Please search past posts on the same topic.