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
