Expert: Arshad Khan - 11/15/2009

Question
QUESTION: Dear Arshad Khan,
I hope that you are fine and keeping good. My questions are
1) We provide beams at plinth level all around the building under the walls. These beams rest directly on ground above lean concrete. Should we also include them in ETabs model???? is this practise correct?
2)I have designed a number of 5 or 6 stories building having one or two basements. When defining automatic UBC seismic loads in Etabs I take the bottom story as the story which is at Natural ground level as for time period height of the building taken should be what it is above natural ground.. is my practise correct.. will be grateful if you further comment

Thanks in advance..


ANSWER: Hi Muhammad,

1. If the plinth beams are contributing to lateral stiffness of the frames, then include them in the model (posibly with spring supports), otherwise do not.

2. Yes, the UBC load shoul normally be applied from ground floor upwards only.

---------- FOLLOW-UP ----------

QUESTION: 1)Obviously they do add to lateral stiffness but how to model them using spring supports..???
how can plinth donot add to the lateral stiffness of a frame????
moreover when plinths are used bottom story drifts change and moment transfer to fix support is also very less.. so I am very confuse on this issue... please provide some guidance

2) One more question, do we need to take cracked section properties of slabs as 0.25 (stiffness modifier) when using a two way slab with beams???? i mean to say that it is not a flat plate or flat slab...

thanks

ANSWER: Hi Muhammad,

The effect of plinth beams on lateral stiffness/deflection depends a lot on the geometry (e.g. depth of foundations)and relative stiffness of plinth beams compared to say the columns and shear walls. So, depending on these variables the effect on lateral stiffness can be minor or major.

To get spring stiffnesses, you will need to do site investigations, etc. If that is not possible, to be on the conservative side, you can assume that beams span from column to column.

Slab modifier 0.25 should only be used to determine maximum deflections, but for strength (rebar design), you can use 1.0

---------- FOLLOW-UP ----------

QUESTION: Thanks man!
Queries regarding plinth are resolved.

I was asking about cracked section of two way slab as using cracked section properties increase the beam design refino in etabs . In UBC it is said that we should use cracked section properties for beams,columns and walls and for Flat plate and flat slabs.. so what do you say regarding using cracked section properties for slabs. I have reviewed some examples by SK Gosh in which he has not taken cracked section properties of slabs into account when doing computer modeling..

One more thing..
While designing building that only have special moment resisting frames as LFR system  and no shear walls and have one or two basements.. now what i do is I model basement walls as Walls(definied by etabs) in the model. and use value of R as provided by UBC table 8.5.. is this correct or due to the addittion of basement wall there is a combination of different systems in vertical direction..
Please shed some light..
thanks for your time and reply to the previous questions.

Answer
Hi Muhammad,

UBC makes the recommendation you have quoted as these members, when cracked, lead to higher sways/deflections of the frame/building. If a slab is supported on beams, then whether the slab is cracked or not makes little difference to lateral stiffness of the building, as that is mainly dependant on the stiffnesses of the beams, columns and walls. So, I repeat what I said earlier- in this case, do not use factor 0.25.

As seismic forces are applied from ground level upwards, having walls in the basements does not have much effect on the superstructure or its R value. So, you can ignore them.