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Exposure Category B

Urban & Suburban
Wind Protection

In Miami-Dade's downtown and suburban neighborhoods, buildings act as giant windbreaks. Watch how urban terrain slows the wind and lowers your wind load calculations.

Exposure B: Urban/Suburban Environment
0.70
Kz at 30 ft
Wind Slowed by 30%
Surface Roughness B - Urban/Suburban

Buildings as Windbreaks

Imagine you're walking through downtown Miami. Tall buildings surround you on all sides. Even on a windy day, you might notice the wind feels calmer at street level than up on the rooftops.

That's Exposure Category B in action! All those buildings, trees, and houses create "surface roughness" that slows down the wind. It's like how water flows slower through a rocky stream than a smooth canal.

In wind load calculations, we use lower Kz values for Exposure B because the rough terrain absorbs some of the wind's energy before it reaches your building.

Exposure B (This Page)
Urban/suburban areas with buildings, trees, surface obstructions
Exposure C
Open terrain, flat farmland, grasslands
Exposure D
Flat unobstructed areas, water surfaces, coastal
2,600 ft minimum
or 20x building height, whichever is greater

The 2,600-Foot Rule

Here's the catch: you can't just look at your immediate neighbors. ASCE 7-22 says you need to check the terrain UPWIND from your building for a specific distance.

For Exposure B, the upwind area must have urban/suburban roughness for at least 2,600 feet OR 20 times your building height - whichever is greater.

This is why coastal properties in Miami-Dade often can't use Exposure B. Even if you're surrounded by buildings, if the ocean is within that fetch distance upwind, you'll need Exposure D instead.

Lower Kz = Lower Wind Loads

The Kz value directly multiplies your wind pressure. At 30 feet height, Exposure B gives you Kz = 0.70 compared to Exposure C's 0.98.

That's a 28% reduction in velocity pressure! For a Miami-Dade building with 180 mph wind speed, this can mean significant savings in structural materials - but ONLY if your site truly qualifies for Exposure B.

Don't try to claim Exposure B if you don't have the required upwind roughness. Building officials will check, and claiming the wrong exposure is both dangerous and code-violating.

Height (ft) Exp. B Exp. C Exp. D
15 0.57 0.85 1.03
30 0.70 0.98 1.09
60 0.81 1.13 1.22
100 0.99 1.27 1.35

Frequently Asked Questions

What is Exposure Category B? +
Exposure Category B applies to urban, suburban, and wooded areas where buildings, trees, and other obstructions slow down and break up wind flow. It provides the lowest wind loads of all exposure categories because surrounding structures act as windbreaks.
What areas of Miami-Dade qualify for Exposure B? +
Inland urban areas of Miami-Dade with dense building patterns can qualify for Exposure B. However, the upwind fetch must have surface roughness B conditions for at least 2,600 feet or 20 times the building height, whichever is greater. Coastal areas typically don't qualify due to the open water upwind.
How does Exposure B affect Kz values? +
Exposure B has the lowest Kz (velocity pressure exposure coefficient) values. For example, at 30 feet height, Exposure B has Kz = 0.70, while Exposure C has Kz = 0.98 and Exposure D has Kz = 1.09. This means Exposure B buildings experience about 30% less wind pressure than Exposure C at the same height.
Can I use Exposure B near the coast in Miami-Dade? +
Generally no. ASCE 7-22 requires that the upwind terrain be evaluated. If any portion of the fetch within the required distance has Exposure C or D characteristics (open water, flat land), you must use the higher exposure category. Most coastal Miami-Dade areas require Exposure D.

Calculate Your Miami-Dade Wind Loads

Our calculator automatically determines the correct Kz values based on your exposure category. Get accurate ASCE 7-22 wind load calculations for your Miami-Dade project.

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