Skyline College Building 2: Means and Methods Column Shoring

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Skyline College is located 1 mile west of the San Andreas fault on a 111-acre site west of Skyline Boulevard and south of Sharp Park Road in San Bruno, California. The community college opened in 1969, and the buildings on campus are low-rise structures, mostly constructed of reinforced concrete in a common “brutalist” architectural style of that era. The original designers of the building were Isadore Thompson, Structural Engineers, and John Carl Warnecke, Architects and Planning Consultants.

Building 2 is a three-story, 53,000-square-foot concrete structure set into a hillside on the west, creating a partial basement. The floor plates are heavy and consist of a combination of flat slabs, waffle slabs, and one-way joists mostly on a conventional 18-foot grid. The roof has a pop-up central clerestory with long-span concrete girders that create space for theater-style seating in a large lecture hall. Column dead loads range from 75 to 300 kips where each column is supported on a single concrete caisson. The loads applied to the columns supporting the pop-up roof have much greater gravity loads than other columns. The caissons are interconnected with small, lightly reinforced concrete grade beams, with some on diagonal lines between columns to distribute the earth pressure from the hillside basement walls throughout the foundation.

A seismic retrofit was necessary to address numerous seismic deficiencies including non-ductile exterior precast concrete shear wall panels and discontinuous interior shear walls. As a community college in California, the project was under the jurisdiction of the Division of the State Architect, and a comprehensive seismic retrofit necessitated compliance with two performance levels based on the Tier 3 methodology described in ASCE 41-17 for a Risk Category III building. This Tier 3 methodology involves a detailed systematic evaluation of a building’s seismic performance using both linear and nonlinear analysis procedure.

Given the proximity to the San Andreas Fault and the heavy seismic mass of the existing concrete structure, the retrofit design involved the addition of concrete shotcrete overlay walls on the existing perimeter precast walls as well as interior concrete infill walls to eliminate vertical discontinuities in the existing lateral system. The existing caisson foundations were adequate for the much higher seismic demands required, so long as the lateral load could be distributed in a way that allowed all the caissons to work together. The original grade beam system was too small and did not provide a continuous load path to all of the existing caissons, so a new concrete grade beam system was required to vertically support the new concrete shear walls while simultaneously acting as a truss to uniformly distribute horizontal loads to all of the existing caissons. In order to directly connect the existing caissons with the new grade beams, the top six feet of the caissons would be removed while maintaining the existing reinforcement. This approach was necessary to avoid supplementing the existing foundations with additional deep foundation elements that were limited by site and building access constraints.

The project was a traditional “hard bid” public works project, so it was paramount that the contractor develop a safe, but economical, shoring solution. The column shoring scheme also needed to allow for the work occurring simultaneously throughout the building.

A common approach to shoring of columns involves point-loaded needle beams that are essentially two closely spaced steel beams attached to the sides of the column, with the beams simple spanning between two temporary supports. When columns are steel, it is easy to weld short transverse beams to the columns that then distribute the load to the steel needle beams. After the construction team calculated the approximate column gravity dead loads (including small construction live load allowances), it became apparent that attachment of steel lugs or bolts into the concrete columns would not work due to the high loads. The college also wanted to be able to expose the columns in the future, so a patched, Swiss cheese look was not something they would agree with.

The next thought was to use conventional post shoring at each column location over the three levels of the building. Due to the mix of gravity load systems within a story and vertically between stories, it became apparent quickly that a system of stacked spreader beams and four posts at each column was going to be difficult to design and time consuming to construct. One experienced engineer took over four hours to design a system for just one column. The building has 70 columns/wall locations supported on caissons, and most are unique in terms of the configuration of the gravity systems they support.

Complicating the post shoring approach was the width and layout of the new grade beams. It was not possible to post down through the grade beams (too much reinforcing steel), so it was necessary to find open spots away from the columns. Unfortunately, putting posts farther away from the columns put them at or near the quarter span locations in the slabs where there is typically little negative steel. If the spreader beams were located there, the floors might be damaged or collapse. Stacked spreader beams were considered, with one set near the columns spanning to another set farther away. But doing this at three levels at each column just did not seem like the right approach: too costly to design and build. Another approach was required.

The design team reached out to colleagues who design construction means and methods for contractors and asked if they had ever been faced with similar challenges. One colleague provided information about a project in which the means and methods engineer had clamped the concrete column tightly with two short wide-flange steel beams stressed with large, high strength bolts that spanned to needle beams. The surface of the steel that would contact the concrete was “roughened” with parallel beads of weld, to create what might be called shear keys—basically a load transfer mechanism akin to shear friction in ACI 318.

This sounded a bit crazy at first, but after some thought it made more sense technically and showed great engineering instincts and innovative thinking. Certainly nothing like this could be found in the building code or other design guidelines. The colleague was able to provide a drawing for the project which showed the column load and the clamping force, and from that it could be determined what the friction per square inch must have been. The project had been successful which provided one data point, but with an unknown coefficient of friction, mu, strength reduction factor, phi, and factor of safety. Anyway, it was a start.

The post approach was abandoned and back came the needle beams. The needle beams needed to span to “something” and this “something” is often drilled micro-piles, installed using low-overhead equipment in a basement. The general contractor client knew this was going to be expensive and asked the team to develop a scheme that was simpler and something they could self-perform. The first thought was timber mats at the existing slab level that would support the ends of the needle beams. The problem with this approach was that the contractor still needed to excavate six feet down to the bottom of the new grade beam. To avoid a slope failure, the needle beams needed to be long. This idea was discarded. It was decided to instead excavate and place the temporary foundations at the lower level. Timber mats (12x12 timbers bolted together) were an obvious solution but some of the mats would need to pass under the new grade beams. Timber was not permitted to be left in place, so this idea was also abandoned. As a result, cast-in-place concrete pads were used.

