The Wonders of Rebar

Reinforced concrete is really a wonderful thing (the same goes for reinforced concrete masonry unit (CMU) block walls).   We take two completely different types of materials (concrete and steel) and combine them to take advantage of the best of both materials.  Concrete (and CMU) elements are strong in compression – like when gravity presses down along the vertical axis of a wall or column – but if you try to pull it apart or bend it, it tends to fracture.   Steel could be strong in compression if you had enough cross-sectional area, but where it really shines is in tension, as when you pull it.   Steel is strong in tension and can bend without breaking.  Think of concrete like a pencil (if you bend a pencil hard enough, it will break) and the steel like a paperclip (you can bend it back and forth quite a few times before it breaks).  The pencil is exhibiting brittle behavior and the paperclip is exhibiting ductile behavior.

So, the beauty of reinforced concrete, or reinforced CMU block walls, is that the steel (rebar) and concrete or CMU are combined so that they are strong in compression (from the concrete/CMU) but they can also bend (to a point) without breaking.   You might ask yourself, when do we bend a beam, or a wall or a column.  Well, there are lots of answers, but the one that we are concerned with is, Explosive Events or Blasts.

An explosion is a rapid release of energy in the form of light, heat, sound and a shock wave, and it is the shock wave that we look at most often in blast resistant design (or when determining the strength of an existing building to a blast).    The shock wave does not generally apply forces in the same direction as gravity, rather it applies the load at 90 degrees to the direction of gravity (in the case of columns and walls) or opposite the direction of gravity (in the case of uplift on slabs and beams).     Since reinforced concrete and reinforced CMU walls have the ability to bend, they can be designed to resist blast loads or  (in the case of existing buildings) they will be able to resist more blast load than an unreinforced element.

 The first photo below shows the failure of unreinforced walls from explosive forces.  Note that the columns and beams that are around the walls faired much better than the walls themselves.  This is because they were constructed of reinforced concrete.  This next photo shows the same type of failure from the interior or occupied space within the building.   The failure of the exterior, unreinforced, wall is catastrophic (that is, it didn’t just bend in and end up curved or out-of-plumb, it blew right into the room).  Injuries from this type of failure can be significant and widespread. The second photo shows the same type of failure from the interior or occupied space within the building.   The failure of the exterior, unreinforced, wall is catastrophic (that is, it didn’t just bend in and end up curved or out-of-plumb, it blew right into the room).  Injuries from this type of failure can be significant and widespread.

All of the above is an attempt to explain why any building, wall, or structure that is supposed to be designed to resist explosions or is being built in a region of the world where there is a threat of explosion (Afghanistan, Yemen, Iraq, Somalia, Nigeria, to name just a very few) should NEVER be built with unreinforced walls.

What Makes An Extraordinary Load Different?

As part of our day-to-day consulting practice, we deal with what can be termed as Extraordinary Loads.   I was recently in a design meeting and when I mentioned what I consider to be a fairly low blast load, the structural engineer’s eyes widened. She said that this load would change the entire design, and she would have been correct, if we were talking about an Ordinary Load.    After this, it occurred to me that perhaps the concepts of Ordinary and Extraordinary Loads bear a little more explanation.

Ordinary conventional loads are based on services loads, a common terminology within the structural engineering community, that refers to a regularly occurring load encountered by a structure.  Service loads include gravity loads, live loads, wind, snow, etc.. A factored combination for these high-frequency service loads take into account serviceability and strength, resulting in structures designed to withstand these loads with a certain safety factor and within a movement limitation for the comfort of building occupants.

The magnitudes of these loads are relatively low (usually measured in pounds per square foot for uniform loads) and they are applied for long durations.    Because of the relatively long or continuing duration of these loads, a static analysis approach is used in design.

On the other hand, Extraordinary Loads, including hurricanes, earthquakes, tsunamis, and explosions, have a significantly lower frequency of occurrence, and buildings designed to resist them are often assumed to experience some level of damage once the loading has passed.  These loads are generally considered as a separate load case without the increase factors used for service loads.    In most cases, life-safety is the main goal in design for these loads, unless the facility is deemed as mission critical and must function after an extreme event.  The magnitudes of these loads can be very high (measured in pounds per square inch for uniform loads) and are applied for a relatively short period of time (measured in seconds or even milliseconds).    Because of the very short duration of Extraordinary Loads, a static analysis approach would generally result in an overly conservative (and therefore cost-ineffective) design.   Instead of a static analysis, a performance–based criterion utilizing a dynamic analysis approach is often used, taking into account the duration, in addition to the magnitude of the load. This distinction allows us to meet the requirements of a project, as well as provide our clients with a feasible and cost-effective design when dealing with Extraordinary Loads.

Check out our upcoming Webinars and Face-to-Face Course for more in-depth information.

Three Critical Elements to Include in RFP and Tender Documents ForBlast Resistant Design

Those of us who have been in the business of designing and building blast resistant buildings for a long time have seen RFP and Tender documents evolve from a single line statement (e.g. “Meet ISC Security Criteria”) to long lists of referenced documents and required approaches.   The problem is, that if not carefully and meticulously developed, these longer long lists can be less informative and more confusing than that single statement we used to get.

RFP and Tender documents, whether they be for design or assessment, for new or existing buildings, are often incomplete and/or self-contradictory.  This can lead to proposals from consultants and contractors that:

• Do not address all of the blast resistant requirements that the owner intended, and
• Are so different from each other that it is impossible for the owner to adequately judge which is the best value offer.

While there are many complexities and nuances that need to be considered, there are a few items that must be included and considered in RFP and Tender Documents.

The three major essentials are:

• Threat Scenarios.  Conveying the applicable threat scenarios is critical because this is what tells the blast engineer what loads are going to be imposed on the building façade and structure.   The threat scenarios can be included as explosive weights and locations, pressures and impulses, or based on a reference to a standards document (available to all bidders) that clearly defines the same.
• Level of Protection.   The level of protection defines the response of the structure or façade elements to the blast loads.  This can be conveyed as an actual level of protection (e.g. Low, Medium, High) as defined by a referenced criteria or standard document, as specific response limits (calculated ductilities and rotations for different structural and façade elements), or in narrative form describing the acceptable types of damage and injuries.  Note that the narrative form can still result in inconsistent tender responses because it leaves the interpretation of the narrative up to each of the different bidders.
• Governing Guidelines, Criteria, or Standards Documents.  Since the September 11^th attacks, the number of government agencies, engineering and design organizations, and private companies who have developed guidelines, standards, and criteria documents is overwhelming.   Just trying to keep an updated list of all the documents (and their most recent versions) can be a full time job.  While all of the documents have the same objective – provide direction on how to implement blast resistant design – they vary widely in their approach and protection philosophies.  Prior to developing an RFP or Tender document, owners should determine which documents (or portions of documents) they want their project to use, and should make sure they understand the implications of that decision from both a cost and a protection standpoint.  By referencing a specific document, owners can provide clear direction to bidders with respect to analytic approach and requirements.

Our next blog post will discuss how to select the appropriate guidelines and criteria documents, or how to develop your own.

Check out our upcoming Webinars and Face-to-Face Course for more in-depth information.