By Mitch Meadows | March 28, 2018
In 2017, DHC was contracted by Commercial Scaffolding, Inc. to provide engineered designs for the Paseo Colorado outdoor mall, in Pasadena, CA. DHC’s scope of work consisted of the structural shoring of existing columns and beams to allow for the installation of (4) new shear walls and shear wall foundations on the P2-lower parking level. The shear wall foundations varied in depth from 4’ to 8’ and extended beyond existing columns and existing column influence lines in all directions. This resulted in the need to relieve the weight off the columns so that no foundation or soil failure occurred during the excavation and pour of the new shear wall foundations. Several column locations also required fiber wrapping for strength increase, which required all beams and decking framing into the column to be cut back, supported, and then reattached after the wrapping was complete.
Project preparation began in January 2017 with conference calls and RFIs confirming shoring locations for demo operations. The first site visit I performed was on January 26th, 2017, in which the structural shoring contractor, demo contractor, general contractor, steel contractor, and concrete contractor all attended. Everyone had specific needs and requirements for shoring to complete their scope of work. It became apparent that the required structural shoring would need to be in place to accommodate all trades. The original shear wall design specified a 12’ max deep foundation, which brought up huge concerns for the concrete contractor, knowing that they could not excavate that deep and keep a 1 to 1 soil slope profile without running into adjacent column footings. Preliminary design approaches included pouring piles prior to excavating, excavating smaller openings for sacrificial shoring posts, and cantilevered shoring. However, due to low clearances above and limited space in the lower garage, these items could not be accomplished as needed. Several people on the site visit even voiced concerns that this project would not be possible, prompting a deeper dive into the end design goal.
At this point, a collaborative design approach with the structural engineer of the building was required, which resulted in the redesign of the shear wall footing to a lower depth, but larger area. Incorporated into the foundation were phased mini foundations to allow for the installation of shoring prior to the full excavation being performed. These foundations were critical to the shoring design for safely transferring loads into the soil and allowing the design process to begin. However, the locations of these foundations still resulted in a combination of the following challenges:
- Cantilevered shoring up to 8’
- 22’ clear spans,
- Multiple level shoring to pick up P1 and Level 1 slabs and beams concurrently
- Hanging columns for the removal of existing footings
- Loads of up to 400 kips per column
- Larger shoring costs and several sleepless nights
Once the design process began, many iterations were required due to the large number of parties involved. Additional items were determined to be needed such as support of concrete moment frame system adjacent to excavation and (2) additional foundations for shoring support.
The Paseo Colorado shear wall shoring encapsulated challenges of all types of structural shoring designs and then complicated them with incredibly large loads, limited working area, and 7,000 superintendents (okay maybe not that much). The existing structure had to be cut off from support columns at (2) levels and supported from foundation level up, requiring not only design of shoring, but also validation of the existing structure to adequately transfer the loads. Excavations prevented loads from going leg for leg down to grade and required the use of beam and tower systems to span away from problem areas. Head clearance, access to underground lower level, and new concrete pours also provided issues. Beams had to be craned in after demolishing holes in the slabs (2) levels down, and in some cases, additional drive path shoring had to be provided just to transport steel members to the foundation level. With the use of steel shoring beams up to 21” deep, existing 3’-deep concrete beams, and multiple levels of shoring beams stacks, towers capable of supporting 200 kip loads at less than a 3’ height were needed. Shoring equipment manufacturers had to be contacted and special PERI MP Props were shipped in just for this project. In addition, shoring towers, existing concrete beams, and existing columns either had to be floated above the excavations or hung with the use steel support beams back to pre-poured specially designed footings to be doweled into the final shear wall foundations. Collaboration from all teams on the project made this shoring possible.
Each individual shoring component had to be rigorously organized between multiple contractors, thoroughly analyzed for strength, and flawlessly installed to execute the scopes of work required on this project. With multiple moving targets and last-minute conferences, additional emergency designs had to be performed to keep on schedule. The structural engineer of the building actually modified the shear wall design and the concrete contractor phased work just to allow for installation of the temporary shoring prior to any demolition work (one of the shear walls also has (2) 4’-long pieces of W18x76 steel beam in it from the temp shoring because the pour location required the beams to pass through). Talk about a great structural engineer!
Once the installation had started, a two-level concrete moment frame was determined to be within the influence line of an excavation that would produce unsafe surcharge loads on the excavation shoring. Honestly, at this point nothing surprised me anymore. To relieve the loads, the plans were modified to span W21x101 steel support beams between previously poured foundations and newly designed foundations to suspend the moment frame weighing over 180 kips. This modification was not only produced the most effective solution, but it also allowed the project to continue without delays. Other modifications such as the need to provide additional foundation designs and the requirement for columns to be completed hung surfaced during the install and were taken care of in a similar process. Overall, the level of complexity of the shoring, organization between all contractors and engineers, and execution of the installation to keep the project safe and on schedule, places this project above all others for the year. Without the speed, success, and ingenuity of the shoring, this project would not have been possible.
The ability to complete the shoring design as the installation was occurring was a direct result of an innovative value analysis. The General Contractor, in coordination with the subs, were able to identify previously unknown critical path shoring items before the installation reached them to allow for our design team to complete them in a preemptive manner. This was innovative as all individual teams could collaborate on orders of design relating to importance to the completion of the scope. For example, once the subs agreed on the items they needed to get done, they relayed that information to our design firm to execute. Although this may seem like the obvious best approach, most construction projects do not rely on a single temporary shoring engineer. Instead, each sub requiring engineer uses their own engineer they have in-house or consulted separately. This common approach could not have been used to complete this project on time, or safely, as each sub has different design concerns. The final result was a safe shoring install and building erection, as well as being awarded the Structural Shoring Project of the Year from SAIA, just one of many DHC has been awarded over the years. This highlights the dedication, expertise, and commitment our company has for the construction industry.