Shoring Considerations in Existing Buildings

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Construction in existing buildings can encompass repair, restoration, horizontal and/or vertical expansion, modification of architectural programs, or changes in occupancy classification to achieve project goals. Typically, the project’s Registered Design Professional (RDP) develops construction documents for the final design configuration which only show permanent design conditions and do not consider temporary loading and support conditions. Contractors are responsible for means, methods, and sequences of construction to achieve the final design intent, allowing them to complete the work in accordance with their own preferences and capabilities.

Some projects require temporary structural modifications, while others may rely on the existing structure to support construction activities. Temporary shoring is required when existing structural elements do not possess sufficient strength to support construction loads. Shoring, typically, is viewed as props, posts, beams, or other members installed temporarily to provide support to an existing building element during construction.

The RDP generally does not provide shoring design with the final design documents as it is not part of the permanent structural system. The responsibility for temporary conditions is left to the Contractor, who engages a Specialty Structural Engineer (SSE) to serve as the Design Professional in Responsible Charge of designing and specifying shoring systems. The SSE must consider many factors when designing shoring including, but not limited to, temporary loads during construction, facility use during the work period, appropriate types of shoring, the available strength of the existing structure, including any modifications as part of the work, and the Contractor's sequences, methods, preferences, and capabilities. This article presents several of these considerations and the challenges they can create for the SSE, with examples of how these obstacles were addressed on previous projects where the authors acted as the SSE for shoring design.

Temporary loads during construction and their associated load combinations vary from the loads and combinations used by the RDP to design the permanent structural system. The American Society of Civil Engineering Standard for Design Loads on Structures During Construction (ASCE/SEI 37), defines these loads and generally considers the following three categories:

Additionally, load factors in ASCE/SEI 37 combinations differ from permanent design load combinations to reflect the range of applications and potential for misuse or errors, as well as the inherent or natural variabilities in loading considered by permanent design load factors. Load factors vary from 0.5 to 2.0 depending on the category of the load (described above), the potential variability in loading, and whether loads are additive or counteractive. The commentary to ASCE/SEI 37 Section 2.2 provides additional information on load combinations to be used during construction.

Construction within occupied spaces requires the existing building to remain functional for occupants and users, which creates restrictions on where shoring and temporary structures can be located and requires design live loads to be considered in part of the structure. Many structures are designed utilizing the full design load leaving little to no reserve member strength. As a result, structures that remain occupied during construction usually require more shoring than if access is restricted to construction activities only, since construction live loads are usually less than the full design live load under occupancy uses, allowing the difference to be used to support the imposed shoring loads.

A project at Chicago’s Union Station Great Hall involved construction of a new skylight and maintenance walkways, over the existing skylight, in the Great Hall Waiting Room. Temporary scaffolds were installed in the building to access high ceiling areas where work occurred. As the location is a major transportation hub, prohibiting public use of this space during construction was infeasible, thus preventing a ground-supported scaffolding system because the supporting posts would have impacted pedestrian flow. To minimize these impacts, scaffolding was suspended from the existing roof structure (Fig. 1). Suspending scaffolding over an occupied floor required significantly more effort by the SSE and contractor to coordinate the scaffold layout and design, existing structure evaluation, scaffold erection, and access methods.

Similarly, a concrete restoration project at a school building in the Midwest required extensive repairs to the mansard roof (French roof) structure during the school year. The unique roof geometry required temporary support by a combination of shoring towers under the roof ridge and spandrel beam along with inclined outriggers at the spandrel beam to resist horizontal thrusts (Fig. 2). This shoring occurred on the second floor, which was closed to occupants allowing a temporary reduction in the live load for that floor. Even considering reduced live loads in restricted-access areas, the second-floor structure did not possess adequate design strength to support shoring, which meant shoring on occupied lower levels was necessary to support the loading. These "re-shores" were concentrated in select locations to minimize impacts on occupied areas. Transfer beams within the second floor restricted-access spaces supported the roof shoring towers and distributed loads to the concentrated re-shores in the first floor and basement, allowing continued use of the lower occupied floors.

Shoring temporarily modifies the structure’s load path as construction is completed. Members can see increased or altered loading during the construction period and need to be assessed for these loads. For example, existing dead loads in a single span beam create tensile stresses at the beam bottom flanges; installing shoring at mid-span of a beam creates a continuous two-span condition and additional load creates tensile stresses at the beam top flange that would otherwise be in compression. The SSE must evaluate the existing members and assess whether there is sufficient strength to resist the altered behavior.

