Loads during construction: what to consider and what methods to use

The challenges

Very often, on industrial projects, design engineers are asked to validate partially completed structures during construction phases in order to comply with specific constructibility requirements, equipment installation limitations or construction sequence requirements.

In practice, some structural engineers may not be aware that construction loads can be used when analyzing temporary conditions and, instead, rely on loads that apply during the structure’s lifecycle. This might lead to expensive design requests for more construction materials and cost increases. Also, loads during construction must be taken into account to ensure the safety of all workers on site, so the important questions to ask are: “What should be considered and what methods should be used?”

It is not always clear what code or reference should be used to apply construction loads adequately. For this reason, BBA structural engineers have reviewed several references in the National Building Code of Canada (NBC), Canadian Standards Association standards, Eurocodes, IBC codes and ASCE standards. We have found that the most appropriate reference for North American conditions is ASCE 37 – Design Loads on Structures during Construction since:

• It clearly defines all loads that may occur during construction and how to apply them.
• U.S. and Canadian construction practices for design, fabrication, erection and onsite execution are identical.

The solutions

Construction loads are defined as loads imposed on a partially completed or temporary structure during the construction process. Construction loads include, but are not limited to, materials, personnel and equipment imposed on the temporary or permanent structure during the construction process. Other than the design loads on structures for permanent use, the following loads during construction must be considered:

Loads due to construction activity

C = construction loads

Loads due to construction include the weight of temporary structures, material loads, construction execution loads and lateral earth pressures. If no information is available in the early stages, the ASCE 37 standard can be used as a guideline for load estimation. All the abovementioned loads must finally be confirmed with the construction contractor.

Environmental loads during construction

W = wind loads

Designing wind loads on structures during construction may be applied using a reduction factor:

Special attention should be given to loads on staging structures and partially enclosed structures, especially near the edges.

T = thermal loads

Provisions must be made for thermal distortions of structures and architectural components when structures are erected under the following conditions:

1. When the product of dimension and temperature difference exceeds 7,000 ft-°F (1,185 m-°C)
2. When portions of the structure, which will be shielded when the structure is completed, are subjected to direct solar radiation in cold or hot weather.
3. Whenever temperature changes create distortions that could damage structural or architectural components.

S = snow loads

When snowfall is expected during the construction period, snow loads must be considered. If construction will not occur during the winter months when snow is expected, snow loads need not be considered. Designing snow loads on structures during construction may be applied using a reduction factor:

R = rain loads

For temporary conditions that persist for one month or less, rain loads need not be considered for construction during months with historical rainfall averages of less than 1 in. (25 mm) per month. Otherwise, rain loads must be considered. Care must be taken to keep drains clear and to provide unobstructed paths for rainwater to flow from structures. Water that accumulates in unfinished structures should be removed.

I = ice loads

For construction during seasons when structures are not susceptible to ice accumulation, ice loads need not be considered. Structures that will be enclosed when construction is complete and are designed for live loads of 1.0 kPa or more need not be considered as ice-sensitive structures while open during construction. Otherwise, ice loads must be considered.

However, should ice accumulate on these structures, it should be removed. Or, the construction and live loads applied to the structure should be reduced by an amount corresponding to the weight of the accumulated ice.

E = earthquake effects

It is not reasonable to require seismic resistance for temporary works where major earthquakes are infrequent or unlikely. This means that earthquake loads need not be considered, unless required by the competent authority and the mapped Risk-Targeted MCER, 5% damped, spectral response acceleration parameter equals or exceeds 0.40 at a period of 1 s, S1, as defined in Section 11.4.1 of the ASCE/SEI 7-10 code. It has to be considered that the seismic data is not equivalent between the ASCE and NBCC codes because of different damping criteria used.

Conclusion

In short, the loads on structures during construction may be considered practically in such a way:

1. Loads due to construction activities must be aligned with the construction strategy and evaluated with the construction team in accordance with the ASCE 37 standard or at least as a guideline.
2. Environmental loads do not need to be considered if they definitely will not happen during construction, otherwise
3. Environmental loads must apply with certain reduction factors based on the construction schedule.

For more information about this topic, please contact us. The BBA team has a solid background in structural engineering, as well as constructability analysis and technical assistance during construction.

References

NBCC – National Building Code of Canada

ASCE 37 – Design loads on Structures during Construction

EN 1991-1-6 – General actions – Actions during execution