Gravity-Induced Interstorey Drift Limits - Eccentric Core - Tall Buildings 1

[Drapes]

In tall buildings with an eccentric core, where gravity-induced long term differential axial shortening leads to the building leaning under its own self-weight, just wondering if the resulting inter-storey drift needs to be added to the inter-storey drift assessed under wind loads? Or if a separate criteria can be used for the long term lean effect. Is there any guidance on what criteria to adopt for gravity-induced inter-storey drifts?

For example, looking at applying an inter-storey drift limit of H/500 for service wind loads alone (as is commonly adopted), and an inter-storey drift limit of H/1000 to H/2000 for the long term lean on its own. Does this sound reasonable? This will result in an overall inter-storey drift of around H/300 to H/400. Alternatively, you limit the overall inter-storey drift to within H/500, but I believe this may be conservative.

Note, my query is specifically for inter-storey drifts. I would still be looking to adopt a total drift limit of H/500 (wind load plus long term lean).

 

[slickdeals]

Buildings that have significant gravity induced lean are compensated for during construction to account for the short term lean. Any further lean resulting from long-term effects, in my view, should be accouted for in combination with lateral loads.

 

[r13]

I am not discounting this question (on tall building tilting), which is very real and interesting. But I think you need first to review the code intent/reason to setup drift limits for lateral load induced displacement/deflection, then see when the tilt is to be considered, and how to handle it in the design.

IMO, It should be included in gravity, maybe seismic, design considerations, but wind.

 

[Drapes]

slickdeals,

Agree that a non-linear staged construction analysis (which includes time-dependant effects) should be used to assess the long term inter-storey drift effects. Any tendency for the building to lean in the short term will be mostly negated during construction.

retired13,

The intent of the code in my view specifically translates to avoiding distress in non-loadbearing elements (curtain walls, lightweight partitions, masonry infill etc). I suspect if these non-loadbearing elements were adequately articulated and detailed to withstand a reduced inter-storey drift limit of say H/400, which is still a commonly adopted criteria in other areas, then this would also be acceptable.

The code in question, AS3600 commentary states that the "maximum limit on the inter-storey lateral drift of 1/500 of the storey height is intended to provide appropriate stiffness and serviceability for most multistorey buildings and unbraced framed structures under the design lateral loads for serviceability."

But lateral loads typically only include wind and seismic, so would the long term lean effect be included in the mix as well? Seeing as the long term lean-induced inter-storey drift is accumulated over a long period of time (i.e. builds up gradually), as opposed to inter-storey drift due to wind and seismic which occurs almost instantaneously, could you justify treating the two effects independently with their own criteria set, as I proposed in my original post? And I'm only talking inter-storey drift here (not total drift). I am yet to come across any guidance on this.

 

[r13]

I think the lean should be considered in the original design (overall layout and sizing), for it will induce instability, and the effects exemplified at seismic event, maybe wind too. I've difficulty to connect the lean with storey drift though. Seems to me, those are two separate effects that don't need to add together.

 

[rapt]

Drapes,

My reading the AS3600 code rule is that it is the deflection at service with the lateral loading applied. Gravity is there at the same time, so it is the total lateral drift, not just the instantaneous drift due to wind.

Creep will be a major influence in a non-symmetric building as the columns will normally be at much higher compression stress than the core walls under permanent gravity loads and will cause very significant horizontal deflections if the walls are at one end of the building.

You then need to add the instantaneous effects to this.