Deflection in BS8110

[NewImhotep]

In BS 8110 there is a modification factor "kt" (table 3-10) ,which multiplied into basic span/depth factor
this factor take the tension steel effect in deflection.
in many calculation this factor become less than 1 !!( .75, .6 etc)
then the S/d factor reduced !

is this factor used for enhancing the span/depth?

it's not giving any sense if this factor less than 1.

does any body agree with me ?


AhmedR

 

[apsix]

Although it's called 'modification factor for tension reinforcement', it is also a factor for compressive stress in the concrete and possibly other effects as well. Therefore higher M/bd2 levels results in greater compressive stress and therefore more deflection, even if the rebar stress remains constant. Using compr. rebar will counteract this effect (table 3.11).

By the way, my 1997 edition doesn't use the notation 'kt', what edition is your's AhmedR?

John

 

[NewImhotep]

John

thanx for ur response .
I know about the other factor "kc" which already more than 1 as the code equ.
u r right its come when M/bd^2 is large (K also be grater than .156),but some times i make max. value of kc (1.5) and final s/d become less than basic too.!!

i think this factors must not taked if k>.156
or beter to calculate actual deflection as part 2 or ACI methode.

Iam using the 2002 copy for BS8110:1997 with Amend. 1&2.
but using kt & kc notations from my books and IStructE documents .
Best regards.

Ahmed

 

[apsix]

Ahmed
I'm no expert but this is my interpretation;
The K factor is used to determine the necessity for compression rebar, for flexural strength purposes. It has no effect on determining the S/d ratio. More rigorous methods such as in part 2 should give more accurate results.

The issue of the final S/d ratio being less than the basic is just a matter of calibration. The code committee could have chosen (min.kt x max.kc) = 1 , with the basic ratios in Table 3.9 reduced to suit.