Structural Engineer

STRUCTURAL ANALYSIS AND DESIGN

Introduction

Aircraft Rescue and Firefighting (ARFF) renovation and expansion includes demolition of portion of existing structural steel. The expansion requires the design of concrete moment resisting frame, owner preferred, with a maximum clear span of 72 feet at the vehicle bay area and smaller spans at both sides for service and storage areas. There is also new mezzanine storage over one of the vehicle bay area.

Building Description

The expansion composed of new building of reinforced concrete frame. Main frame is a gable type of 24” by 48” concrete columns with 72 feet clear span beam measuring 48 inches at the column support tapering down to 36 inches at mid span intersection. The building has side frames that support stability and resisting lateral loads while continuous frame were used in the longitudinal direction. In general, the building is a special moment resisting space frame. The concrete roof slab in between frames transfers the lateral forces to the main frames down to the foundation. This will achieve the proper load path of this high one story but massive concrete structure.

The building has a perimeter parapet along the main frames. Side parapet has as high as 10 feet while front and back parapet has as high as 15 feet. Openings at the front and back are provided with Bi-fold type doors while roll-up doors were provided to the side frames. Cranes with 5 ton capacity were also provided at the service area.

Proposed Aircraft Rescue and Firefighting Renovation and Expansion вЂ" Isometric View

Because of the high allowable bearing capacity of the soil at the site, the foundations for the extension are to be on spread footing. Frames adjacent to existing building were to rest on the footing of the existing building that has to be strengthened depending on the additional loads imposed by the new concrete frame.

The renovation consists of replacing the existing pre-engineered building’s metal deck roofing with concrete roof slab and the existing metal cladding with concrete masonry (CMU) walls.

There are 2 inches gap between the new and existing structures to allow for movement during strong earthquake and typhoon.

Building Site

The building occupancy category falls under essential facility. The seismic design will be in accordance with Chapter 19 of the UBC 1997. Building lateral forces are calculated based upon LRFD load combinations in UBC section 1612.2.2 and the base shear equations in section 1630.2. The building is located in Seismic Zone No. 3 with the following site conditions:

Z = 0.30 (Table 16-I)

Soil Profile Type = Sd (Table 16-J)

I = 1.25 (Table 16-K)

Seismic Source Type = B (Table 16-U)

Ca = 0.36 (Table 16-Q)

Near Field Factor Na = 1.00 (Table 16-S)

Near Field Velocity Factor, Nv = 1.00 (Table 16-T)

Rw = 8.5 (Table 16-N)

The seismic forces used in this building are based upon a “regular shaped” building. However, a separate dynamic analysis and design were provided as a spot check of the main frames resisting lateral forces.

Wind Loads

The design wind pressures will be in accordance with Division III Chapter 16 of the UBC 1997 edition of UBC. The structure is considered to be at locations of most severe exposure with basic wind speeds of 80 mi/hr or greater. Design wind speed is 170 mph. The building is located in Exposure D with the following site conditions:

Ce = 1.39, minimum (Table 16-G)

Cq = Method 1 (Table 16-H)

I = 1.15 (Table 16-K)

Qs = 73.984 psf (Table 16-U)

UBC 1997 design wind pressure, P

P = Ce Cq Qs I

Height brackets and velocity pressures will be as follows.

Height above ground

Feet

Velocity PressureÐ'¬Ð'¬Ð'¬Ð'¬Ð'¬Ð'¬Ð'¬Ð'¬Ð'¬Ð'¬ Ð'¬Ð'¬Ð'¬Ð'¬Ð'¬

pounds-force per square foot

0 - 15 118

20 123

25 127

30 131

40 137

The above velocity pressures are average values for the indicated height brackets. The design wind pressures will be determined by multiplying the velocity pressures by the appropriate pressure coefficients given in UBC Table No. 16-H.

If wind design governs, the detailing requirements and limitations in the UBC 1997 seismic provisions will also be followed.

Modeling Assumptions

Staad.Pro structural software was used for analysis and design of the building. The building was analyzed as a three dimensional frames subjected to earthquake and wind lateral forces in both orthogonal directions. Several trials were made to arrive at the most optimal beam and column sizes to properly support gravity and lateral loadings and to satisfy the allowable building drift caused by earthquake load. Lateral drift caused by wind load was limited to L/300 because there are no critical non-structural elements attached to the main lateral resisting frames. The following assumptions were used in the modeling process:

• Frames were analyzed in two orthogonal directions, North-South and East-West.

• The bases of columns were assumed pinned but the torsional effect was considered.

• All beams were assumed to be