Building Use: Education and Classroom

Typology/Structural Form: Short Span Portal Frames

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Single-storey portal frame structures are a common classroom design format (especially for upcoming off-site build programmes). Historically, these types of structures have been designed and built using steel portals. A mass timber alternative offers real benefits in this application.  

This case study “snapshot” demonstrates the benefits of swapping the steel portals out with Glulam portals and the impacts of doing this.  Although the reduction in embodied CO2-eq may be moderate per building, the accumulated emissions reduction over repeated classroom builds would be substantial.  

The summary table below compares the 3 options of concrete/steel, hybrid and all timber and the impacts of each.

Easy Gains

In this simple example, the typical portal frames are compared. Portal frames are one of the most standard and common construction solutions for single-storey structures. Typically, when looking at Glulam portals frames as an alternative, the focus is on the maximum span or performance in the 12m - 15m + range.

This case study “snapshot” addresses the following questions: 

  • What is the performance in cost, embodied carbon and weight for shorter spans (circa 8-10m) commonly used in classrooms?
  • What is the current cost, carbon and weight results for a typical build using steel portals and steel DHS purlins?
  • What are the changes/impacts of swapping the steel portals for Glulam portals?
  • What are the additional changes/impacts of also swapping out the steel purlins for LVL timber purlins? 

Results Summary

By switching to Glulam portals there is, in fact, a minor cost saving (hybrid option).  

Whilst achieving a saving across the 5 portals of circa $3k there is also a reduction in upfront embodied carbon in the core structure of 58%. This building typology can be easily extended; for example,2 buildings of this type joined together, each comprising of 10 portal frames. The benefits simply keep multiplying out as the 5-portal module is repeated. 


Glulam portal frames are included in the assessment. LVL is possible and is cost-neutral, although for durability reasons, it should not be left exposed. Steel portal legs typically require a cladding system externally for aesthetic reasons. Glulam can be left exposed, although treatments must be applied for weather and UV protection (durability performance). 

History / Project Brief

This classroom typology has been replicated across the country, built in identical fashion, over the past 10 years. Typically, they average around 200m2 and include 5 portal frames spanning 8m. The flexible design allows for additional bays to be added where schools require a larger GFA. A key reason for the portal frame is to allow for the raking ceiling and high-level clerestory windows. The alternative is a flat ceiling under trusses which is a very different type of space with a different feel. Ministry of Education is moving into a large programme of off-site construction. The building designs seen to date for off-site built classrooms are very similar to this design. The volume of this type of construction will likely increase rapidly / exponentially. 

Green - Full Mass Timber
Brown - Hybrid
Blue - Concrete/Steel