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Modeling intermediate slab floor perimeters

asked 2020-04-01 23:46:27 -0500

Krishnan gravatar image

updated 2020-04-02 11:33:14 -0500

In multi-storey building model with store-front curtain wall systems, what is the best strategy to represent the intermediate floor slab construction as part of the exterior envelope. I am currently trying to reprsent a 9" thick concrete slab as a solid concrete wall of 9" thick. This is not really a wall, but I am not convinced that the curtain wall area be extended to cover the slab either. The total envelope surface area of the intermediate floors in 35 floors with 200 ft of slab edge makes it a significant amount of slab permieter envelope area that needs to be modeled correctly. Please share any standard practice or workaround.

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answered 2020-04-07 11:15:29 -0500

Jono gravatar image

The standard practice is to treat this as a "thermal bridge". The standard way to model a thermal bridge is to derate the assembly according to the extra thermal transmittance this discontinuity would add.

There are different ways to input thermal bridges depending on the software used, though generally you have to sum the total thermal transmittance of the clear field assembly and subtract the additional thermal transmittance that would be caused by the thermal bridge, then enter this adjusted thermal transmittance into the model.

In the example you describe, the slab would be assigned a "psi" value (W/mK), which then can be multiplied per unit length to get a W/K transmittance. This W/K transmittance can be subtracted from an assembly in the model to derate it.

This process is described in much more detail in the references below. They should get you on the right track.

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You can use the TBD measure, which automates the ISO/BETBG derating process (as described by Jono), within OpenStudio.

Denis Bourgeois gravatar image Denis Bourgeois  ( 2021-04-23 04:59:45 -0500 )edit

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Asked: 2020-04-01 23:46:27 -0500

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Last updated: Apr 07 '20