Moments check

[Snatch]

I am designing a one storey reinforced concrete frame building (9.15m by 30.39m). Height 4.42 m. Walls on exterior only
Nine frames I set up are at 3.811m intervals, consisting of 2 columns each and 1 beam spanning 9.15m. The loading for interior frames are as follows:
1. seismic 398 kN at roof level
2. Dead and live 424 kN (11.28kN/m2 + beam self weight)
3. beam dimensions assumed 610mm x250mm
At column-beam joint: For the seismic loading I am getting a fixed end moment of 880kNm and for the UDL 31.91kNm.
One value seems very large and the other small. Also I am not sure how to treat these values in the design of the beam and column. Please advise.

 

[ishvaaag]

At first sight it is your seismic load that looks big. You have horizontal earthaquake force exceeding the weight of your roof wholly loaded by the live load, this shouldn't happen I think but in extremely rare strong erathquake solicitations and with concomitant stiff structures plus maybe a requirement of adopting values very close to the elastic response for design.

In any case, at your plan dimensions, depending upon your short façades you can put in them either bracing, eccentric bracing or shearwalls able to take the lateral force. Of course the roof needs then be made to work as a unit able to pass the forces to ends.

 

[Snatch]

Thanks for your reply. I must have made a mistake that is the total base shear I should have divided it along the frames that is 398 kN by nine in the long direction and 398kN by 2 in the short direction. Is this correct?
The exterior wall are concrete block walls but not specifically designed as shear walls but would provide some resistance in the long direction. My other concern is that in the short direction, that is the single bay frames spanning 9.15m, only the frames at the far ends have walls so only small resistance is provided and the column beam system would take most of the lateral load. Please advise.

 

[ishvaaag]

On the first part I have to say that about 10% of the acting weight looks to me far more natural a seismic load. The 1 g lateral loads very rare case, I assume, yet maybe possible in the highly uncommon circumnstances aforementioned.

Your plan dimensions have a 1/3 or so ratio. Depending on how you build the roof -and specially if including a concrete slab- the end walls may end taking the whole brunt of the lateral load (and even this be convenient). The roof will act as a spreading beam and it will be the combined stiffness of the frames plues end wall elements what will give displacement.

However, if you control for good behaviour each inner frame to tolerable lateral displacement, most disregard the check of the ends, they are nonstructural as you say.