Hearst Tower, NY: Impact of corner diagrid-nodes on floor slabs due to radial forces?

[Tommyankee]

Considering the diagrid-system of the Hearst Tower, especially the "bird`s mouth" at the corners of the building, I wonder if the radial (better: horizontal) forces due to the sloping corner-diagonals had any impact on the slabs!
Are all of these horizontal forces directed through the floor slabs?




Furthermore, I don´t understand why the structure needed a second lateral system (braced frame at the service core area) in addition to the diagrid-system!

Any suggestions!?!

[Kelvin]

Almost all modern buildings rely on floor slabs as "diaphragms" to efficiently collect and distribute horizontal forces to other parts of the building lateral force resisting system. The idea being that the more this is done, the more uniformly the building will move. Both the floor slab and the horizontal members comprising the diagrid framing will act to transfer these forces. Having additional bracing at the core serves to to several things:

a) reduce the total load passing through the exterior grid and thus the size of the members required (ultimately making it appear lighter),

b) reduce the magnitude of shear force carried by the core columns. Without the core diagonals, any horizontal force in the floor (carried into the columns) would have to be carried as shear - using diagonals reduces the amount significantly, and

c) contributes to the overall lateral stiffness of the building and probably helps keep the torsional centroid close to the centre of the building as well - important in any highrise.

[Tommyankee]

Originally Posted by Kelvin

Almost all modern buildings rely on floor slabs as "diaphragms" to efficiently collect and distribute horizontal forces to other parts of the building lateral force resisting system. The idea being that the more this is done, the more uniformly the building will move. Both the floor slab and the horizontal members comprising the diagrid framing will act to transfer these forces. Having additional bracing at the core serves to to several things:

a) reduce the total load passing through the exterior grid and thus the size of the members required (ultimately making it appear lighter),

b) reduce the magnitude of shear force carried by the core columns. Without the core diagonals, any horizontal force in the floor (carried into the columns) would have to be carried as shear - using diagonals reduces the amount significantly, and

c) contributes to the overall lateral stiffness of the building and probably helps keep the torsional centroid close to the centre of the building as well - important in any highrise.

Thanks Kelvin for this quick reply!

Considering that these horzontal forces due to the sloping corner-diagonals pass partly through the floor slab and partly through the comprising members of the diagrid, is it important to know exactly how much of these forces go through the floor slab?
Since forces going through the floor slab do have an impact on the core (zigzag-moment distribution).
In the case of the Hearst Tower the construction of the floor slabs doesn´t seem to pay to much attention about guiding these forces from the corners
to the core...

Second lateral system: What about additional stiffening for the diagonals between the nodal levels?