Many times, existing members lack sufficient strength leading to potential restrictions of variable loads, additional shoring, or differing sequencing. In buildings with high dead load-to-live load ratios that were originally designed using Allowable Stress Design (ASD), utilizing Load and Resistance Factor Design (LRFD) methodology can sometimes yield favorable results because, where ASD uses a blended safety factor for different load types, LRFD acknowledges that certain loads such as dead loads are known to a greater degree of certainty and can utilize a lower load factor. This more precise approach where load factors are consistent with the probability of exceedance can realize some additional reserve strength.

The existing roof girders supporting the suspended scaffold at the Union Station project initially showed allowable design strengths significantly lower than the required strengths. The SSE analyzed the girders using an LRFD methodology due to the high dead-to-live load ratio to realize additional reserve strength. This analysis also showed the girders were controlled by lateral-torsional buckling due to their infrequent compression flange bracing. Temporary compression flange bracing was added to realize additional design strength. The SSE and contractor also modified the construction sequence to temporarily reduce existing dead loads, prior to installing the suspended scaffold platforms, and limited platform live loads to hand tools and personnel. These steps allowed the work to continue without the need for significant or permanent modifications to the existing structure.

In another area of Union Station where pedestrian access could be restricted, ground-supported scaffolding was utilized. Initial analyses of the supporting floor structure used historical material properties available at the time of construction. Though multiple material strengths may be available depending on the construction timeframe, many industry guidelines recommend conservatively using the lowest available material strength when no additional information is available. The SSE’s initial analyses indicated local overstresses in the floor structure at some areas; however, a slightly higher material strength would have eliminated the overstresses. A sampling and testing program was developed and material samples from low-stress regions were obtained and tested to determine the actual material strength, which was sufficient to resist the construction loading.

Another technique that can be utilized is temporary unloading which, in this article, refers to the removal of a portion of the existing load on a structural element prior to performing work. The most common reason for temporary unloading is to allow the use of the entire cross section of the repaired member to resist the applied loading. For example, dead loads are typically resisted by unrepaired sections while additional loading is resisted by a combination of the unrepaired and repaired sections. This could result in localized overstress in the unrepaired sections. Temporarily unloading de-stresses the unrepaired sections prior to repairs and, upon removal, allows dead loads to be carried on the full repaired section.

During temporary unloading, the existing structure should be monitored for signs of distress. Visual cues such as (1) excessive or unexpected deformation, (2) cracking, delamination, and spalling in concrete members, or (3) buckling and warping in steel members indicate that temporary unloading operations should be stopped. Auditory signs of distress including popping, cracking, or tearing also can be indicators. If unanticipated distress to existing building elements occurs, further investigation into the existing structural strength may be required, loads may need to be released, or alternate/additional shoring may be required to avoid additional unacceptable distress. Non-structural building components, e.g., mechanical and plumbing systems and installed finishes, should also be monitored to ensure temporary unloading operations do not unexpectedly affect these elements.

The existing concrete beams at the Midwest elementary school were temporarily unloaded to allow the concrete repairs and additional strengthening to be effective in resisting the structure’s dead load. The beams were unloaded mechanically by incrementally turning screw jacks at the base of the vertical shoring towers supporting the mansard roof ridge. The vertical shoring towers were supported on simply supported steel transfer beams, elevated above the existing second finished floor level. This allowed measurement of the actual deflection of the steel transfer beams to determine the load in the shoring system. The SSE was able to confirm the existing concrete beams were adequately unloaded once the measured steel transfer beam deflections matched the expected deflections.

Construction in existing buildings typically involves limited accessibility within and around the building. The SSE should work with the Contractor to determine how shoring will be brought to the project site, stored, transported through the building to its final location, and erected. The final placement of shoring should also avoid interfering with the Contractor's operations and execution of work.

It is often infeasible for machinery to transport shoring elements through buildings for a variety of reasons. Shoring members are typically transported to their final location and lifted into place manually or using small-scale lifts or jacking equipment brought in through passenger or freight elevators. Shorter length and lower weight shoring members improve maneuverability and reduce the necessary equipment to lift pieces in place, but may require additional work in the field.

