Discussion: View Thread

Hello,

We design and install meteorological guyed and self-supported masts. Most of our masts have service lives of 1 to 3 years. Since we consider them risk category I, we use a 300-year mean recurrence interval. However, we are curious to know whether it is an acceptable practice to assume much lower return periods, given that the structure is temporary and has a maximum life of 3 years.

For example, before accounting for the importance factor and the wind load factor, can we use 25-year mean recurrence interval instead of 50, then apply the wind load factor and the importance factor?

Assem

This is an excellent question, that has been considered, but not answered many times by the ASCE 7 committee.  There are two relevant points of view:

1- Since a temporary structure will exist only for a limited time period, if it is designed for the same load return periods as permanent structures, it will have a much lower risk of failure, during its lifetime.  This is true and an argument for using reduced return period loads for design.

2- If a person enters a temporary structure (or stands next to it), that has been designed for reduced return period loads, then that person will be at higher risk of injury while in (or next to ) the temporary structure than if that person were in (or next to) a permanent structure.  This argues that to provide equal safety to occupants, all structures should be deisgned for the same return period loads.

A third point is that many structres that are intended to be "temporary" end up being used far beyond their expected life times.  For example, following the great San Francisco earthquake of 1906, government agencies provided shelter for homeless residents of the City in the form of inexpensive, modular wood frame buildings, the then equivalent of "FEMA Trailers."  Not surprisingly, some of these stuctures are still in service today,nearly 120 years later. As a society, we have little control over the temporary nature of construction.

As you are likely aware, ASCE 37 provide loads for design of structures during construction that are substantially reduced from tthe requirements for permanent conditions.  THese loads are reduced in part becasue the time frame of the construction period is short, but also because construction sites have relatively few occupants, there is an implied risk accepted by construction workers and sites can be shut down if a large storm is known to be approaching.  It is therefore not applicable to the types of structures you are working with.

The ASCE 7 committee will continue to wrestle with temporary structures and may some day have more definitive guidance.

Hello,

We design and install meteorological guyed and self-supported masts. Most of our masts have service lives of 1 to 3 years. Since we consider them risk category I, we use a 300-year mean recurrence interval. However, we are curious to know whether it is an acceptable practice to assume much lower return periods, given that the structure is temporary and has a maximum life of 3 years.

For example, before accounting for the importance factor and the wind load factor, can we use 25-year mean recurrence interval instead of 50, then apply the wind load factor and the importance factor?

Assem

This is an excellent question, that has been considered, but not answered many times by the ASCE 7 committee.  There are two relevant points of view:

1- Since a temporary structure will exist only for a limited time period, if it is designed for the same load return periods as permanent structures, it will have a much lower risk of failure, during its lifetime.  This is true and an argument for using reduced return period loads for design.

2- If a person enters a temporary structure (or stands next to it), that has been designed for reduced return period loads, then that person will be at higher risk of injury while in (or next to ) the temporary structure than if that person were in (or next to) a permanent structure.  This argues that to provide equal safety to occupants, all structures should be deisgned for the same return period loads.

A third point is that many structres that are intended to be "temporary" end up being used far beyond their expected life times.  For example, following the great San Francisco earthquake of 1906, government agencies provided shelter for homeless residents of the City in the form of inexpensive, modular wood frame buildings, the then equivalent of "FEMA Trailers."  Not surprisingly, some of these stuctures are still in service today,nearly 120 years later. As a society, we have little control over the temporary nature of construction.

As you are likely aware, ASCE 37 provide loads for design of structures during construction that are substantially reduced from tthe requirements for permanent conditions.  THese loads are reduced in part becasue the time frame of the construction period is short, but also because construction sites have relatively few occupants, there is an implied risk accepted by construction workers and sites can be shut down if a large storm is known to be approaching.  It is therefore not applicable to the types of structures you are working with.

The ASCE 7 committee will continue to wrestle with temporary structures and may some day have more definitive guidance.

Hello,

We design and install meteorological guyed and self-supported masts. Most of our masts have service lives of 1 to 3 years. Since we consider them risk category I, we use a 300-year mean recurrence interval. However, we are curious to know whether it is an acceptable practice to assume much lower return periods, given that the structure is temporary and has a maximum life of 3 years.

For example, before accounting for the importance factor and the wind load factor, can we use 25-year mean recurrence interval instead of 50, then apply the wind load factor and the importance factor?