Structural Design – Discussion for EWB Reviewer – building A
Codes Used
For load information ASCE 7-05 is used to determine vertical loads on the roof, wind loads, and
seismic loads. Concrete elements are designed per ACI 318-08. Cold formed steel is designed per
AISI 2007. Masonry walls are designed per IBC 2006. Both ultimate strength design and allowable
stress design are used in this project for the design of structural elements. However, care is taken to
match the method of design with the correct load combinations and load factors.
Vertical loads
Per ASCE 7-05, roof live loads are taken as 20 psf. Additional vertical loads are due to dead loads
caused by the weight of the structure and all permanently affixed elements. Where appropriate
vertical wind pressures are considered in the design of the structures for this project.
Vertical Load System
The vertical load system is as follows. Vertical loads travel through the structural elements in order
as follows, metal roofing, roof beams or trusses, to concrete frames and masonry walls, footings and
finally to the ground. When the structure is complete the walls are the primary vertical load bearing
elements. However, in the concrete frames are designed as if the walls were not present. In many
cases, this results in minimum steel requirements for temperature and shrinkage being sufficient
also for strength purposes. In those locations where more than minimums are required, the
increase is not significantly more expensive.
Lateral Loads
Wind – A document was found on the internet that gives wind values for regions in the
Caribbean and Central America. In particular, the document contains basic wind speed contours for
Honduras and Puerto Rico. ASCE 7-05 contains values for Puerto Rico, but not Honduras. Using the
previous document and ASCE7-05, and knowing the relationship between these two documents for
Puerto Rico, a value for Honduras consistent with ASCE 7-05 is obtained. Having the basic wind
speed, the design proceeds according to the simplified design procedure contained in ASCE 7-05.
The basic wind speed for the project site is 120 mph. The exposure chosen for the building site is
exposure C, although one could argue that the less severe case of exposure B is warranted. The
wind pressures are in the range of to psf. More details are provided in the calculations.
Seismic – A U.S. military document on the internet provides S1 and SD values for seismic design
in Puerto Rico and Honduras. Similar values are provided for Puerto Rico in ASCE 7-05. Using a
procedure similar to that explained for wind, values for Honduras are obtained. Then, having
obtained the values for Honduras, seismic design proceeds using the procedures described in ASCE
7-05. The calculated seismic coefficient for the structure designed as part of this project is 0.38.
Lateral Load System
For both wind and seismic design the concrete frames are designed as if the masonry block is not
present. (Although, in seismic design the concrete frames are designed as if the masonry block
weight contributes to the seismic forces.) This is usually a conservative approach for the concrete
frames. For many cases the stiffest elements are the masonry walls and not the frames and hence
the walls will resist the lateral loads. The walls are verified for adequacy as well. Therefore, the
system is fairly redundant, but not overly costly for the extra redundancy.
Foundations
On site investigations reveal very hard soil material. The clay is also found to be high. Spread
footings are used at the concrete columns and continuous footings are included under all walls. It
would be helpful to receive feedback regarding the problem of high clay content.
EWB
We will appreciate having a review by EWB. We look forward to comments and suggestions that
may be new to us regarding construction in a third world country and if our material specifications
are reasonable.
structural design summary.doc