Retrofit of a Residential Building - A Case Study

AB Consulting Engineers were engaged by a client to conduct a Structural assessment and design strengthening as required for one of their properties. The structure in question comprised two buildings:

• A three-storey main house

• A single-storey granny flat joining the main two storey building.

The existing building had been designed and detailed by another party, with a significant portion of the buildings already constructed on site. However, the client required a new PS1 certificate from a qualified Professional Engineer - Structural Engineer, as the credentials supporting the original PS1 were deemed questionable.

Our methodology involved a rationalized assessment of the existing structure to prevent unnecessary structural interventions while ensuring structural safety was not compromised.

This project was divided into two primary stages:

• Assessing the existing structure to identify any potential deficiencies

• Developing a strengthening strategy to address the identified deficiencies.

Stage 1: Assessment of the Existing Structure and Identification of Structural Inadequacies

This project required a thorough examination of the record drawings and documents to comprehend the original design and construction details. AB Consulting Engineers conducted an independent evaluation of the structure, interpreting the information obtained from the existing drawings and site inspection. Our assessment revealed several deficiencies in the original structural design.

The primary deficiencies identified are listed below:

·       Inadequacy in the roof ridge beam

·       Insufficient capacity in the floor beams at the loft floor level

·       Deficiencies in the design for posi-strut floor beams

·       Inadequate capacity of lintels

·       Insufficient lateral bracing

Stage 2: Strengthening Design

The second phase of this project focused on developing solutions to reinforce the structure, addressing the deficiencies identified in Stage 1.

The primary challenge was that the structure was nearly fully constructed, leaving limited options for modifications. We collaborated closely with stakeholders, including clients and the preferred contractor, to ensure that our proposed structural interventions would minimize impact on functional performance while also being cost-effective and minimally disruptive. For instance, the floor beams in the loft space were undersized for the specified loading in the original design. Strengthening for the original load would have been costly due to the extensive floor area impacted and limited room to add new floor beams. Instead, we worked with the client and the local building consent department at the council to determine that the optimal approach was to restrict allowable loading on that floor, thus avoiding the need for reinforcement in that area. This solution was discussed and approved by the Council before proceeding with the strengthening works.

Given these constraints, we developed an appropriate strengthening scheme that included:

·       Restraints for roof beams

·       Loading restrictions on the loft floor

·       Introduction of new portal frames

·       Additional posi-strut below the first floor to increase beam capacity

Some of the sketches prepared are shown below

Ridge Beam Strengthening - Residential Building

We have successfully identified and achieved the key objectives for this project. The primary KPIs were:

1. Implement strengthening only where necessary.

Achieved. Although additional effort was required to maximize the use of the existing structure, it was worthwhile as we minimized the extent of structural intervention needed. A rational approach to reducing the loading on the loft floor is one example, and avoiding additional lateral bracing in the granny flat is another.

2. Conduct strengthening interventions with minimal disruption to the functional use of the space.

Achieved. Through careful coordination and optimization, we avoided significant impacts on the functional use of the building's interior. Installing steel portal frames within an existing wall serves as an example.

3. Ensure the strengthening process is straightforward to implement.

Achieved. It was crucial for both us and the client that the proposed strengthening activities could be executed without significantly damaging the existing structure. All components had to be of a size and weight manageable manually, as the building envelope was already in place. This was accomplished by providing multiple splices with bolted details, allowing smaller pieces to be brought into the space and assembled in their final positions. For example, by providing splices in the columns at ceiling and floor levels, the contractor could install them with ease.

Column Splice - Portal Frame - Residential Building Strengthening

AB Consulting Engineers offers pragmatic and practical solutions to on-site constraints, as demonstrated in the project example above. We invite you to contact us for your upcoming project.