huangyhg

deflection after long time in etabs
hi everybody
i want to ask how can i get the value for the deflection after long time as per aci 318-99 using e tab programm ?
why not just take the initial deflection (based on sustained loads - dl + some reasonable %ll) per your program and apply the multiplier per aci 318-05 section 9.5.2.5 (eq. 9-11)?
i think you have to do what strleit said. make another load combination.
if you're using safe, i think it does a little more to help with this.
swelm,
you need to allow for cracking and long term effects.
the effect of cracking is variable along a span and also dependent on how heavily stressed and reinforced the member is.
the long term effects can be allowed for very approximately by using the code long term multiplier but remember in using this that shrinkage deflection is unrelated to the load deflection and creep deflection is very variable. the use of a single multiplier, 2, to allow for these is very very approximate. to then adjust this multiplier by a factor to allow for "compression" reinforcement is even more inaccurate. for instance, in an 8" slab with normal levels of reinforcement, the compression face reinforcement will do very little for deflection because it probably is not even in compression.
the only way to do it properly is to calculate the effects of cracking, shrinkage, creep and reinforcement in the section design. the total effect of cracking plus long term effects could be anywhere between 5 and 12 times the elastic deflections that safe or etabs will give you.
if you want to do this properly to get an idea of the real deflections you can expect, there is a program called rapt which we develop which does this for you for rc and pt members (and no, it is not free or cheap).

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