Evaluating scaffolding hoist vibrations: the impact of normal construction activities on the existing building structure is an important part of any restoration project

Masonry Construction, May, 2003

The Cathedral of Immaculate Conception in Albany, N.Y., is currently undergoing an extensive restoration. One of the studies performed to ensure a safe and successful project was determining the impact of operating a scaffolding hoist on the north tower (Fig. 1).

[FIGURE 1 OMITTED]

Background

The Cathedral is located next to the Governor's Mansion and just east of The Empire State Plaza, and is a highly visible and easily recognized part of Albany's skyline. It was the second Roman Catholic cathedral built in New York state and is listed on local, state, and national historic registers. The Cathedral was designed by Patrick Charles Keeley and serves the Albany Diocese.

The construction of the Cathedral began in 1848 and was completed in 1892. The building takes the form of a Neo-Gothic cathedral. There is a tower on each side of the main entrance, which is located on the east side. The towers extend about 185-feet above the ground. The north tower houses 10 bells on the sixth level (about 80-feet above the ground).

The Cathedral walls are constructed of clay brick faced with Connecticut River Valley Portland Brownstone (red sandstone). The stone facing (veneer) is typically 4- to 8-inches thick. Some of the unique profiled stones project out from the outside face of the wall and are thicker than 8 inches. The clay brick on the north tower varies from 24- to 36-inches thick.

Problem area

Sandstone is a sedimentary stone formed in horizontal layers called bedding planes. In building construction, it is important to install the stone so that the orientations of the bedding planes are the same as the orientation in which the stone is formed. However, the original stone on the Cathedral was installed with the bedding planes vertical. Over the years, freeze-thaw cycles delaminated the bedding planes, which resulted in portions of the stone falling to the ground.

This failure called for significant areas of the stone veneer to be replaced. The areas selected for replacement were prioritized, with the worst sections picked for Phase One. This work included the entire north tower.

A hoist was incorporated into the scaffolding system used on the job, which created one important question: Would the vibrations caused by operating a loaded hoist damage the overall structure of the tower? A vibration study was conducted to answer this question.

This instance was not the first time that man-made construction vibrations were a concern for the tower. Vibration monitors were installed at selected locations of the tower in 1999 during the pile-driving operation for a neighboring parking garage.

Also, there are a number of "tell-tale" 3-inch-diameter plaster disks installed in the tower over existing cracks. It is believed that the "tell-tales" were installed in the 1970s as a means of monitoring movement during the construction of the Empire State Plaza directly to the east.

Material movement

One of the challenges encountered in removing the existing delaminated stone and replacing it with the new stone was the logistics of moving the materials up and down the tower. The scaffolding and hoist were installed on the north side of the tower to facilitate the material handling (Fig. 2). Typical anticipated loads on the hoist ranged from 500 to 750 pounds, which is based on the weight of material that two workers are able to maneuver on the scaffolding.

[FIGURE 2 OMITTED]

In 2002, the outside wythe of stone was removed from the seventh level, which is about 80-feet above grade. The exterior face of the tower walls is inset at this elevation, which results in a "self-relieving" detail that allowed the stone below to be removed without extensive shoring to support the stone above. The upper portion of the tower will be addressed during the 2003 construction season.

Prior to beginning the construction work, a survey of the existing conditions was conducted. Two major observations were noted relative to the vibrations on the tower.

First, the interior masonry of the tower appeared to be in fair-to-good condition. Some vertical cracking was observed, but it appeared localized and relatively minor. There was no horizontal cracking noted, which is important because this type could indicate an overturning stability problem.

Second, three of the bells were supported on their original peals (frames that allow the bells to swing). The three bells weigh between 560 and 3000 pounds. Originally, the bells were rung by swinging, which caused significant vibrations in the tower. The swinging bells were decommissioned in the 1980s and they are now rung by an electronic clangor.

Analyzing the problem

Vibration measurements were recorded using an accelerometer and a Hewlett-Packard 3560A two-channel FFT dynamic signal analyzer. The equipment recorded particle accelerations (vibrations) of the masonry and the wood floors on the fourth and sixth levels. Horizontal accelerations were measured perpendicular to the masonry walls, and vertical accelerations were measured on the floors.