June 03, 2011
Attacking the Problem from Every Angle: Shear Sliding
by Hinman Team
The retrofit of existing buildings to satisfy protection requirements involves a delicate balance between cost and benefit. Often times, renovation plans are overburdened by design efforts to satisfy client and stakeholder driven goals alongside codified building system performance requirements. The addition of blast services to a seemingly overloaded project scope can easily feel like trying to force a filled to capacity suitcase shut. Furthermore, the retrofit of a facility whose original design utilized out-dated physical security criteria, at best, can impose significant financial strains on a project budget. Consequently, the role of design professionals addressing blast and other extreme loading hazardous events walks a fine line between engineer and consultant.
At the outset of a renovation project, the most crucial service a blast engineer can provide is to educate the project owners and design team to establish a clear understanding of how performance goals translate into dollars and cents and how design solutions to protect the building will impact other design goals. This frequently results in a redefinition of threat criteria and related project requirements to address more significant levels of "acceptable" damage and risk in deference to aesthetic/architectural goals or other project work that is a higher priority with respect to the project budget. In other cases, project owners and stakeholders are as concerned with higher levels of protection as they are with an intricate façade or advancements to technological infrastructure and utility systems. In these instances, it can quickly become apparent that blindly following the language of blast criteria documents results in design solutions that are at odds with the efforts of other design disciplines. To be truly effective, blast engineers need to attack a design challenge from every angle and optimize their work so that minimal retrofit work results in maximum protection enhancements.
Although the analytical and design work associated with this objective is complex, it is easily captured in the adage "If it's not broken, don't fix it". But what really constitutes broken? Often times analysis of existing building systems seem to indicate expected failure in response to design basis threat scenarios. While the knee-jerk reaction of any engineer is to indicate a required retrofit, this approach side steps a whole list of important questions… Is my element really failing? What is controlling the failure response? Is failure acceptable? It quickly becomes evident that the explicit analytical approach taken results in an undesirable response. However, maybe there's another way to the look at the problem all together.
A perfect example of this scenario, is an approach we take at Hinman when analyzing concrete elements exposed to air-blast loading and controlled by shear "failure". In these cases, the element is considered to be too strong in flexure as it can sustain rotational and deflection responses well beyond realization of the section's shear capacity. To re-examine this problem we allow the element to "fail" in shear through shear sliding. This mechanism considers a reduction of the element's bending capacity to match the shear capacity (i.e., shear controlled element) rather than the reverse scenario, which represents the typical analytical approach (see Figure 2).
Figure 2 – Reduced Section Moment Capacity
Evaluation of sliding shear is most ideal for concrete columns as a result of the axial load, which is considered to restrain the column. Shear forces begin to develop at the supports and the column begins to displace (or slide) out of the supports, but the column is restrained by the axial load and friction forces (see Figure 3). However, the displacement response must be closely monitored as there is potential for abrupt failure. In general, shear sliding is considered relevant for conditions where:
- Vertical load-bearing elements have significant longitudinal steel reinforcement.
- Shear demand due to blast loading exceeds shear capacity of the element. (Per Table 4-1 of the Department of Defense UFC 3-340-02 blast criteria document, it is acceptable to allow the shear demand to exceed element capacity by 10% based on the design range of the threat.)
- Introduction of additional shear reinforcement is not feasible due to constructability issues.
- Existing conditions would make retrofit difficult and costly.
Figure 3 – Column "Displacement" due to Shear Sliding
This is just one example of a much larger tool kit available when evaluating existing facilities for blast criteria. With each project, we are able to further develop our understanding of how our work impacts the project design efforts at large so that we can contribute to a truly collaborative and versatile design that addresses project needs and goals in as elegant a manner as possible.