public static class Building
{
    public static readonly List<Beam> Beams = new List<Beam>();
}

public class Beam
{
    public string Story;
    public double Elevation;
}

<ComboBox x:Name="cmbBuilding"  ItemsSource="{Binding}">
    <ComboBox.ItemTemplate>
        <DataTemplate>
            <Grid Width="300">
                <TextBlock Width="150"  Text="{Binding Path=Story }"/>
                <TextBlock Width="150" Text="{Binding Path=Elevation}"/>
            </Grid>
        </DataTemplate>
    </ComboBox.ItemTemplate>
</ComboBox>

var b1 = new Beam { Elevation = 320, Story = "ST1" };
var b2 = new Beam { Elevation = 640, Story = "ST2" };
Building.Beams.Add(b1);
Building.Beams.Add(b2);

<Grid>
    <ComboBox x:Name="cmbBuilding" Width="100" Height="25" ItemsSource="{Binding Path=Beams}">
        <ComboBox.ItemTemplate>
            <DataTemplate>
                <Grid Width="300">
                    <TextBlock Width="150" Text="{Binding Path=Story}" HorizontalAlignment="Left" />
                    <TextBlock Width="150" Text="{Binding Path=Elevation}" HorizontalAlignment="Right" />
                </Grid>
            </DataTemplate>
        </ComboBox.ItemTemplate>
    </ComboBox>

    <Button Content="Add item" VerticalAlignment="Top" Click="Button_Click" />
</Grid>

public partial class MainWindow : Window
{
    Building building = new Building();

    public MainWindow()
    {
        InitializeComponent();

        building.Beams = new List<Beam>();

        building.Beams.Add(new Beam
                           {
                               Elevation = 320,
                               Story = "ST1"
                           });

        this.DataContext = building;
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        var b1 = new Beam { Elevation = 320, Story = "ST1" };
        var b2 = new Beam { Elevation = 640, Story = "ST2" };

        building.Beams.Add(b1);
        building.Beams.Add(b2);

        cmbBuilding.Items.Refresh();
    }
}

public class Building
{
    public List<Beam> Beams
    {
        get;
        set;
    }
}

public class Beam
{
    public string Story
    {
        get;
        set;
    }

    public double Elevation
    {
        get;
        set;
    }
}

public class Beam
{
    public string Story { get; set;}
    public double Elevation { get; set;}
}

public static readonly ObservableCollection<Beam> Beams
                                   = new ObservableCollection<Beam>();