Deflection at the Apex

[D.Jaya]

Hi Structural Engineers
I designed a small RCC building (for a Buddhist temple). A basic RCC frame drawing used for analysis is attached herewith
It having a building footprint of 6.5m x 6.5m ,with Ground+2 floors and with a RCC dual portal framed(different slopes)type roof where the 2 roof frames(4 RCC roof members) connecting at a single apex point.(at the top).Also 4 nos of additional horizontal beams(at the region where the roof slope chnages)also provided for stability of the roof.
The roof to be covered with light weight steel roof claddings and after analysis also found that 225x300mm RCC colums are sufficient for the building structure and roof portal RCC member sizes also to be 225x300mm.Assumed Grade 25 concrete to be used.
I can determine the apex point vertical(downward)deflection by using the same analysis software progarm but I need to know that value is within the acceptable range.
In otherwords I want to know the value of maximum allowable deflection limit for this type of RCC roof frame structure
Thanks,
Dushyantha

 

[r13]

1) There is no code imposed deflection on roof other than flat, I believe.
2) Any deflection should be small if the 4 inclined members below and the ties are of adequate strength.
3) Don't mean picky, "RCC" is usually understand as "Roller Compacted Concrete", the proper abbreviation for reinforced concrete structure is "RC" structure.

 

[JLNJ]

The only appreciable deflection you will get is from the members pulling themselves apart at the joints if they are not connected properly. Some of those thrust forces you have to resist will be pretty big.

Lateral stability and strength looks like it might be a challenge as well, especially with such small members, some of which are crooked.

 

[JAE]

Agree with JLNJ - some of your beams near the corner columns are rotated 45 degrees which can induce a huge amount of moment in the columns due to unbalanced live loads on the floors.
This doesn't appear very stable to me at all. The roof is fine - it is those weird 45 degree corner bent beams that makes me want to know where this building is, so I can avoid going inside it.

 

[D.Jaya]

Dear retired13,JLNJ & JAE
Thanks a lot for your valuable comments
Also please note the following:
45 degree bend length of the beams at the 2 front columns are approximately 0.71 meters each only.(These beams were bend due to incorporate an Architectural feature)Also note that this is a small building and column to column span was only 3.81 meters.
Only outer walls are clad with brickwork and in this design we select small RC sections because at the critical load combination the Maximum BM at a beam was 120KNm only. Max Axial force and BM of the Center column(which you may not see in the given diagram) were 450KN & 42 KNm respectively & for a edge column the Max Axial force & BM to be 219KN & 60KNm respectively.
We observed 300mmx 225mm sections can be adequately reinforced to carry those forces & moments in beams & columns.In addition I included additional reinforcement to the beams (which having a 45 degree bend at the edge) to counteract the torsional effects.
Foundations were designed as individual RC pad footings connected with tie beams and soil condition at the site was quite OK, therefore 200KN/m2 soil bearing pressure was assumed.
The building (Yet to be construct) located at a suburb of Colombo,Sri Lanka
Thanks,
Dushyantha

 

[JAE]

In otherwords I want to know the value of maximum allowable deflection limit for this type of RCC roof frame structure

Deflection limitations are most always based on:
1. The ability of finished surfaces/materials to tolerate deflections
2. The comfort level of occupants in tolerating deflections (and sometimes the resulting vibrations).

With brick as a finish, the tolerances should be tight to avoid brick cracking. In the US, the vertical deflection limit for members supporting masonry is L/600 where L is the span of the member.
That would be deflection in the same plane as the brick.

For out-of-plane bending for masonry, it ranges between L/360 and L/600 depending on your own level of conservatism.