Loads and Codes

Loads and Codes

Before any kind of structural analysis or design can be performed on a building or structure two things must happen:

  • Required Codes must be determined or specified
  • Loads must be determined

Specific Codes for this Discussion

Using Pennsylvania as an example, the required code is the state-wide building code known as the Pennsylvania Uniform Construction Code (PA UCC).   While the PA UCC creates a number of requirements specific to Pennsylvania, the loads and structural requirements are addressed by incorporating the International Building Code (IBC) as part of the requirements.  IBC structural sections are similar to the industry standard American Society of Civil Engineers (ASCE) Manual 7, Minimum Design Loads for Buildings and Other Structures.

As this particular installment is written to complement AE 210, the specific code years being referenced are IBC 2009 and ASCE 7-05 with some discussion of ASCE 7-10 (the latest version). Regardless of the edition, the basic material and self-weight based loads (Dead Loads) are not dictated by the codes (with a few minor exceptions) because they are a function of the density of the material or the actual weight of the assembly. Basic Occupancy-based loads (Live Loads) do not vary much from one code year to the next however; Live Loads that describe loading conditions such as wind or seismic events are frequently changed or refined in their magnitude or method of application.

Local Code Requirements

It is imperative that the Architect and Structural Engineer on a project discuss and coordinate the specific codes that will be used.  In addition, all design professionals on a project must check with the local code agency to determine if any local ordinances have been adopted that alter the determination, magnitude or method of application of a load.  For example, State College PA (Centre Region Code) has adopted a different ground snow load than is specified in IBC and ASCE 7.  This came about primarily as a result of a local roof collapse that occurred one winter from snow and a desire to have a higher factor of safety against snow related collapses in general.

Dead Loads

Dead Loads (DL) is the term used to describe loads that are a result of the materials used in the typical construction of a building.  Generally they are stationary and relatively easy to predict because they represent the self-weight of the materials or assemblies being used. The direction of the load is the same as the earth’s gravity which sometimes leads to the loads being referred to as Gravity Loads.  Although they act toward gravity, it is important to realize that the loads follow the actual assembly of the material in the structure and not necessarily the horizontal projection.  Thus, an assembly such as a gable roof will contain dead loads represented by the sloped surface of the roof and not the horizontal projection or span.

The following are examples of typical items that contribute DL to the structure:

  • Self-weight of the structural framing (beams, columns, concrete slabs, metal roof deck, etc.)
  • Finishes such as carpet, floor tile, drywall, plaster ceilings, roof shingles, etc.
  • Bearing and non-bearing walls including concrete masonry units (CMU) commonly called concrete blocks, bricks, wood framing, stucco, gypsum board (drywall), siding and sheathing, etc.
  • Items related to building engineering systems such as mechanical, electrical, plumbing, fire protection (sprinkler lines) commonly referred to as MEP.  MEP items are often a little harder to predict because they can relate to the type of systems selected rather than the type of occupancy.  Many engineers use an allowance or range to cover MEP on a typical project.

If you examine a set of plans for a building, you generally will not see a list of DL as they vary with the amount of building materials used in any particular location or assembly.  It is necessary to look at the architectural, structural and MEP plans, sections and details to determine the type, locations, thicknesses and orientation of the DL for a particular structure.

Self weights of many materials are often listed by their density such as Steel which is 490 lbs/cu ft (pcf) or reinforced concrete which is generally accepted to be 150 pcf (145 pcf for unreinforced).  Normally for ease of reference and calculations, we convert and sometimes “smear” these loads over an area representing part of the structural framing plan.  For example, the DL for a solid 3” unreinforced concrete slab would be found by:  DL = 3/12 * 145 pcf  = 37.5 psf

Dead loads can contribute to a category of loads called Construction Loads which are discussed in more detail later in this post.

Live Loads

Live Loads (LL) are commonly related to occupancy (residential, office, school) and type of use of a building (classroom, apartment bedroom, manufacturing assembly line, movie theater, etc.).  LL are considered to be variable, often transient, changing or varying with time, usually of short duration (compared to DL) and often less predictable than a DL.  For example from an occupancy standpoint, the students in a classroom come and go, sometimes the seats are filled – sometimes not.  Each student is a different weight and carries different items etc.

Most often LL are represented in lbs /sq. ft (psf) and considered as acting in a uniformly distributed manner across the entire surface area involved, somewhat like laying a weighted blanket across the area.  However, LL can be concentrated or point loads as in the case of a piece of heavy machinery, the wheel of a truck or a large heavy object such as a piece of medical equipment.

Another general group or source of LL is the category of environmental loads such as wind, seismic (earthquake), rain, snow, or even waves.  Although these types of LL are modified for structural size and type before being used in an analysis, they have geographic location as their primary basis.  For example, the codes provide maps for ground snow loads, wind speeds, and seismic accelerations among others.

In general, codes such as IBC provide tables or charts that specify the minimum LL that must be used in the design of a particular structure.  LL can vary throughout the structure depending on the exact occupancy type or the activity.  For example, an educational building could have any number of LL including, but not limited to the values noted below from the 2009 IBC:

  • Classroom:                                     LL= 40 psf
  • Corridors above first floor:       LL= 80 psf
  • Auditorium or Assembly:        LL= 100 psf
  • Library Stack Room:                  LL= 150 psf  (See code for additional specifics for stacks.)

While the LL listed in the various codes such as ASCE-7 are minimum required values, they have proven over time to be reliable under normal circumstances.  The actual values of LL on many structures when checked by on-site surveys is often lower than the code minimum (which is a good thing).  For example, in the commentary of ASCE 7-10, Table C4-10, the Survey Load for a classroom is noted to be 12.0 psf indicating a sustained load well below the code requirement.  However, not represented in that value is the transient loads that take place.  For example, a public assembly or well attended standing room only meeting held in a classroom could increase the actual LL considerably.   Design professionals must design for the worst case load that can reasonably be anticipated to occur over the lifetime of the structure.  A discussion of Live Loads and additional ASCE load survey results can be found in a posting called Live Loads Explained for Structural Design by Nishkian including a photo of what is noted as a Live Load overload of the Golden Gate Bridge.

The reader is referred to the LL tables in IBC or ASCE-7 for a more comprehensive list of required LL values.

 

Construction Load Situations

It is often the case that a structure is most highly loaded during the construction phase of a project.  Buildings are subjected to equipment and operations that may not be present, nor covered by the specified LL.  At the same time, structures may be subjected to regular loads such as code required wind loads but while they are in an incomplete state of completion whereby their permanent bracing may not be in place.  It is the responsibility of the contractor to address these temporary load situations by coordinating the stacking and storage of building materials and constructing temporary bracing and shoring as necessary to prevent overloads.  General guidelines for construction loads are available and are being developed by a number of organizations; however, specific construction load values for design are not currently required by model codes such as IBC.

 Code is Minimum Standard

It is important to realize that the code mandated Live Loads are minimum values.  If in the judgment of the design professional a situation exists where the code does not specifically discuss a particular load situation or there is a legitimate reason to establish a higher standard of design, it is acceptable practice to use a higher value.

Related Posts

You may also be interested in the following related educational posts:

Load Paths – Tributary Area Concepts

 

This post been prepared in conjunction with AE 210, Introduction to Architectural Structures as a summary review to be used in conjunction with the student’s lecture and practicum notes as well as related information contained on the Angel course management website. 

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