A project at Chicago’s Navy Pier involved dismantling the old Ferris Wheel to install a larger one. The project constructed new cast-in-place reinforced concrete shear walls in the east-west direction, under the north and south feet of the new Ferris Wheel. The new shear walls spanned vertically from existing precast concrete inverted tee beams at the pier parking level (lower level) to a new 12-inch-thick concrete mat above the pier park level (upper level), under the new Ferris Wheel. To construct the new shear walls, the existing precast concrete inverted tee beams at the upper level had to be removed. The upper-level inverted tee beams supported precast concrete double-tee floor beams that required shoring prior to demolition of the inverted tee beams (Fig. 3).

Complicating the shoring design was the fact that the lower level was rated for vehicular loads only (40 psf) which was less than the self-weight of the double tees above (approximately 90 psf). Additional constraints included minimizing contractor operations to place concrete for the new shear walls. Ultimately, a topside shoring approach utilizing steel transfer beams supported on concrete columns at the upper level, with cantilevered beams and hanging supports was developed to support the upper-level precast concrete double tees (Fig. 4). This approach provided unrestricted access to the lower level for construction activities and did not rely on the lower level for support.

When available, original structural and architectural drawings often provide the most direct and reliable information regarding the existing building. Many older buildings are renovated during their life, so site visits should be used to validate information shown on existing drawings and document modifications to the structure since the original construction.

When drawings are not available, literature reviews of historical materials, and non-destructive and destructive testing can be carried out to obtain necessary information about the base structure. Following is a list of references commonly used to obtain additional information about existing structures as well as to evaluate existing structures and design temporary shoring.

Considering potential construction sequences during design of the permanent structure may minimize the need for temporary shoring. Examples are a design that allows permanent construction to be installed prior to demolition (e.g., at slab or wall openings), designing permanent components to permit un-shored construction (e.g., beam spacing at infill slabs), and detailing optional splices for large or heavy members. In some cases, accounting for potential construction sequences during design can help contractor bids to be more evenly evaluated and can realize cost savings.

While the contractor ultimately remains responsible for shoring design, indicating the need for shoring in the construction documents helps delineate scope and responsibility and allows the contractor to obtain appropriate construction cost estimates. ACI's Code Requirements for Assessment, Repair, and Rehabilitation of Existing Concrete Structures (ACI 562, Section 9.2), recognizes that shoring is an inherent part of repair and rehabilitation and requires that shoring and bracing criteria be shown on the construction documents. The AISC Code of Standard Practice (AISC 303, Section 7.10) requires identification of special erection conditions or considerations required by the design concept that could not otherwise be known by the erector.

The RDP can take different approaches to indicate shoring needs on the construction documents, based on the complexity of the process and potential effects on the permanent construction. Arrows and notes graphically indicating permanent components requiring temporary support allows the RDP to convey scope, while allowing the contractor to design and select shoring materials, systems, and methods to their preferences. Another approach for more complex construction can be to outline a conceptual shoring sequence, as a basis of design for the permanent structure and to provide general guidance to the construction team. In this case, the construction documents should clearly indicate that conceptual shoring shown is for general information only and the contractor is still responsible for the design and specification of the final shoring system.

Construction in existing buildings that modifies or relies on the existing structure for support may require temporary shoring. Many project-specific factors and considerations influence shoring design, which if not considered can increase the difficulty of executing the work. These considerations include:

Working with the contractor to understand their means and methods will result in a successful shoring design and a completed project achieving the project goals. ■

About the Author

Ezra B. Arif Edwin, SE, is a Consulting Engineer with Simpson Gumpertz & Heger Inc. (Chicago). He received his B. Eng. from the University of Auckland and his M.S. from Virginia Polytechnic Institute and State University. Arif Edwin has over nine years of industry experience in the design of new structures, and the renovation and repair of existing structures. (ebarifedwin@sgh.com)

Kevin Conroy, SE, PE, P.Eng. is an Associate Principal with Simpson Gumpertz & Heger, Inc. (Chicago). He received his B.S. and M.S. from the Illinois Institute of Technology and has over 20 years of industry experience specializing in investigations, evaluations, and the development and implementation of repair designs for deteriorated structures. (kconroy@sgh.com)

R. Brett Holland, Ph.D., PE, SE is an Associate Principal with Simpson Gumpertz & Heger, Inc. (Waltham, MA). He received his B.S., M.S. and Ph.D. in Civil Engineering from the Georgia Institute of Technology. Holland has over 15 years of research and industry experience. He specializes in developing and implementing concrete for high-performance applications, concrete placement troubleshooting, specialized durability requirements, and investigating and repairing structures. (RBHolland@sgh.com)