Karlos80
Structural
- Mar 29, 2013
- 29
Hi,
I am involved in design of extensive refurbishment and upgrade works of an existing shopping centre.
The original building consists of a light weight cladding roof on cold formed purlins on precast 4' pitch roof concrete beams on 300x300mm precast columns. The building is a single storey portal frame building with three different concrete frame arrangements. It consists of 9 bays with three spans in each frame and 12 bays with two spans. The three span concrete frames span approx. 15mm, 22.5m, and 22.5m and two span frames with equal span of approx. 22.5m. The precast roof beams are a triangular shape with a slim web. The precast roof beams appear to be pin-jointed on to precast columns. This would suggest that the stability of the existing building is provided by means of fixed foundations and cantilever action of the columns.
The precast columns are slender (300mm square , approx. 5500mm in height). I checked the frame as unbraced /sway assuming all columns are pure cantilever (BS8110 coefficient B=2.2) and the frame is failing under the Notional load and gravity load combination only. For the notional load I took 1.5% of the characteristic dead load. I cant pass the existing building. but it standing there for 30 years (client point of view).
There are a few full length internal walls in the centre of the shop and I suspect that the lateral forces are also resisted by the internal walls and diaphragm roof action. The client wants to remove all the infill blockwalls.
We were thinking to replace the existing walls with some sort of a steel picture frame to provide some lateral stability and brace the frame laterally. But unless we provide some massive steel heavy sections it wont replace the walls in plane stiffness.
The question is how to determine if a pin-jointed frame is sway or non-sway? So the columns can be considered as braced elements reducing the effective length.
Should the notional load be combined with the wind load case (there is a big difference between BS and EN codes)? And if the existing frame fails under the notional loads, does it mean that all the stresses due to the notional load are already in the columns and any additional bracing will not be effective? What would be a standard strengthening method for the columns?
Thanks
Karlos
I am involved in design of extensive refurbishment and upgrade works of an existing shopping centre.
The original building consists of a light weight cladding roof on cold formed purlins on precast 4' pitch roof concrete beams on 300x300mm precast columns. The building is a single storey portal frame building with three different concrete frame arrangements. It consists of 9 bays with three spans in each frame and 12 bays with two spans. The three span concrete frames span approx. 15mm, 22.5m, and 22.5m and two span frames with equal span of approx. 22.5m. The precast roof beams are a triangular shape with a slim web. The precast roof beams appear to be pin-jointed on to precast columns. This would suggest that the stability of the existing building is provided by means of fixed foundations and cantilever action of the columns.
The precast columns are slender (300mm square , approx. 5500mm in height). I checked the frame as unbraced /sway assuming all columns are pure cantilever (BS8110 coefficient B=2.2) and the frame is failing under the Notional load and gravity load combination only. For the notional load I took 1.5% of the characteristic dead load. I cant pass the existing building. but it standing there for 30 years (client point of view).
There are a few full length internal walls in the centre of the shop and I suspect that the lateral forces are also resisted by the internal walls and diaphragm roof action. The client wants to remove all the infill blockwalls.
We were thinking to replace the existing walls with some sort of a steel picture frame to provide some lateral stability and brace the frame laterally. But unless we provide some massive steel heavy sections it wont replace the walls in plane stiffness.
The question is how to determine if a pin-jointed frame is sway or non-sway? So the columns can be considered as braced elements reducing the effective length.
Should the notional load be combined with the wind load case (there is a big difference between BS and EN codes)? And if the existing frame fails under the notional loads, does it mean that all the stresses due to the notional load are already in the columns and any additional bracing will not be effective? What would be a standard strengthening method for the columns?
Thanks
Karlos
