Innovation In Structural Investigation


GBG has been commissioned by Robert Silman Associates, a leading structural engineering practice with a strong track record in working on historic buildings, to carry out a non-destructive survey (NDT) of a private estate residence in the heart of Washington DC.

Built in 1988, the home is an ornately decorated two-storey mansion. The façade is of reinforced cast stone block and the main walls are formed of reinforced CMU (concrete masonry units). The building has oval windows, balconies, a projecting cornice, and an ornate roof parapet with bottle balustrade.

Since changing ownership the house has shown significant signs of water infiltration, which appears to be occurred over several years. As the house was recently purchased and problems had already been identified it was decided (by RSA and the new owner) that further investigation was required to confirm the construction arrangement and condition of various items, specifically the parapet walls, exterior walls and chimney stacks.

GBG’s team of engineers, working in partnership with Robert Silman Associates, identified six areas of the building to survey, using a combination of NDT techniques, including impulse radar and metal detection, the latter being used to confirm the location of vertical reinforcement bars. The main aim of this investigative work was to confirm the construction against existing design details so as to reduce the level of assumption regarding the structure when designing repairs.

The results of the survey indicated that the main exterior walls and chimney stacks were consistent with the original design intentions, but that the parapet wall construction differed considerably from the design specifications. In particular, the introduction of a steel angle to the rear face of the parapet wall – used to secure the projecting cornice stones – had required other parts of the design to be changed, most notably above the steel angel where no parapet reinforcement had been used.

GBG recommended a small probe be taken above a section of steel angle specifically to confirm the fixing arrangement between the angle and cornice stones. GBG also recommended one additional small probe beneath the angle to confirm the size of the reinforcement bars.

The results of this investigation confirmed much of the building design and provided sufficiently detailed information regarding certain parts of the structure that extensive probing was no longer required.

For further information, please contact:

Chas Bransby Zachary()

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