At this point the team thought they had a solid approach. The columns would be clamped with roughened wide flange beams, with the clamping beams (eventually called “grippers”) bearing on spreader beams spanning to needle beams. The needle beams needed to be placed above the new grade beams, so the needle beams were supported on steel posts bearing on the concrete pads. Although the building is supported on concrete caissons, the soil was found to be reasonably good for bearing based on recommendations from the project geotechnical engineer. Each building column had two pads (mostly 5 feet by 10 feet in plan by 1.5 feet thick), with each pad supporting one or two posts. Given the column loads and what was known about the skin friction that could be developed, it was concluded that two sets of grippers would be required at each building column so that all four column surfaces could be engaged.

To address the economics issue, it was determined that two wide-flange beam sizes, W24x117 and W18x76, would address the range of column loads and needle beam spans. The same grippers, fabricated from W14x159 sections, would be used at each column location regardless of load. Although posts could be re-used, the concrete pads would be left in place.

Where concrete bearing walls existed rather than columns (at stair and elevator cores, the building perimeter and certain interior shear walls), it was possible to punch holes in the walls and support the walls directly on the needle beams through bearing.

With only one data point and no solid available data for steel to concrete friction, the design needed to be tested. The general contractor client was reluctant at first but eventually came to understand what was known and what was not with respect to project risks (quite high) relative to the cost of the tests (quite modest).

The test set-up consisted of a concrete pad footing with a concrete column extending upward 8 feet. The column dimensions matched those in the building, and the columns were reinforced so that the column tension strength was twice that of the highest column gravity load that needed to be lifted. The two sets of grippers could theoretically provide more capacity than needed.

The general contractor had by then decided to hire a specialty subcontractor to install the shoring, do the actual column jacking, and assist with the monitoring. The subcontractor also assisted with the testing and developed the procedure for tightening the bolts to create a reliable normal force, akin to tightening the lug nuts on an automobile wheel. Hydraulic jacks were placed between the top of the footing and the grippers, and then the jacks were extended to load the column, while monitoring for clamp slippage. Slippage was a major concern since no alternative support system for the column could be quickly installed. The sub-contractor also recommended placing a thin, ¼-inch thick piece of plywood between the steel grippers and the contact surface on the concrete column. The benefit of the plywood was not apparent other than perhaps creating a more uniform bearing surface; the plywood might actually reduce the clamping friction.

The first test was a success since the column failed in tension, demonstrating that the grippers were able to transfer the required force. However, this test provided only one more data point, so another test set-up was constructed, this one including the plywood. The second test was successful too.

After the fact, it was learned that the sub-contractor had utilized this column clamping approach before and was very comfortable with the proposed design. If only this had been known at the outset! As the project progressed, it was observed that the weld beads had pressed through the plywood and embedded themselves in the surfaces of the columns, thereby contributing greatly to the working of the grippers.

Shoring up the entire building, or even a large part of the building, at one time would be too risky. Counter-balancing considerations were: 1) it would be prohibitively expensive to have enough steel to do major portions at the same time; 2) there was not enough space to store the excavated soil; 3) moving materials around the work area would be difficult; and 4) not enough workers were available.

After considering various solutions with input from their structural engineers regarding the design criteria in ASCE-37 Design Loads on Buildings During Construction (wind and seismic loads for temporary construction) and the earth pressure load from the hillside, the general contractor decided on an eight-phase approach that worked around the perimeter and from north to south, ending with the eighth phase in the main access point on the east side of the building. This permitted enough of the building caissons to be engaged to resist the wind and seismic loads at all times. The temporary wind loads surprisingly controlled the lateral force design even though the building is only a mile from the San Andreas fault. The west basement wall needed new waterproofing, so rather than excavate the west side soil in phases coinciding with the shoring phases, all of the soil was excavated at once to eliminate the soil pressure issue entirely. As the work progressed, the soil was replaced when adequate lateral load capacity had been restored.

The specialty shoring sub-contractor developed shop drawings for review by the designers. They frequently made recommendations for small changes that improved the basic design or allowed for the re-use of materials. In the end, the collaboration between the contractor, sub-contractor and designers was excellent and played a major factor in the project’s success.
The possibility of clamp slippage or the loss of grip was a constant concern. At the beginning, the general contractor hired a surveyor to monitor the column elevations each day. This was expensive and it became apparent this could not be done for the life of the project. But any slippage could not go unnoticed. After some research, the general contractor purchased and installed an electronic survey system linked to their computers that provided a continuous record of the column elevations. After working out the kinks and going through a few panicked and false alarms, the system worked well.

No project is perfect, but only two hiccups occurred.

The first was breaking of the clamping bolts. Special ASTM A193 Grade B7 bolts were required to develop enough clamping force. In one instance, the wrong (lesser grade) bolts were inadvertently transported to the site and installed. Fortunately, the bolts failed during the tensioning process rather than when the building was supported.

The second was gripper slippage during the final phase, just when everyone thought the end of the tunnel could be seen. There had been plaster on the column which had been left in place, so that was removed. Slippage occurred again, so the column faces were bush hammered. This proved to be the solution. The clamping load and vertical jacking load were applied and left in place over a weekend just to be sure, before the demolition of the columns could commence.

The planning, design, and testing began in June 2023. Construction of the first phase started in December 2023, with the work completed on March 10, 2025, with the last concrete pour. As with all construction means and methods projects, this was the time to finally relax! ■

About the Authors

James Enright, SE, is an Associate Principal with Element Structural Engineers, Newark and Oakland, California.

John Dal Pino, SE, is a Principal with Claremont Engineers Inc., Oakland, California and the Chair of the STRUCTURE Editorial Board.